Re: btrfs reported used space doesn't correspond with space occupied by the files themselves

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

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On 2019/9/10 下午12:41, Chris Murphy wrote:
> On Mon, Sep 9, 2019 at 10:16 PM Daniel Martinez
> <danielsmartinez@gmail.com> wrote:
>>
>> Hello,
>>
>> I've recently converted my root 32GB ext4 partition to btrfs (using
>> btrfs-progs 5.2). After that was done, I made a snapshot and tried to
>> update the system. Unfortunately I didn't have enough free space to
>> fit the whole update on that small partition, so it failed. I then
>> realized my mistake and deleted not only that newly made snapshot, but
>> also ext2_saved and some random files on the filesystem, totaling
>> about 5GB. For my surprise, the update still failed due to ENOSPC.
>>
>> At this point, I tried running a balance, but it also failed with
>> ENOSPC. I tried the balance -dusage X with X increasing from zero, but
>> to my surprise again, it also failed.
>>
>> Data, single: total=28.54GiB, used=28.34GiB
>> System, single: total=32.00MiB, used=16.00KiB
>> Metadata, single: total=1.00GiB, used=807.45MiB
>> GlobalReserve, single: total=41.44MiB, used=0.00B
>>
>> Looking at btrfs filesystem df, it looks like those 5GB of data I
>> deleted are still occupying space. In fact, ncdu claims all the files
>> on that drive sum up to only 19GB.

That's not uncommon.

Since convert make the ext2 image first, then reflink files to use part
of the extents of that image.

So just deleting the image subvolume won't ensure to free all space, as
part of the space is still used by the converted data.

You need to delete some files to free up some space first, make sure
there is no snapshot of your current subvolume, then do a full defrag.

The balance won't really do much help, you need to defrag to free up the
space wasted by the ext*->btrfs convert.

Thanks,
Qu

>>
>> I tried adding a second 2GB drive but that still wasn't enough to run
>> a full data balance (metadata runs fine).
>>
>> This is what filesystem usage looks like:
>>
>> Overall:
>>     Device size:                  31.59GiB
>>     Device allocated:             29.57GiB
>>     Device unallocated:            2.03GiB
>>     Device missing:                  0.00B
>>     Used:                         29.13GiB
>>     Free (estimated):              2.22GiB      (min: 2.22GiB)
>>     Data ratio:                       1.00
>>     Metadata ratio:                   1.00
>>     Global reserve:               41.44MiB      (used: 0.00B)
>>
>> Data,single: Size:28.54GiB, Used:28.34GiB
>>    /dev/sda7     768.00MiB
>>    /dev/sdb1      27.79GiB
>>
>> Metadata,single: Size:1.00GiB, Used:807.45MiB
>>    /dev/sdb1       1.00GiB
>>
>> System,single: Size:32.00MiB, Used:16.00KiB
>>    /dev/sdb1      32.00MiB
>>
>> Unallocated:
>>    /dev/sda7       1.03GiB
>>    /dev/sdb1       1.00GiB
>>
>>
>> I then made a read-only snapshot of the root filesystem and used btrfs
>> send/receive to transfer it to another btrfs filesystem, and when it
>> got there its also only occupying 19GB.
>>
>> So there seems 10GB got lost somewhere in the process and I can't find
>> a way to get them back (other thank mkfs'ing and restoring a backup),
>> which in this case is about 30% of the available disk space.
>>
>> What may be causing this?
> 
> 
> Since the 4.6 convert rewrite, I'm not sure off hand if a defragment
> is still suggested after the conversion. Qu can answer it.
> 
> There is an edge case where extents can get pinned when modified after
> a snapshot, and not released even after the snapshot is deleted. But
> what you're describing would be a really extreme version of this, and
> isn't one I've come across before. It could be an unintended artifact
> of conversion from ext4. Hard to say.
> 
> I suggest 'btrfs-image -c9 -t4 -ss /dev/ /path/to/file' and keep it
> handy in case a developer asks for it. Metadata is only 800MiB so it
> should compress down to less than 400 MiB. Also report back what
> kernel verion is being used.
> 
> In the meantime, I suggest deleting all snapshots to give Btrfs a
> chance to clean up unused extents. And then you could try to force a
> clean up of unused extents by recursive defragment. The system is so
> full right now that it's likely this will fail also with ENOSPC. COW
> requires a completely successful write to a new location before old
> extents can be freed. So whether delete or defragment, space is
> consumed before it can be later freed up. But you might have some luck
> at selectively defragmenting directories that you know do not have big
> files. Like, start out with /etc/ and /usr - maybe you have VM images
> in /var? If not then /var can be next. Maybe big files in /home? So do
> that last, or do in such a way to avoid the big files until last.
> 
> 


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