Query: XFS reflink unique block measurement / snapshot accounting

Query: XFS reflink unique block measurement / snapshot accounting

[Priya PM]

Hi,

I was using reflinks to create snapshots of an XFS filesystem;
however, I’m looking for ways to determine the unique snapshot usage
or perform snapshot accounting.

We tried using filefrag, but it doesn’t help when editing files with
vi, as vi changes the block numbers with every write operation. This
causes the snapshot accounting to show unique blocks even though there
are shared blocks with the original file.

Can anyone suggest a better method for snapshot accounting on XFS?
Thanks in advance for your help!

Thanks!

Re: Query: XFS reflink unique block measurement / snapshot accounting

[Christoph Hellwig]

On Sat, Jul 12, 2025 at 01:19:19PM +0530, Priya PM wrote:
> Hi,
> 
> I was using reflinks to create snapshots of an XFS filesystem;
> however, I’m looking for ways to determine the unique snapshot usage
> or perform snapshot accounting.

Can you explain what you are trying to measure?  How many blocks in a
given file are refcounted blocks with other users?  Or the difference
between i_blocks for all files combined vs actual space usage?

There isn't really such a thing as snapshot accouting in XFS.

Re: Query: XFS reflink unique block measurement / snapshot accounting

[Dave Chinner]

On Sat, Jul 12, 2025 at 01:19:19PM +0530, Priya PM wrote:
> Hi,
> 
> I was using reflinks to create snapshots of an XFS filesystem;
> however, I’m looking for ways to determine the unique snapshot usage
> or perform snapshot accounting.

XFS doesn't have any "snapshot" functionality in it. What are you
trying to measure, exactly?

-Dave.
-- 
Dave Chinner
david@fromorbit.com

Re: Query: XFS reflink unique block measurement / snapshot accounting

[Priya PM]

For example,
/mnt/fs1 has file1 of size 40KB
Do a reflink cp of /mnt/fs1 to /mnt/.snap/1
Now, when I check the size of /mnt/.snap/1/, it should show only snap
usage. In this case, it should be 0, as no blocks are unique to this
snap.
Modify file1 in /mnt/fs1, modify 2 blocks of block size 4 KB.
Take snapshot /mnt/snap/2 --.> This should show only the unique snap
usage as 8KB.

I understand there is no snap accounting in XFS. However, to achieve
the above things, is there any XFS command that can help?

Thanks in advance for your time.

On Mon, Jul 14, 2025 at 11:28 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Sat, Jul 12, 2025 at 01:19:19PM +0530, Priya PM wrote:
> > Hi,
> >
> > I was using reflinks to create snapshots of an XFS filesystem;
> > however, I’m looking for ways to determine the unique snapshot usage
> > or perform snapshot accounting.
>
> Can you explain what you are trying to measure?  How many blocks in a
> given file are refcounted blocks with other users?  Or the difference
> between i_blocks for all files combined vs actual space usage?
>
> There isn't really such a thing as snapshot accouting in XFS.
>

Re: Query: XFS reflink unique block measurement / snapshot accounting

[Christoph Hellwig]

On Tue, Jul 15, 2025 at 07:21:22PM +0530, Priya PM wrote:
> For example,
> /mnt/fs1 has file1 of size 40KB
> Do a reflink cp of /mnt/fs1 to /mnt/.snap/1
> Now, when I check the size of /mnt/.snap/1/, it should show only snap
> usage. In this case, it should be 0, as no blocks are unique to this
> snap.
> Modify file1 in /mnt/fs1, modify 2 blocks of block size 4 KB.
> Take snapshot /mnt/snap/2 --.> This should show only the unique snap
> usage as 8KB.
> 
> I understand there is no snap accounting in XFS. However, to achieve
> the above things, is there any XFS command that can help?

reflinked blocks are not owned by either file, so the usage is shared
by them.

There's a few things you could theoretically do:

 - check how many non-shared blocks a file has.  The easiest way to
   do that would be to call fiemap on the inode, and count the extents
   that don't have the shared flag set on them.  You'd probably need to
   write that at the C level, as the filefrag and xfs_io fiemap commands
   aren't all that great
 
 - look how many shared vs unshared block a file system has in total.
   For thay you'd want a rmap-enabled file system and look at the fsmap
   output.