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btrfs: Introduce support for FSID change without metadata rewrite

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This field is going to be used when the user wants to change the UUID
of the filesystem without having to rewrite all metadata blocks. This
field adds another level of indirection such that when the FSID is
changed what really happens is the current UUID (the one with which the
fs was created) is copied to the 'metadata_uuid' field in the superblock
as well as a new incompat flag is set METADATA_UUID. When the kernel
detects this flag is set it knows that the superblock in fact has 2
UUIDs:

1. Is the UUID which is user-visible, currently known as FSID.
2. Metadata UUID - this is the UUID which is stamped into all on-disk
   datastructures belonging to this file system.

When the new incompat flag is present device scanning checks whether
both fsid/metadata_uuid of the scanned device match any of the
registered filesystems. When the flag is not set then both UUIDs are
equal and only the FSID is retained on disk, metadata_uuid is set only
in-memory during mount.

Additionally a new metadata_uuid field is also added to the fs_info
struct. It's initialised either with the FSID in case METADATA_UUID
incompat flag is not set or with the metdata_uuid of the superblock
otherwise.

This commit introduces the new fields as well as the new incompat flag
and switches all users of the fsid to the new logic.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ minor updates in comments ]
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Nikolay Borisov 2018-10-30 16:43:23 +02:00 • committed by David Sterba
parent ce9f967f31
commit 7239ff4b2b
8 changed files with 104 additions and 28 deletions
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1
include/uapi/linux/btrfs.h
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@ -269,6 +269,7 @@ struct btrfs_ioctl_fs_info_args {
#define BTRFS_FEATURE_INCOMPAT_RAID56 (1ULL << 7)
#define BTRFS_FEATURE_INCOMPAT_SKINNY_METADATA (1ULL << 8)
#define BTRFS_FEATURE_INCOMPAT_NO_HOLES (1ULL << 9)
#define BTRFS_FEATURE_INCOMPAT_METADATA_UUID (1ULL << 10)
struct btrfs_ioctl_feature_flags {
__u64 compat_flags;