Rsphinx.addnodesdocument)}( rawsourcechildren]( translations LanguagesNode)}(hhh](h pending_xref)}(hhh]docutils.nodesTextChinese (Simplified)}parenthsba attributes}(ids]classes]names]dupnames]backrefs] refdomainstdreftypedoc reftarget%/translations/zh_CN/filesystems/erofsmodnameN classnameN refexplicitutagnamehhh ubh)}(hhh]hChinese (Traditional)}hh2sbah}(h]h ]h"]h$]h&] refdomainh)reftypeh+ reftarget%/translations/zh_TW/filesystems/erofsmodnameN classnameN refexplicituh1hhh ubh)}(hhh]hItalian}hhFsbah}(h]h ]h"]h$]h&] refdomainh)reftypeh+ reftarget%/translations/it_IT/filesystems/erofsmodnameN classnameN refexplicituh1hhh ubh)}(hhh]hJapanese}hhZsbah}(h]h ]h"]h$]h&] refdomainh)reftypeh+ reftarget%/translations/ja_JP/filesystems/erofsmodnameN classnameN refexplicituh1hhh ubh)}(hhh]hKorean}hhnsbah}(h]h ]h"]h$]h&] refdomainh)reftypeh+ reftarget%/translations/ko_KR/filesystems/erofsmodnameN classnameN refexplicituh1hhh ubh)}(hhh]hPortuguese (Brazilian)}hhsbah}(h]h ]h"]h$]h&] refdomainh)reftypeh+ reftarget%/translations/pt_BR/filesystems/erofsmodnameN classnameN refexplicituh1hhh ubh)}(hhh]hSpanish}hhsbah}(h]h ]h"]h$]h&] refdomainh)reftypeh+ reftarget%/translations/sp_SP/filesystems/erofsmodnameN classnameN refexplicituh1hhh ubeh}(h]h ]h"]h$]h&]current_languageEnglishuh1h hh _documenthsourceNlineNubhcomment)}(h SPDX-License-Identifier: GPL-2.0h]h SPDX-License-Identifier: GPL-2.0}hhsbah}(h]h ]h"]h$]h&] xml:spacepreserveuh1hhhhhh?/var/lib/git/docbuild/linux/Documentation/filesystems/erofs.rsthKubhsection)}(hhh](htitle)}(h&EROFS - Enhanced Read-Only File Systemh]h&EROFS - Enhanced Read-Only File System}(hhhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhhhhhKubh)}(hhh](h)}(hOverviewh]hOverview}(hhhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhhhhhKubh paragraph)}(hEROFS (Enhanced Read-Only File System) is a modern, efficient, and secure read-only kernel filesystem designed for various use cases including immutable system images, container images, application sandbox images, and dataset distribution.h]hEROFS (Enhanced Read-Only File System) is a modern, efficient, and secure read-only kernel filesystem designed for various use cases including immutable system images, container images, application sandbox images, and dataset distribution.}(hhhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK hhhhubh)}(hXAn immutable image filesystem can be regarded as an enhanced archive format which allows golden images to be built once and mounted everywhere -- images are bit-for-bit identical across all deployments and can be verified, audited, or shared without concerns about runtime modifications (in this model, all user writes should be redirected into another trusted filesystem, for example, via overlayfs for copy-on-write-style redirection, by design).h]hXAn immutable image filesystem can be regarded as an enhanced archive format which allows golden images to be built once and mounted everywhere -- images are bit-for-bit identical across all deployments and can be verified, audited, or shared without concerns about runtime modifications (in this model, all user writes should be redirected into another trusted filesystem, for example, via overlayfs for copy-on-write-style redirection, by design).}(hhhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhhhhubh)}(hXEROFS is a dedicated implementation of the image filesystem idea above, with a flexible, hierarchical on-disk design so that needed features can be enabled on demand. Filesystem data in the core format is strictly block-aligned in order to perform optimally on all kinds of storage media, including block devices and memory-backed devices. The on-disk format is easy to parse and purposely avoids the unnecessary metadata redundancy found in generic writable filesystems, which can suffer from extra inconsistency issues -- making it ideal for security auditing and untrusted remote access. In addition, designs such as inline data, inline/shared extended attributes, and optimized (de)compression provide better space efficiency while maintaining high performance.h]hXEROFS is a dedicated implementation of the image filesystem idea above, with a flexible, hierarchical on-disk design so that needed features can be enabled on demand. Filesystem data in the core format is strictly block-aligned in order to perform optimally on all kinds of storage media, including block devices and memory-backed devices. The on-disk format is easy to parse and purposely avoids the unnecessary metadata redundancy found in generic writable filesystems, which can suffer from extra inconsistency issues -- making it ideal for security auditing and untrusted remote access. In addition, designs such as inline data, inline/shared extended attributes, and optimized (de)compression provide better space efficiency while maintaining high performance.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhhhhubh)}(hDIn short, EROFS aims to be a better fit for the following scenarios:h]hDIn short, EROFS aims to be a better fit for the following scenarios:}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK!hhhhubh block_quote)}(hX- As part of a secure immutable storage solution, where it needs to be immutable and bit-for-bit identical to the official golden image for each individual copy, in order to meet security, data sharing, and/or other requirements; - Minimizing storage overhead with guaranteed end-to-end performance by using compact (meta)data layout, optimized transparent data compression, deduplication and direct access, especially for those embedded devices with limited memory and high-density hosts with numerous containers. h]h bullet_list)}(hhh](h list_item)}(hAs part of a secure immutable storage solution, where it needs to be immutable and bit-for-bit identical to the official golden image for each individual copy, in order to meet security, data sharing, and/or other requirements; h]h)}(hAs part of a secure immutable storage solution, where it needs to be immutable and bit-for-bit identical to the official golden image for each individual copy, in order to meet security, data sharing, and/or other requirements;h]hAs part of a secure immutable storage solution, where it needs to be immutable and bit-for-bit identical to the official golden image for each individual copy, in order to meet security, data sharing, and/or other requirements;}(hj9hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK#hj5ubah}(h]h ]h"]h$]h&]uh1j3hj0ubj4)}(hXMinimizing storage overhead with guaranteed end-to-end performance by using compact (meta)data layout, optimized transparent data compression, deduplication and direct access, especially for those embedded devices with limited memory and high-density hosts with numerous containers. h]h)}(hXMinimizing storage overhead with guaranteed end-to-end performance by using compact (meta)data layout, optimized transparent data compression, deduplication and direct access, especially for those embedded devices with limited memory and high-density hosts with numerous containers.h]hXMinimizing storage overhead with guaranteed end-to-end performance by using compact (meta)data layout, optimized transparent data compression, deduplication and direct access, especially for those embedded devices with limited memory and high-density hosts with numerous containers.}(hjQhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK(hjMubah}(h]h ]h"]h$]h&]uh1j3hj0ubeh}(h]h ]h"]h$]h&]bullet-uh1j.hhhK#hj*ubah}(h]h ]h"]h$]h&]uh1j(hhhK#hhhhubh)}(hHere is the list of highlights:h]hHere is the list of highlights:}(hjshhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK-hhhhubj))}(hX- Little endian on-disk design with 48-bit block addressing, supporting up to 1 EiB filesystem capacity with 4 KiB block size; - Two compact inode metadata layouts for space and performance efficiency: ======================== ======== ====================================== compact extended ======================== ======== ====================================== Inode core metadata size 32 bytes 64 bytes Max file size 4 GiB 16 EiB (also limited by max. vol size) Max uids/gids 65536 4294967296 Nanosecond timestamps no yes Max hardlinks 65536 4294967296 ======================== ======== ====================================== - Support tailpacking inline data for better space efficiency and reduce unneeded I/O amplification; - Block-based and file-backed distribution are both supported; - Multiple devices to reference external data blobs: inode data can be optionally placed into external blobs, which enables image layering and data sharing among different filesystems; - Inline and shared extended attributes with an optional bloom filter that speeds up negative extended attribute lookups; - POSIX.1e ACLs by using extended attributes; - Transparent data compression as an option: Supported algorithms (LZ4, MicroLZMA, DEFLATE and Zstandard) can be selected on a per-inode basis. Both the on-disk metadata and decompression runtime have been heavily optimized to minimize the overhead for better performance. - Merging tail-end data into a special inode as fragments; - Chunk-based deduplication and rolling-hash compressed data deduplication; - Direct I/O and FSDAX support on uncompressed inodes for use cases such as secure containers, loop devices, and ramdisks that do not need page caching; - Page cache sharing among inodes with identical content fingerprints on the same machine. h]j/)}(hhh](j4)}(h}Little endian on-disk design with 48-bit block addressing, supporting up to 1 EiB filesystem capacity with 4 KiB block size; h]h)}(h|Little endian on-disk design with 48-bit block addressing, supporting up to 1 EiB filesystem capacity with 4 KiB block size;h]h|Little endian on-disk design with 48-bit block addressing, supporting up to 1 EiB filesystem capacity with 4 KiB block size;}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK/hjubah}(h]h ]h"]h$]h&]uh1j3hjubj4)}(hXVTwo compact inode metadata layouts for space and performance efficiency: ======================== ======== ====================================== compact extended ======================== ======== ====================================== Inode core metadata size 32 bytes 64 bytes Max file size 4 GiB 16 EiB (also limited by max. vol size) Max uids/gids 65536 4294967296 Nanosecond timestamps no yes Max hardlinks 65536 4294967296 ======================== ======== ====================================== h](h)}(hHTwo compact inode metadata layouts for space and performance efficiency:h]hHTwo compact inode metadata layouts for space and performance efficiency:}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK2hjubhtable)}(hhh]htgroup)}(hhh](hcolspec)}(hhh]h}(h]h ]h"]h$]h&]colwidthKuh1jhjubj)}(hhh]h}(h]h ]h"]h$]h&]colwidthKuh1jhjubj)}(hhh]h}(h]h ]h"]h$]h&]colwidthK&uh1jhjubhthead)}(hhh]hrow)}(hhh](hentry)}(hhh]h}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubah}(h]h ]h"]h$]h&]uh1jhjubhtbody)}(hhh](j)}(hhh](j)}(hhh]h)}(hInode core metadata sizeh]hInode core metadata size}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK7hjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(h32 bytesh]h32 bytes}(hj1hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK7hj.ubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(h64 bytesh]h64 bytes}(hjHhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK7hjEubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(h Max file sizeh]h Max file size}(hjhhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK8hjeubah}(h]h ]h"]h$]h&]uh1jhjbubj)}(hhh]h)}(h4 GiBh]h4 GiB}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK8hj|ubah}(h]h ]h"]h$]h&]uh1jhjbubj)}(hhh]h)}(h&16 EiB (also limited by max. vol size)h]h&16 EiB (also limited by max. vol size)}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK8hjubah}(h]h ]h"]h$]h&]uh1jhjbubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(h Max uids/gidsh]h Max uids/gids}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK9hjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(h65536h]h65536}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK9hjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(h 4294967296h]h 4294967296}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK9hjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(hNanosecond timestampsh]hNanosecond timestamps}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK:hjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(hnoh]hno}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK:hjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(hyesh]hyes}(hj2hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK:hj/ubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(h Max hardlinksh]h Max hardlinks}(hjRhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK;hjOubah}(h]h ]h"]h$]h&]uh1jhjLubj)}(hhh]h)}(h65536h]h65536}(hjihhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK;hjfubah}(h]h ]h"]h$]h&]uh1jhjLubj)}(hhh]h)}(h 4294967296h]h 4294967296}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK;hj}ubah}(h]h ]h"]h$]h&]uh1jhjLubeh}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]colsKuh1jhjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1j3hjubj4)}(hcSupport tailpacking inline data for better space efficiency and reduce unneeded I/O amplification; h]h)}(hbSupport tailpacking inline data for better space efficiency and reduce unneeded I/O amplification;h]hbSupport tailpacking inline data for better space efficiency and reduce unneeded I/O amplification;}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhK>hjubah}(h]h ]h"]h$]h&]uh1j3hjubj4)}(h=Block-based and file-backed distribution are both supported; h]h)}(h h]h)}(h9linux-erofs mailing list h](hlinux-erofs mailing list <}(hj.hhhNhNubj)}(hlinux-erofs@lists.ozlabs.orgh]hlinux-erofs@lists.ozlabs.org}(hj6hhhNhNubah}(h]h ]h"]h$]h&]refuri#mailto:linux-erofs@lists.ozlabs.orguh1jhj.ubh>}(hj.hhhNhNubeh}(h]h ]h"]h$]h&]uh1hhhhKhhj*ubah}(h]h ]h"]h$]h&]uh1j3hj'hhhhhNubah}(h]h ]h"]h$]h&]jkjluh1j.hhhKhhhhhubeh}(h]overviewah ]h"]overviewah$]h&]uh1hhhhhhhhKubh)}(hhh](h)}(h Mount optionsh]h Mount options}(hjghhhNhNubah}(h]h ]h"]h$]h&]uh1hhjdhhhhhKkubj)}(hhh]j)}(hhh](j)}(hhh]h}(h]h ]h"]h$]h&]colwidthKuh1jhjxubj)}(hhh]h}(h]h ]h"]h$]h&]colwidthK?uh1jhjxubj)}(hhh](j)}(hhh](j)}(hhh]h)}(h(no)user_xattrh]h(no)user_xattr}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKnhjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(hgSetup Extended User Attributes. Note: xattr is enabled by default if CONFIG_EROFS_FS_XATTR is selected.h]hgSetup Extended User Attributes. Note: xattr is enabled by default if CONFIG_EROFS_FS_XATTR is selected.}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKnhjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(h(no)aclh]h(no)acl}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKphjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(hjSetup POSIX Access Control List. Note: acl is enabled by default if CONFIG_EROFS_FS_POSIX_ACL is selected.h]hjSetup POSIX Access Control List. Note: acl is enabled by default if CONFIG_EROFS_FS_POSIX_ACL is selected.}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKphjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(hcache_strategy=%sh]hcache_strategy=%s}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKrhjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](h)}(h7Select a strategy for cached decompression from now on:h]h7Select a strategy for cached decompression from now on:}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKrhjubj)}(hhh]j)}(hhh](j)}(hhh]h}(h]h ]h"]h$]h&]colwidthK uh1jhj.ubj)}(hhh]h}(h]h ]h"]h$]h&]colwidthK-uh1jhj.ubj)}(hhh](j)}(hhh](j)}(hhh]h)}(hdisabledh]hdisabled}(hjNhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKuhjKubah}(h]h ]h"]h$]h&]uh1jhjHubj)}(hhh]h)}(h In-place I/O decompression only;h]h In-place I/O decompression only;}(hjehhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKuhjbubah}(h]h ]h"]h$]h&]uh1jhjHubeh}(h]h ]h"]h$]h&]uh1jhjEubj)}(hhh](j)}(hhh]h)}(h readaheadh]h readahead}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKvhjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(hCache the last incomplete compressed physical cluster for further reading. It still does in-place I/O decompression for the rest compressed physical clusters;h]hCache the last incomplete compressed physical cluster for further reading. It still does in-place I/O decompression for the rest compressed physical clusters;}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKvhjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjEubj)}(hhh](j)}(hhh]h)}(h readaroundh]h readaround}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKzhjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(hCache both ends of incomplete compressed physical clusters for further reading. It still does in-place I/O decompression for the rest compressed physical clusters.h]hCache both ends of incomplete compressed physical clusters for further reading. It still does in-place I/O decompression for the rest compressed physical clusters.}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKzhjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjEubeh}(h]h ]h"]h$]h&]uh1jhj.ubeh}(h]h ]h"]h$]h&]colsKuh1jhj+ubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(hdax={always,never}h]hdax={always,never}}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhj ubj)}(hhh]h)}(hJUse direct access (no page cache). See Documentation/filesystems/dax.rst.h]hJUse direct access (no page cache). See Documentation/filesystems/dax.rst.}(hj)hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj&ubah}(h]h ]h"]h$]h&]uh1jhj ubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(hdaxh]hdax}(hjIhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjFubah}(h]h ]h"]h$]h&]uh1jhjCubj)}(hhh]h)}(h5A legacy option which is an alias for ``dax=always``.h](h&A legacy option which is an alias for }(hj`hhhNhNubhliteral)}(h``dax=always``h]h dax=always}(hjjhhhNhNubah}(h]h ]h"]h$]h&]uh1jhhj`ubh.}(hj`hhhNhNubeh}(h]h ]h"]h$]h&]uh1hhhhKhj]ubah}(h]h ]h"]h$]h&]uh1jhjCubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(h device=%sh]h device=%s}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(h6Specify a path to an extra device to be used together.h]h6Specify a path to an extra device to be used together.}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(hdirectioh]hdirectio}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(hs(For file-backed mounts) Use direct I/O to access backing files, and asynchronous I/O will be enabled if supported.h]hs(For file-backed mounts) Use direct I/O to access backing files, and asynchronous I/O will be enabled if supported.}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(h domain_id=%sh]h domain_id=%s}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(hSpecify a trusted domain ID. Filesystems sharing the same domain ID can share page cache across mounts when inode page sharing is enabled. (not shown in mountinfo output)h]hSpecify a trusted domain ID. Filesystems sharing the same domain ID can share page cache across mounts when inode page sharing is enabled. (not shown in mountinfo output)}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(h fsoffset=%lluh]h fsoffset=%llu}(hj9hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj6ubah}(h]h ]h"]h$]h&]uh1jhj3ubj)}(hhh]h)}(h?Specify block-aligned filesystem offset for the primary device.h]h?Specify block-aligned filesystem offset for the primary device.}(hjPhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjMubah}(h]h ]h"]h$]h&]uh1jhj3ubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(h inode_shareh]h inode_share}(hjphhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjmubah}(h]h ]h"]h$]h&]uh1jhjjubj)}(hhh]h)}(hEnable inode page sharing for this filesystem. Inodes with identical content within the same domain ID can share the page cache.h]hEnable inode page sharing for this filesystem. Inodes with identical content within the same domain ID can share the page cache.}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhjjubeh}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh](j)}(hhh]h)}(h source=%sh]h source=%s}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhjubj)}(hhh]h)}(he(For file-backed mounts) Specify the backing image as a path or as an already-opened file descriptor.h]he(For file-backed mounts) Specify the backing image as a path or as an already-opened file descriptor.}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjubah}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjubeh}(h]h ]h"]h$]h&]uh1jhjxubeh}(h]h ]h"]h$]h&]colsKuh1jhjuubah}(h]h ]h"]h$]h&]uh1jhjdhhhhhNubeh}(h] mount-optionsah ]h"] mount optionsah$]h&]uh1hhhhhhhhKkubh)}(hhh](h)}(hFile-backed mountsh]hFile-backed mounts}(hjhhhNhNubah}(h]h ]h"]h$]h&]uh1hhjhhhhhKubh)}(hWhen CONFIG_EROFS_FS_BACKED_BY_FILE is enabled, EROFS file-backed images can be mounted directly without a loopback block device. The backing file can be given either as a path, or as an already-opened file descriptor.h]hWhen CONFIG_EROFS_FS_BACKED_BY_FILE is enabled, EROFS file-backed images can be mounted directly without a loopback block device. The backing file can be given either as a path, or as an already-opened file descriptor.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjhhubh)}(hWhen a file descriptor is used, the kernel resolves its path and records it so that /proc/mounts and similar interfaces can still report the mount source.h]hWhen a file descriptor is used, the kernel resolves its path and records it so that /proc/mounts and similar interfaces can still report the mount source.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjhhubh)}(hOnly regular files are accepted as backing files; to mount an image that resides on a block device, use the traditional block device mount path instead.h]hOnly regular files are accepted as backing files; to mount an image that resides on a block device, use the traditional block device mount path instead.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjhhubeh}(h]file-backed-mountsah ]h"]file-backed mountsah$]h&]uh1hhhhhhhhKubh)}(hhh](h)}(h Sysfs Entriesh]h Sysfs Entries}(hj9 hhhNhNubah}(h]h ]h"]h$]h&]uh1hhj6 hhhhhKubh)}(hInformation about mounted erofs file systems can be found in /sys/fs/erofs. Each mounted filesystem will have a directory in /sys/fs/erofs based on its device name (i.e., /sys/fs/erofs/sda). (see also Documentation/ABI/testing/sysfs-fs-erofs)h]hInformation about mounted erofs file systems can be found in /sys/fs/erofs. Each mounted filesystem will have a directory in /sys/fs/erofs based on its device name (i.e., /sys/fs/erofs/sda). (see also Documentation/ABI/testing/sysfs-fs-erofs)}(hjG hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj6 hhubeh}(h] sysfs-entriesah ]h"] sysfs entriesah$]h&]uh1hhhhhhhhKubh)}(hhh](h)}(hOn-disk detailsh]hOn-disk details}(hj` hhhNhNubah}(h]h ]h"]h$]h&]uh1hhj] hhhhhKubh)}(hhh](h)}(hSummaryh]hSummary}(hjq hhhNhNubah}(h]h ]h"]h$]h&]uh1hhjn hhhhhKubh)}(hfDifferent from other read-only file systems, an EROFS volume is designed to be as simple as possible::h]heDifferent from other read-only file systems, an EROFS volume is designed to be as simple as possible:}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjn hhubh literal_block)}(h |-> aligned with the block size ____________________________________________________________ | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | |_|__|_|_____|__________|_____|______|__________|_____|______| 0 +1Kh]h |-> aligned with the block size ____________________________________________________________ | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | |_|__|_|_____|__________|_____|______|__________|_____|______| 0 +1K}hj sbah}(h]h ]h"]h$]h&]hhuh1j hhhKhjn hhubh)}(hAll data areas should be aligned with the block size, but metadata areas may not. All metadata can be now observed in two different spaces (views):h]hAll data areas should be aligned with the block size, but metadata areas may not. All metadata can be now observed in two different spaces (views):}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjn hhubj))}(hX1. Inode metadata space Each valid inode should be aligned with an inode slot, which is a fixed value (32 bytes) and designed to be kept in line with compact inode size. Each inode can be directly found with the following formula: inode offset = meta_blkaddr * block_size + 32 * nid :: |-> aligned with 8B |-> followed closely + meta_blkaddr blocks |-> another slot _____________________________________________________________________ | ... | inode | xattrs | extents | data inline | ... | inode ... |________|_______|(optional)|(optional)|__(optional)_|_____|__________ |-> aligned with the inode slot size . . . . . . . . . . . . .____________________________________________________|-> aligned with 4B | xattr_ibody_header | shared xattrs | inline xattrs | |____________________|_______________|_______________| |-> 12 bytes <-|->x * 4 bytes<-| . . . . . . . . . . ._______________________________.______________________. | id | id | id | id | ... | id | ent | ... | ent| ... | |____|____|____|____|______|____|_____|_____|____|_____| |-> aligned with 4B |-> aligned with 4B Inode could be 32 or 64 bytes, which can be distinguished from a common field which all inode versions have -- i_format:: __________________ __________________ | i_format | | i_format | |__________________| |__________________| | ... | | ... | | | | | |__________________| 32 bytes | | | | |__________________| 64 bytes Xattrs, extents, data inline are placed after the corresponding inode with proper alignment, and they could be optional for different data mappings. _currently_ total 5 data layouts are supported: == ==================================================================== 0 flat file data without data inline (no extent); 1 fixed-sized output data compression (with non-compacted indexes); 2 flat file data with tail packing data inline (no extent); 3 fixed-sized output data compression (with compacted indexes, v5.3+); 4 chunk-based file (v5.15+). == ==================================================================== The size of the optional xattrs is indicated by i_xattr_count in inode header. Large xattrs or xattrs shared by many different files can be stored in shared xattrs metadata rather than inlined right after inode. 2. Shared xattrs metadata space Shared xattrs space is similar to the above inode space, started with a specific block indicated by xattr_blkaddr, organized one by one with proper align. Each share xattr can also be directly found by the following formula: xattr offset = xattr_blkaddr * block_size + 4 * xattr_id h]henumerated_list)}(hhh](j4)}(hX Inode metadata space Each valid inode should be aligned with an inode slot, which is a fixed value (32 bytes) and designed to be kept in line with compact inode size. Each inode can be directly found with the following formula: inode offset = meta_blkaddr * block_size + 32 * nid :: |-> aligned with 8B |-> followed closely + meta_blkaddr blocks |-> another slot _____________________________________________________________________ | ... | inode | xattrs | extents | data inline | ... | inode ... |________|_______|(optional)|(optional)|__(optional)_|_____|__________ |-> aligned with the inode slot size . . . . . . . . . . . . .____________________________________________________|-> aligned with 4B | xattr_ibody_header | shared xattrs | inline xattrs | |____________________|_______________|_______________| |-> 12 bytes <-|->x * 4 bytes<-| . . . . . . . . . . ._______________________________.______________________. | id | id | id | id | ... | id | ent | ... | ent| ... | |____|____|____|____|______|____|_____|_____|____|_____| |-> aligned with 4B |-> aligned with 4B Inode could be 32 or 64 bytes, which can be distinguished from a common field which all inode versions have -- i_format:: __________________ __________________ | i_format | | i_format | |__________________| |__________________| | ... | | ... | | | | | |__________________| 32 bytes | | | | |__________________| 64 bytes Xattrs, extents, data inline are placed after the corresponding inode with proper alignment, and they could be optional for different data mappings. _currently_ total 5 data layouts are supported: == ==================================================================== 0 flat file data without data inline (no extent); 1 fixed-sized output data compression (with non-compacted indexes); 2 flat file data with tail packing data inline (no extent); 3 fixed-sized output data compression (with compacted indexes, v5.3+); 4 chunk-based file (v5.15+). == ==================================================================== The size of the optional xattrs is indicated by i_xattr_count in inode header. Large xattrs or xattrs shared by many different files can be stored in shared xattrs metadata rather than inlined right after inode. h](h)}(hInode metadata spaceh]hInode metadata space}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubh)}(hEach valid inode should be aligned with an inode slot, which is a fixed value (32 bytes) and designed to be kept in line with compact inode size.h]hEach valid inode should be aligned with an inode slot, which is a fixed value (32 bytes) and designed to be kept in line with compact inode size.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubhdefinition_list)}(hhh]hdefinition_list_item)}(hqEach inode can be directly found with the following formula: inode offset = meta_blkaddr * block_size + 32 * nid h](hterm)}(h aligned with 8B |-> followed closely + meta_blkaddr blocks |-> another slot _____________________________________________________________________ | ... | inode | xattrs | extents | data inline | ... | inode ... |________|_______|(optional)|(optional)|__(optional)_|_____|__________ |-> aligned with the inode slot size . . . . . . . . . . . . .____________________________________________________|-> aligned with 4B | xattr_ibody_header | shared xattrs | inline xattrs | |____________________|_______________|_______________| |-> 12 bytes <-|->x * 4 bytes<-| . . . . . . . . . . ._______________________________.______________________. | id | id | id | id | ... | id | ent | ... | ent| ... | |____|____|____|____|______|____|_____|_____|____|_____| |-> aligned with 4B |-> aligned with 4Bh]hX |-> aligned with 8B |-> followed closely + meta_blkaddr blocks |-> another slot _____________________________________________________________________ | ... | inode | xattrs | extents | data inline | ... | inode ... |________|_______|(optional)|(optional)|__(optional)_|_____|__________ |-> aligned with the inode slot size . . . . . . . . . . . . .____________________________________________________|-> aligned with 4B | xattr_ibody_header | shared xattrs | inline xattrs | |____________________|_______________|_______________| |-> 12 bytes <-|->x * 4 bytes<-| . . . . . . . . . . ._______________________________.______________________. | id | id | id | id | ... | id | ent | ... | ent| ... | |____|____|____|____|______|____|_____|_____|____|_____| |-> aligned with 4B |-> aligned with 4B}hj sbah}(h]h ]h"]h$]h&]hhuh1j hhhKhj ubh)}(hyInode could be 32 or 64 bytes, which can be distinguished from a common field which all inode versions have -- i_format::h]hxInode could be 32 or 64 bytes, which can be distinguished from a common field which all inode versions have -- i_format:}(hj" hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubj )}(hX __________________ __________________ | i_format | | i_format | |__________________| |__________________| | ... | | ... | | | | | |__________________| 32 bytes | | | | |__________________| 64 bytesh]hX __________________ __________________ | i_format | | i_format | |__________________| |__________________| | ... | | ... | | | | | |__________________| 32 bytes | | | | |__________________| 64 bytes}hj0 sbah}(h]h ]h"]h$]h&]hhuh1j hhhKhj ubh)}(hXattrs, extents, data inline are placed after the corresponding inode with proper alignment, and they could be optional for different data mappings. _currently_ total 5 data layouts are supported:h]hXattrs, extents, data inline are placed after the corresponding inode with proper alignment, and they could be optional for different data mappings. _currently_ total 5 data layouts are supported:}(hj> hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubj)}(hhh]j)}(hhh](j)}(hhh]h}(h]h ]h"]h$]h&]colwidthKuh1jhjO ubj)}(hhh]h}(h]h ]h"]h$]h&]colwidthKDuh1jhjO ubj)}(hhh](j)}(hhh](j)}(hhh]h)}(h0h]h0}(hjo hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjl ubah}(h]h ]h"]h$]h&]uh1jhji ubj)}(hhh]h)}(h/flat file data without data inline (no extent);h]h/flat file data without data inline (no extent);}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubah}(h]h ]h"]h$]h&]uh1jhji ubeh}(h]h ]h"]h$]h&]uh1jhjf ubj)}(hhh](j)}(hhh]h)}(h1h]h1}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubah}(h]h ]h"]h$]h&]uh1jhj ubj)}(hhh]h)}(hAfixed-sized output data compression (with non-compacted indexes);h]hAfixed-sized output data compression (with non-compacted indexes);}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubah}(h]h ]h"]h$]h&]uh1jhj ubeh}(h]h ]h"]h$]h&]uh1jhjf ubj)}(hhh](j)}(hhh]h)}(h2h]h2}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubah}(h]h ]h"]h$]h&]uh1jhj ubj)}(hhh]h)}(h9flat file data with tail packing data inline (no extent);h]h9flat file data with tail packing data inline (no extent);}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubah}(h]h ]h"]h$]h&]uh1jhj ubeh}(h]h ]h"]h$]h&]uh1jhjf ubj)}(hhh](j)}(hhh]h)}(h3h]h3}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubah}(h]h ]h"]h$]h&]uh1jhj ubj)}(hhh]h)}(hDfixed-sized output data compression (with compacted indexes, v5.3+);h]hDfixed-sized output data compression (with compacted indexes, v5.3+);}(hj+ hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj( ubah}(h]h ]h"]h$]h&]uh1jhj ubeh}(h]h ]h"]h$]h&]uh1jhjf ubj)}(hhh](j)}(hhh]h)}(h4h]h4}(hjK hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhjH ubah}(h]h ]h"]h$]h&]uh1jhjE ubj)}(hhh]h)}(hchunk-based file (v5.15+).h]hchunk-based file (v5.15+).}(hjb hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj_ ubah}(h]h ]h"]h$]h&]uh1jhjE ubeh}(h]h ]h"]h$]h&]uh1jhjf ubeh}(h]h ]h"]h$]h&]uh1jhjO ubeh}(h]h ]h"]h$]h&]colsKuh1jhjL ubah}(h]h ]h"]h$]h&]uh1jhj ubh)}(hThe size of the optional xattrs is indicated by i_xattr_count in inode header. Large xattrs or xattrs shared by many different files can be stored in shared xattrs metadata rather than inlined right after inode.h]hThe size of the optional xattrs is indicated by i_xattr_count in inode header. Large xattrs or xattrs shared by many different files can be stored in shared xattrs metadata rather than inlined right after inode.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubeh}(h]h ]h"]h$]h&]uh1j3hj ubj4)}(hX>Shared xattrs metadata space Shared xattrs space is similar to the above inode space, started with a specific block indicated by xattr_blkaddr, organized one by one with proper align. Each share xattr can also be directly found by the following formula: xattr offset = xattr_blkaddr * block_size + 4 * xattr_id h](h)}(hShared xattrs metadata spaceh]hShared xattrs metadata space}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubh)}(hShared xattrs space is similar to the above inode space, started with a specific block indicated by xattr_blkaddr, organized one by one with proper align.h]hShared xattrs space is similar to the above inode space, started with a specific block indicated by xattr_blkaddr, organized one by one with proper align.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubj )}(hhh]j )}(hEach share xattr can also be directly found by the following formula: xattr offset = xattr_blkaddr * block_size + 4 * xattr_id h](j )}(hEEach share xattr can also be directly found by the following formula:h]hEEach share xattr can also be directly found by the following formula:}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1j hhhKhj ubj )}(hhh]h)}(h8xattr offset = xattr_blkaddr * block_size + 4 * xattr_idh]h8xattr offset = xattr_blkaddr * block_size + 4 * xattr_id}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhKhj ubah}(h]h ]h"]h$]h&]uh1j hj ubeh}(h]h ]h"]h$]h&]uh1j hhhKhj ubah}(h]h ]h"]h$]h&]uh1j hj ubeh}(h]h ]h"]h$]h&]uh1j3hj ubeh}(h]h ]h"]h$]h&]enumtypearabicprefixhsuffix.uh1j hj ubah}(h]h ]h"]h$]h&]uh1j(hhhKhjn hhubj )}(hXT |-> aligned by 4 bytes + xattr_blkaddr blocks |-> aligned with 4 bytes _________________________________________________________________________ | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... |________|_____________|_____________|_____|______________|_______________h]hXT |-> aligned by 4 bytes + xattr_blkaddr blocks |-> aligned with 4 bytes _________________________________________________________________________ | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... |________|_____________|_____________|_____|______________|_______________}hj sbah}(h]h ]h"]h$]h&]hhuh1j hhhMhjn hhubeh}(h]summaryah ]h"]summaryah$]h&]uh1hhj] hhhhhKubh)}(hhh](h)}(h Directoriesh]h Directories}(hj+ hhhNhNubah}(h]h ]h"]h$]h&]uh1hhj( hhhhhM ubh)}(hXXAll directories are now organized in a compact on-disk format. Note that each directory block is divided into index and name areas in order to support random file lookup, and all directory entries are _strictly_ recorded in alphabetical order in order to support improved prefix binary search algorithm (could refer to the related source code).h]hXXAll directories are now organized in a compact on-disk format. Note that each directory block is divided into index and name areas in order to support random file lookup, and all directory entries are _strictly_ recorded in alphabetical order in order to support improved prefix binary search algorithm (could refer to the related source code).}(hj9 hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhM hj( hhubj )}(hX ___________________________ / | / ______________|________________ / / | nameoff1 | nameoffN-1 ____________.______________._______________v________________v__________ | dirent | dirent | ... | dirent | filename | filename | ... | filename | |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| \ ^ \ | * could have \ | trailing '\0' \________________________| nameoff0 Directory blockh]hX ___________________________ / | / ______________|________________ / / | nameoff1 | nameoffN-1 ____________.______________._______________v________________v__________ | dirent | dirent | ... | dirent | filename | filename | ... | filename | |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| \ ^ \ | * could have \ | trailing '\0' \________________________| nameoff0 Directory block}hjG sbah}(h]h ]h"]h$]h&]hhuh1j hhhMhj( hhubh)}(hNote that apart from the offset of the first filename, nameoff0 also indicates the total number of directory entries in this block since it is no need to introduce another on-disk field at all.h]hNote that apart from the offset of the first filename, nameoff0 also indicates the total number of directory entries in this block since it is no need to introduce another on-disk field at all.}(hjU hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhM hj( hhubeh}(h] directoriesah ]h"] directoriesah$]h&]uh1hhj] hhhhhM ubh)}(hhh](h)}(hChunk-based filesh]hChunk-based files}(hjn hhhNhNubah}(h]h ]h"]h$]h&]uh1hhjk hhhhhM%ubh)}(hXIn order to support chunk-based data deduplication, a new inode data layout has been supported since Linux v5.15: Files are split in equal-sized data chunks with ``extents`` area of the inode metadata indicating how to get the chunk data: these can be simply as a 4-byte block address array or in the 8-byte chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more details.)h](hIn order to support chunk-based data deduplication, a new inode data layout has been supported since Linux v5.15: Files are split in equal-sized data chunks with }(hj| hhhNhNubji)}(h ``extents``h]hextents}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1jhhj| ubh area of the inode metadata indicating how to get the chunk data: these can be simply as a 4-byte block address array or in the 8-byte chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more details.)}(hj| hhhNhNubeh}(h]h ]h"]h$]h&]uh1hhhhM&hjk hhubh)}(h;By the way, chunk-based files are all uncompressed for now.h]h;By the way, chunk-based files are all uncompressed for now.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhM-hjk hhubeh}(h]chunk-based-filesah ]h"]chunk-based filesah$]h&]uh1hhj] hhhhhM%ubh)}(hhh](h)}(h%Long extended attribute name prefixesh]h%Long extended attribute name prefixes}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhj hhhhhM0ubh)}(hThere are use cases where extended attributes with different values can have only a few common prefixes (such as overlayfs xattrs). The predefined prefixes work inefficiently in both image size and runtime performance in such cases.h]hThere are use cases where extended attributes with different values can have only a few common prefixes (such as overlayfs xattrs). The predefined prefixes work inefficiently in both image size and runtime performance in such cases.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhM1hj hhubh)}(hThe long xattr name prefixes feature is introduced to address this issue. The overall idea is that, apart from the existing predefined prefixes, the xattr entry could also refer to user-specified long xattr name prefixes, e.g. "trusted.overlay.".h]hThe long xattr name prefixes feature is introduced to address this issue. The overall idea is that, apart from the existing predefined prefixes, the xattr entry could also refer to user-specified long xattr name prefixes, e.g. “trusted.overlay.”.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhM5hj hhubh)}(hXWhen referring to a long xattr name prefix, the highest bit (bit 7) of erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole represent the index of the referred long name prefix among all long name prefixes. Therefore, only the trailing part of the name apart from the long xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if the full xattr name matches exactly as its long xattr name prefix.h]hXWhen referring to a long xattr name prefix, the highest bit (bit 7) of erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole represent the index of the referred long name prefix among all long name prefixes. Therefore, only the trailing part of the name apart from the long xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if the full xattr name matches exactly as its long xattr name prefix.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhM:hj hhubh)}(hXAll long xattr prefixes are stored one by one in the packed inode as long as the packed inode is valid, or in the meta inode otherwise. The xattr_prefix_count (of the on-disk superblock) indicates the total number of long xattr name prefixes, while (xattr_prefix_start * 4) indicates the start offset of long name prefixes in the packed/meta inode. Note that, long extended attribute name prefixes are disabled if xattr_prefix_count is 0.h]hXAll long xattr prefixes are stored one by one in the packed inode as long as the packed inode is valid, or in the meta inode otherwise. The xattr_prefix_count (of the on-disk superblock) indicates the total number of long xattr name prefixes, while (xattr_prefix_start * 4) indicates the start offset of long name prefixes in the packed/meta inode. Note that, long extended attribute name prefixes are disabled if xattr_prefix_count is 0.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhMAhj hhubh)}(hXEach long name prefix is stored in the format: ALIGN({__le16 len, data}, 4), where len represents the total size of the data part. The data part is actually represented by 'struct erofs_xattr_long_prefix', where base_index represents the index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for "trusted.overlay." long name prefix, while the infix string keeps the string after stripping the short prefix, e.g. "overlay." for the example above.h]hXEach long name prefix is stored in the format: ALIGN({__le16 len, data}, 4), where len represents the total size of the data part. The data part is actually represented by ‘struct erofs_xattr_long_prefix’, where base_index represents the index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for “trusted.overlay.” long name prefix, while the infix string keeps the string after stripping the short prefix, e.g. “overlay.” for the example above.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhMHhj hhubeh}(h]%long-extended-attribute-name-prefixesah ]h"]%long extended attribute name prefixesah$]h&]uh1hhj] hhhhhM0ubh)}(hhh](h)}(hData compressionh]hData compression}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhj hhhhhMPubh)}(hXEROFS implements fixed-sized output compression which generates fixed-sized compressed data blocks from variable-sized input in contrast to other existing fixed-sized input solutions. Relatively higher compression ratios can be gotten by using fixed-sized output compression since nowadays popular data compression algorithms are mostly LZ77-based and such fixed-sized output approach can be benefited from the historical dictionary (aka. sliding window).'h]hXEROFS implements fixed-sized output compression which generates fixed-sized compressed data blocks from variable-sized input in contrast to other existing fixed-sized input solutions. Relatively higher compression ratios can be gotten by using fixed-sized output compression since nowadays popular data compression algorithms are mostly LZ77-based and such fixed-sized output approach can be benefited from the historical dictionary (aka. sliding window).}(hj" hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhMQhj hhubh)}(hXIn details, original (uncompressed) data is turned into several variable-sized extents and in the meanwhile, compressed into physical clusters (pclusters). In order to record each variable-sized extent, logical clusters (lclusters) are introduced as the basic unit of compress indexes to indicate whether a new extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now fixed in block size, as illustrated below::h]hXIn details, original (uncompressed) data is turned into several variable-sized extents and in the meanwhile, compressed into physical clusters (pclusters). In order to record each variable-sized extent, logical clusters (lclusters) are introduced as the basic unit of compress indexes to indicate whether a new extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now fixed in block size, as illustrated below:}(hj0 hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhMXhj hhubj )}(hX |<- variable-sized extent ->|<- VLE ->| clusterofs clusterofs clusterofs | | | _________v_________________________________v_______________________v________ ... | . | | . | | . ... ____|____._________|______________|________.___ _|______________|__.________ |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-| (HEAD) (NONHEAD) (HEAD) (NONHEAD) . . CBLKCNT . . . . . . . . _______._____________________________.______________._________________ ... | | | | ... _______|______________|______________|______________|_________________ |-> big pcluster <-|-> pcluster <-|h]hX |<- variable-sized extent ->|<- VLE ->| clusterofs clusterofs clusterofs | | | _________v_________________________________v_______________________v________ ... | . | | . | | . ... ____|____._________|______________|________.___ _|______________|__.________ |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-| (HEAD) (NONHEAD) (HEAD) (NONHEAD) . . CBLKCNT . . . . . . . . _______._____________________________.______________._________________ ... | | | | ... _______|______________|______________|______________|_________________ |-> big pcluster <-|-> pcluster <-|}hj> sbah}(h]h ]h"]h$]h&]hhuh1j hhhM_hj hhubh)}(hX A physical cluster can be seen as a container of physical compressed blocks which contains compressed data. Previously, only lcluster-sized (4KB) pclusters were supported. After big pcluster feature is introduced (available since Linux v5.13), pcluster can be a multiple of lcluster size.h]hX A physical cluster can be seen as a container of physical compressed blocks which contains compressed data. Previously, only lcluster-sized (4KB) pclusters were supported. After big pcluster feature is introduced (available since Linux v5.13), pcluster can be a multiple of lcluster size.}(hjL hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhMohj hhubh)}(hXFor each HEAD lcluster, clusterofs is recorded to indicate where a new extent starts and blkaddr is used to seek the compressed data. For each NONHEAD lcluster, delta0 and delta1 are available instead of blkaddr to indicate the distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is also a HEAD lcluster except that its data is uncompressed. See the comments around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.h]hXFor each HEAD lcluster, clusterofs is recorded to indicate where a new extent starts and blkaddr is used to seek the compressed data. For each NONHEAD lcluster, delta0 and delta1 are available instead of blkaddr to indicate the distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is also a HEAD lcluster except that its data is uncompressed. See the comments around “struct z_erofs_vle_decompressed_index” in erofs_fs.h for more details.}(hjZ hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhMthj hhubh)}(hX+If big pcluster is enabled, pcluster size in lclusters needs to be recorded as well. Let the delta0 of the first NONHEAD lcluster store the compressed block count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy to understand its delta0 is constantly 1, as illustrated below::h]hX,If big pcluster is enabled, pcluster size in lclusters needs to be recorded as well. Let the delta0 of the first NONHEAD lcluster store the compressed block count with a special flag as a new called CBLKCNT NONHEAD lcluster. It’s easy to understand its delta0 is constantly 1, as illustrated below:}(hjh hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhM{hj hhubj )}(hX' __________________________________________________________ | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD | |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_| |<----- a big pcluster (with CBLKCNT) ------>|<-- -->| a lcluster-sized pcluster (without CBLKCNT) ^h]hX' __________________________________________________________ | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD | |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_| |<----- a big pcluster (with CBLKCNT) ------>|<-- -->| a lcluster-sized pcluster (without CBLKCNT) ^}hjv sbah}(h]h ]h"]h$]h&]hhuh1j hhhMhj hhubh)}(hIf another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT, but it's easy to know the size of such pcluster is 1 lcluster as well.h]hIf another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT, but it’s easy to know the size of such pcluster is 1 lcluster as well.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhMhj hhubh)}(hSince Linux v6.1, each pcluster can be used for multiple variable-sized extents, therefore it can be used for compressed data deduplication.h]hSince Linux v6.1, each pcluster can be used for multiple variable-sized extents, therefore it can be used for compressed data deduplication.}(hj hhhNhNubah}(h]h ]h"]h$]h&]uh1hhhhMhj hhubeh}(h]data-compressionah ]h"]data compressionah$]h&]uh1hhj] hhhhhMPubeh}(h]on-disk-detailsah ]h"]on-disk detailsah$]h&]uh1hhhhhhhhKubeh}(h]$erofs-enhanced-read-only-file-systemah ]h"]&erofs - enhanced read-only file systemah$]h&]uh1hhhhhhhhKubeh}(h]h ]h"]h$]h&]sourcehuh1hcurrent_sourceN current_lineNsettingsdocutils.frontendValues)}(hN generatorN datestampN source_linkN source_urlN toc_backlinksjfootnote_backlinksK sectnum_xformKstrip_commentsNstrip_elements_with_classesN strip_classesN report_levelK halt_levelKexit_status_levelKdebugNwarning_streamN tracebackinput_encoding utf-8-siginput_encoding_error_handlerstrictoutput_encodingutf-8output_encoding_error_handlerj error_encodingutf-8error_encoding_error_handlerbackslashreplace language_codeenrecord_dependenciesNconfigN id_prefixhauto_id_prefixid dump_settingsNdump_internalsNdump_transformsNdump_pseudo_xmlNexpose_internalsNstrict_visitorN_disable_configN_sourcehnj _destinationN _config_files]7/var/lib/git/docbuild/linux/Documentation/docutils.confafile_insertion_enabled raw_enabledKline_length_limitM'pep_referencesN pep_base_urlhttps://peps.python.org/pep_file_url_templatepep-%04drfc_referencesN rfc_base_url&https://datatracker.ietf.org/doc/html/ tab_widthKtrim_footnote_reference_spacesyntax_highlightlong smart_quotessmartquotes_locales]character_level_inline_markupdoctitle_xform docinfo_xformKsectsubtitle_xform image_loadinglinkembed_stylesheetcloak_email_addressessection_self_linkenvNubreporterNindirect_targets]substitution_defs}substitution_names}refnames}refids}nameids}(j j jaj^jjj3 j0 jZ jW j j j% j" jh je j j j j j j u nametypes}(j jajj3 jZ j j% jh j j j uh}(j hj^hjjdj0 jjW j6 j j] j" jn je j( j jk j j j j u footnote_refs} citation_refs} autofootnotes]autofootnote_refs]symbol_footnotes]symbol_footnote_refs] footnotes] citations]autofootnote_startKsymbol_footnote_startK id_counter collectionsCounter}Rparse_messages]transform_messages] transformerN include_log] decorationNhhub.