Re: 'btrfs filesystem defragment' makes files explode in size, especially fallocated ones

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2024/8/4 18:50, i.r.e.c.c.a.k.u.n+kernel.org@gmail.com 写道:
> (Originally reported on Kernel.org Bugzilla: 
> https://bugzilla.kernel.org/show_bug.cgi?id=219033)
> 
> There is a very weird quirk I found with 'btrfs filesystem defragment' 
> command. And no, it's not about reflinks removal, I'm aware of that.
> 
> It is kinda hard to replicate, but I found a somewhat reliable way. It 
> reaches extremes with fallocated files specifically.
> 
> 1. Create a file on a Btrfs filesystem using 'fallocate' and fill it. 
> The easy way to do that is just to copy some files with 'rsync 
> --preallocate'.
> 
> 2. Check compsize info:

Mind to dump the filemap output (xfs_io -c "fiemap -v") before and after 
the defrag?

Thanks,
Qu
> 
> # compsize foo
> Processed 71 files, 71 regular extents (71 refs), 0 inline.
> Type       Perc     Disk Usage   Uncompressed Referenced
> TOTAL      100%      630M         630M         630M
> none       100%      630M         630M         630M
> 
> All is fine here for now. 1 extent per 1 file, "Disk Usage" = "Referenced".
> 
> 3. Run defragment:
> 
> # btrfs filesystem defragment -r foo
> 
> 4. Check compsize again:
> 
> # compsize foo
> Processed 71 files, 76 regular extents (76 refs), 0 inline.
> Type       Perc     Disk Usage   Uncompressed Referenced
> TOTAL      100%      638M         638M         630M
> none       100%      638M         638M         630M
> 
> Oops, besides the fact that the amount of extents is actually increased, 
> which means 'btrfs filesystem defragment' actually made fragmentation 
> worse, physical disk usage increased for no reason. And I didn't find 
> any way to shrink it back.
> 
> ---
> 
> The end result seems to be random though. But I managed to achieve some 
> truly horrifying results.
> 
> # compsize foo
> Processed 45 files, 45 regular extents (45 refs), 0 inline.
> Type       Perc     Disk Usage   Uncompressed Referenced
> TOTAL      100%      360M         360M         360M
> none       100%      360M         360M         360M
> 
> # btrfs filesystem defragment -r -t 1G foo
> 
> # compsize foo
> Processed 45 files, 144 regular extents (144 refs), 0 inline.
> Type       Perc     Disk Usage   Uncompressed Referenced
> TOTAL      100%      716M         716M         360M
> none       100%      716M         716M         360M
> 
> Yikes! Triple the extents! Double increase in size!
>