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>
This commit is contained in:
Qu Wenruo 2020-01-23 15:44:50 +08:00 committed by David Sterba
parent 16fbf79b0f
commit 30b3688e1f

View File

@ -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(