Re: exclusive subvolume space missing

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

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On 2017年12月02日 00:15, Tomasz Pala wrote:
> Hello,
> 
> I got a problem with btrfs running out of space (not THE
> Internet-wide, well known issues with interpretation).
> 
> The problem is: something eats the space while not running anything that
> justifies this. There were 18 GB free space available, suddenly it
> dropped to 8 GB and then to 63 MB during one night. I recovered 1 GB
> with rebalance -dusage=5 -musage=5 (or sth about), but it is being eaten
> right now, just as I'm writing this e-mail:
> 
> /dev/sda2        64G   63G  452M 100% /
> /dev/sda2        64G   63G  365M 100% /
> /dev/sda2        64G   63G  316M 100% /
> /dev/sda2        64G   63G  287M 100% /
> /dev/sda2        64G   63G  268M 100% /
> /dev/sda2        64G   63G  239M 100% /
> /dev/sda2        64G   63G  230M 100% /
> /dev/sda2        64G   63G  182M 100% /
> /dev/sda2        64G   63G  163M 100% /
> /dev/sda2        64G   64G  153M 100% /
> /dev/sda2        64G   64G  143M 100% /
> /dev/sda2        64G   64G   96M 100% /
> /dev/sda2        64G   64G   88M 100% /
> /dev/sda2        64G   64G   57M 100% /
> /dev/sda2        64G   64G   25M 100% /
> 
> while my rough calculations show, that there should be at least 10 GB of
> free space. After enabling quotas it is somehow confirmed:
> 
> # btrfs qgroup sh --sort=excl / 
> qgroupid         rfer         excl 
> --------         ----         ---- 
> 0/5          16.00KiB     16.00KiB 
> [30 snapshots with about 100 MiB excl]
> 0/333        24.53GiB    305.79MiB 
> 0/298        13.44GiB    312.74MiB 
> 0/327        23.79GiB    427.13MiB 
> 0/331        23.93GiB    930.51MiB 
> 0/260        12.25GiB      3.22GiB 
> 0/312        19.70GiB      4.56GiB 
> 0/388        28.75GiB      7.15GiB 
> 0/291        30.60GiB      9.01GiB <- this is the running one
> 
> This is about 30 GB total excl (didn't find a switch to sum this up). I
> know I can't just add 'excl' to get usage, so tried to pinpoint the
> exact files that occupy space in 0/388 exclusively (this is the last
> snapshots taken, all of the snapshots are created from the running fs).

I assume there is program eating up the space.
Not btrfs itself.

> 
> 
> Now, the weird part for me is exclusive data count:
> 
> # btrfs sub sh ./snapshot-171125
> [...]
>         Subvolume ID:           388
> # btrfs fi du -s ./snapshot-171125 
>      Total   Exclusive  Set shared  Filename
>   21.50GiB    63.35MiB    20.77GiB  snapshot-171125

That's the difference between how sub show and quota works.

For quota, it's per-root owner check.
Means even a file extent is shared between different inodes, if all
inodes are inside the same subvolume, it's counted as exclusive.
And if any of the file extent belongs to other subvolume, then it's
counted as shared.

For fi du, it's per-inode owner check. (The exact behavior is a little
more complex, I'll skip such corner case to make it a little easier to
understand).

That's to say, if one file extent is shared by different inodes, then
it's counted as shared, no matter if these inodes belong to different or
the same subvolume.

That's to say, "fi du" has a looser condition for "shared" calculation,
and that should explain why you have 20+G shared.

Thanks,
Qu


> 
> 
> How is that possible? This doesn't even remotely relate to 7.15 GiB
> from qgroup.~The same amount differs in total: 28.75-21.50=7.25 GiB.
> And the same happens with other snapshots, much more exclusive data
> shown in qgroup than actually found in files. So if not files, where
> is that space wasted? Metadata?
> 
> btrfs-progs-4.12 running on Linux 4.9.46.
> 
> best regards,
> 


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