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| author | Qu Wenruo <wqu@suse.com> | 2020-01-23 15:44:50 +0800 |
|---|---|---|
| committer | David Sterba <dsterba@suse.com> | 2020-02-10 14:40:25 +0100 |
| commit | ad4803241dcbe836a656376278c4fb01c6a968d0 (patch) | |
| tree | 63a8c9c1a9ba0414e971f1cab331c82aa17a27f8 | |
| parent | bb6d3fb354c5ee8d6bde2d576eb7220ea09862b9 (diff) | |
| download | btrfs-next-ad4803241dcbe836a656376278c4fb01c6a968d0.tar.gz | |
btrfs: Add overview of device replace
The overview of btrfs dev-replace. It mentions some corner cases caused
by the write duplication and scrub based data copy.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ adjust wording ]
Signed-off-by: David Sterba <dsterba@suse.com>
| -rw-r--r-- | fs/btrfs/dev-replace.c | 40 |
1 files changed, 40 insertions, 0 deletions
diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c index 2ca2a09d0e238e..2aad07cdaea884 100644 --- a/fs/btrfs/dev-replace.c +++ b/fs/btrfs/dev-replace.c @@ -22,6 +22,46 @@ #include "dev-replace.h" #include "sysfs.h" +/* + * Device replace overview + * + * [Objective] + * To copy all extents (both new and on-disk) from source device to target + * device, while still keeping the filesystem read-write. + * + * [Method] + * There are two main methods involved: + * + * - Write duplication + * + * All new writes will be written to both target and source devices, so even + * if replace gets canceled, sources device still contans up-to-date data. + * + * Location: handle_ops_on_dev_replace() from __btrfs_map_block() + * Start: btrfs_dev_replace_start() + * End: btrfs_dev_replace_finishing() + * Content: Latest data/metadata + * + * - Copy existing extents + * + * This happens by re-using scrub facility, as scrub also iterates through + * existing extents from commit root. + * + * Location: scrub_write_block_to_dev_replace() from + * scrub_block_complete() + * Content: Data/meta from commit root. + * + * Due to the content difference, we need to avoid nocow write when dev-replace + * is happening. This is done by marking the block group read-only and waiting + * for NOCOW writes. + * + * After replace is done, the finishing part is done by swapping the target and + * source devices. + * + * Location: btrfs_dev_replace_update_device_in_mapping_tree() from + * btrfs_dev_replace_finishing() + */ + static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info, int scrub_ret); static void btrfs_dev_replace_update_device_in_mapping_tree( |