Re: filefrag and btrfs filesystem defragment and maybe snapshots

[Duncan <1i5t5.duncan@cox.net>]

Sandy McArthur posted on Thu, 01 Aug 2013 17:18:50 -0400 as excerpted:

> 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 can't answer the snapshot angle, but do you have btrfs compression 
turned on?  I've read that filefrag always sees btrfs compressed files of 
sufficient size as fragmented, due to the way btrfs compression works.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
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