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author | Darrick J. Wong <darrick.wong@oracle.com> | 2016-01-05 10:31:50 +1100 |
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committer | Dave Chinner <david@fromorbit.com> | 2016-01-05 10:31:50 +1100 |
commit | e95c1616411935dbc54119051235b71f006d7cb8 (patch) | |
tree | 0d1fb32feab2e829d3c1c694ea125f3c51ea10b5 | |
parent | 372d53f774091bba060f811ea57bd8ea4d775c51 (diff) | |
download | xfs-documentation-e95c1616411935dbc54119051235b71f006d7cb8.tar.gz |
add a chapter discussing v5 disk format
Add a chapter to give an overview of the enhanced metadata integrity
fields present on a v5 filesystem.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>
-rw-r--r-- | design/XFS_Filesystem_Structure/metadata_integrity.asciidoc | 36 | ||||
-rw-r--r-- | design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc | 2 |
2 files changed, 38 insertions, 0 deletions
diff --git a/design/XFS_Filesystem_Structure/metadata_integrity.asciidoc b/design/XFS_Filesystem_Structure/metadata_integrity.asciidoc new file mode 100644 index 0000000..f948d5e --- /dev/null +++ b/design/XFS_Filesystem_Structure/metadata_integrity.asciidoc @@ -0,0 +1,36 @@ += Metadata Integrity + +Prior to version 5, most XFS metadata blocks contained a magic number that +could provide a minimal sanity check that a block read off the disk contained +the same type of data that the code thought it was reading off the disk. +However, this was insufficient -- given a correct type code, it was still +impossible to tell if the block was from a previous filesystem, or happened to +be owned by something else, or had been written to the wrong location on disk. +Furthermore, not all metadata blocks had magic numbers -- remote extended +attributes and extent symbolic links had no protection at all. + +Therefore, the version 5 disk format introduced larger headers for all metadata +types, which enable the filesystem to check information being read from the +disk more rigorously. Metadata integrity fields now include: + +* *Magic* numbers, to classify all types of metadata. This is unchanged from v4. +* A copy of the filesystem *UUID*, to confirm that a given disk block is connected to the superblock. +* The *owner*, to avoid accessing a piece of metadata which belongs to some other part of the filesystem. +* The filesystem *block number*, to detect misplaced writes. +* The *log serial number* of the last write to this block, to avoid replaying obsolete log entries. +* A CRC32c *checksum* of the entire block, to detect minor corruption. + +Metadata integrity coverage has been extended to all metadata blocks in the +filesystem, with the following notes: + +* Inodes can have multiple ``owners'' in the directory tree; therefore the record contains the inode number instead of an owner or a block number. +* Superblocks have no owners. +* The disk quota file has no owner or block numbers. +* Metadata owned by files list the inode number as the owner. +* Per-AG data and B+tree blocks list the AG number as the owner. +* Per-AG header sectors don't list owners or block numbers, since they have fixed locations. +* Remote attribute blocks are not logged and therefore the LSN must be -1. + +This functionality enables XFS to decide that a block contents are so +unexpected that it should stop immediately. Unfortunately checksums do not +allow for automatic correction. Please keep regular backups, as always. diff --git a/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc b/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc index 9f4c096..8aacbb7 100644 --- a/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc +++ b/design/XFS_Filesystem_Structure/xfs_filesystem_structure.asciidoc @@ -46,6 +46,8 @@ log items which are formatted in host order. include::overview.asciidoc[] +include::metadata_integrity.asciidoc[] + include::common_types.asciidoc[] // return titles to normal |