Re: filefrag and btrfs filesystem defragment and maybe snapshots

[Liu Bo <bo.li.liu@oracle.com>]

On Thu, Aug 01, 2013 at 05:18:50PM -0400, Sandy McArthur wrote:
> While exploring some btrfs maintenance with respect to defragmenting I
> ran the following commands:
> 
> # filefrag /path/to/34G.file /path/to/5.7G.file
> /path/to/34G.file: 2406 extents found
> /path/to/5.7G.file: 572 extents found
> 
> Thinking those mostly static files could be less fragmented I ran:
> # btrfs filesystem defragment -c /path/to/34G.file
> # btrfs filesystem defragment -c /path/to/5.7G.file
> 
> and to my surprise the number of fragments/extends doubled:
> 
> # filefrag /path/to/34G.file /path/to/5.7G.file
> /path/to/34G.file: 6324 extents found
> /path/to/5.7G.file: 1079 extents found
> 
> Did I actually improve these files?
> 
> I do have a number rolling readonly snapshots on the subvolume these
> files are on. I can imagine how that might be related but I'm not
> sure. When the pre-defrag snapshots are purged will the filefrag
> extents count drop.

I guess the reason is that you're doing the defragment with compression,
ZLIB in default case, and btrfs compression has a maximum length limit, 128k.
So if the original file's extents have a length larger than 128k, it's possible
to get the results of 'filefrag' doubled.

But I have no possible answer for
'why filefrag extents count drop after purging snapshots'.

-liubo