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Questions about "cp --reflink" (I have never used that option so far, but it sounds useful). Quoting "man": "When --reflink[=always] is specified, perform a

Questions about "cp --reflink" (I have never used that option so far, but it sounds useful). Quoting "man": "When --reflink[=always] is specified, perform a lightweight copy, where the data blocks are copied only when modified. If this is not possible the copy fails" Q1: this is copy-on-write, right? Q2: once/if the command completes successfully, are there any (potential) dangers (e.g. if I then immediately delete the original file) or can the new file be treated 100% as if I had copied it the classical way (without that option)? Thx

simcop2387 7y ago
1) Yes, it's copy on write, and requires such support from the underlying filesystem (Btrfs, maybe XFS not sure of what others support it) 2) It's intended to be treated 100% as if it had been copied the classical way. It's not a hardlink or a symlink and can be treated as a completely new file
- cmurf 7y ago
  Support for reflinks in XFS for a while now, although it requires a mkfs time option to enable it. That option was just enabled by default in xfsprogs-5.1.0 which is now in Fedora 31 (still prerelease).
  - simcop2387 7y ago
    Yea my memory was fuzzy and I knew it was planned and being worked on but couldn't remember if it had happened yet. I think F2FS and bcachefs are also supposed to support it but I'm not sure about their level of support either.
  - zepearl 7y ago
    Cool => I'll then probably reformat on my root server the partition hosting the images of my VMs from EXT4 to XFS => then to make 100% backups from time to time I would just have to shut them down for a minute, do a "cp --reflink" of their image-file on the host/dom0, then I could start them up again and do anytime later the slow download of their "copied" file. Any recommendations about a utility (working on a layer higher than the filesystem) that can synchronize only the chunks of a specific file that has changed since the last sync and which generates as final result a "normal" file (no behind-the-curtains database - just copy the identical data from the local/old file and the changes/new data from the remote location to get a normal up-to-date local file)?
    - caf 7y ago
      I believe rsync is the utility you are looking for.
      - zepearl 7y ago
        Damn, now I'm ashamed of myself - it's since forever that I'm using rsync... :) I think that when I tested this with rsync a long time back I did a similar mistake like this one https://stackoverflow.com/questions/28819379/block-level-copying-and-rsync https://stackoverflow.com/questions/28819379/block-level-cop... (doing a sync not over a network) and/or this one https://www.reddit.com/r/linux/comments/3nhx0p/rsync_block_level_incremental_backup/ https://www.reddit.com/r/linux/comments/3nhx0p/rsync_block_l... (doing a sync by adding/removing some bytes instead of just changing their values, as it would happen in the preallocated img-file of the VMs). I'll test it again - thanks a lot for the info!!
cmurf 7y ago
It's essentially duplicating the file metadata into a new inode with shared data extents and incrementing the extent backref count so that the extent tree with those extents shows that there are multiple referrers. If either file is written to, it's COW. You can treat both files as if they are normal files, as if they're identical copies. It's just that the duplication doesn't take up additional space other than metadata. A possible caution, just in general, on Btrfs is that each subvolume gets its own stash of inodes. In a subvolume, inode numbers are unique, you won't see the same inum in use more than once. But inode numbers aren't unique on the volume. So two completely unrelated files can have the same inum but they would be in different subvolumes (including snapshots where this is particularly noticable because a snapshot is really just a pointer to the parent subvolume and all the inums are unchanged.)
cat199 7y ago
Q2: (not 100%) this probably wouldn't apply if crossing fs boundaries, and so cloning to another filesystem would probably occupy more space than the source one. Also, many save out files to temporary copies, remove the old file, and move the new one into place. That likely wouldn't track either. Not really 'immediate safety' of the file, but secondary concerns.. Similar caveats apply for traditional soft/hardlinks and sparse files.

simcop2387 7y ago

1) Yes, it's copy on write, and requires such support from the underlying filesystem (Btrfs, maybe XFS not sure of what others support it) 2) It's intended to be treated 100% as if it had been copied the classical way. It's not a hardlink or a symlink and can be treated as a completely new file

cmurf 7y ago
Support for reflinks in XFS for a while now, although it requires a mkfs time option to enable it. That option was just enabled by default in xfsprogs-5.1.0 which is now in Fedora 31 (still prerelease).
- simcop2387 7y ago
  Yea my memory was fuzzy and I knew it was planned and being worked on but couldn't remember if it had happened yet. I think F2FS and bcachefs are also supposed to support it but I'm not sure about their level of support either.
- zepearl 7y ago
  Cool => I'll then probably reformat on my root server the partition hosting the images of my VMs from EXT4 to XFS => then to make 100% backups from time to time I would just have to shut them down for a minute, do a "cp --reflink" of their image-file on the host/dom0, then I could start them up again and do anytime later the slow download of their "copied" file. Any recommendations about a utility (working on a layer higher than the filesystem) that can synchronize only the chunks of a specific file that has changed since the last sync and which generates as final result a "normal" file (no behind-the-curtains database - just copy the identical data from the local/old file and the changes/new data from the remote location to get a normal up-to-date local file)?
  - caf 7y ago
    I believe rsync is the utility you are looking for.
    - zepearl 7y ago
      Damn, now I'm ashamed of myself - it's since forever that I'm using rsync... :) I think that when I tested this with rsync a long time back I did a similar mistake like this one https://stackoverflow.com/questions/28819379/block-level-copying-and-rsync https://stackoverflow.com/questions/28819379/block-level-cop... (doing a sync not over a network) and/or this one https://www.reddit.com/r/linux/comments/3nhx0p/rsync_block_level_incremental_backup/ https://www.reddit.com/r/linux/comments/3nhx0p/rsync_block_l... (doing a sync by adding/removing some bytes instead of just changing their values, as it would happen in the preallocated img-file of the VMs). I'll test it again - thanks a lot for the info!!

cmurf 7y ago

It's essentially duplicating the file metadata into a new inode with shared data extents and incrementing the extent backref count so that the extent tree with those extents shows that there are multiple referrers. If either file is written to, it's COW. You can treat both files as if they are normal files, as if they're identical copies. It's just that the duplication doesn't take up additional space other than metadata. A possible caution, just in general, on Btrfs is that each subvolume gets its own stash of inodes. In a subvolume, inode numbers are unique, you won't see the same inum in use more than once. But inode numbers aren't unique on the volume. So two completely unrelated files can have the same inum but they would be in different subvolumes (including snapshots where this is particularly noticable because a snapshot is really just a pointer to the parent subvolume and all the inums are unchanged.)

cat199 7y ago

Q2: (not 100%) this probably wouldn't apply if crossing fs boundaries, and so cloning to another filesystem would probably occupy more space than the source one. Also, many save out files to temporary copies, remove the old file, and move the new one into place. That likely wouldn't track either. Not really 'immediate safety' of the file, but secondary concerns.. Similar caveats apply for traditional soft/hardlinks and sparse files.