Re: btrfs thinks fs is full, though 11GB should be still free

Qu Wenruo
2023-12-12T02:37:03Z

On 2023/12/12 11:28, Qu Wenruo wrote:
>
>
> On 2023/12/12 10:42, Christoph Anton Mitterer wrote:
>> On Tue, 2023-12-12 at 10:24 +1030, Qu Wenruo wrote:
>>> Then the last thing is extent bookends.
>>>
>>> COW and small random writes can easily lead to extra space wasted by
>>> extent bookends.
>>
>> Is there a way to check this? Would I just seem maaany extents when I
>> look at the files with filefrag?

IIRC compsize can do it.

https://github.com/kilobyte/compsize

Thanks,
Qu
>>
>>
>> I mean Prometheus, continuously collects metrics from a number of nodes
>> an (sooner or later) writes them to disk.
>> I don't really know their code, so I have no idea if they already write
>> every tiny metric, or only large bunches thereof.
>>
>> Since they do maintain a WAL, I'd assume the former.
>>
>> Every know and then, the WAL is written to chunk files which are rather
>> large, well ~160M or so in my case, but that depends on how many
>> metrics one collects. I think they always write data for a period of
>> 2h.
>> Later on, they further compact that chunks (I think after 8 hours and
>> so on), in which case some larger rewritings would be done.
>> Though in my case this doesn't happen, as I run Thanos on top of
>> Prometheus, and for that one needs to disable Prometheus' own
>> compaction.
>>
>>
>> I've had already previously looked at the extents for these "compacted"
>> chunk files, but the worst file had only 32 extents (as reported by
>> filefrag).
>
> Filefrag doesn't work that well on btrfs AFAIK, as btrfs is emitting
> merged extents to fiemap ioctl, but for fragmented one, filefrag should
> be enough to detect them.
>>
>> Looking at the WAL files:
>> /data/main/prometheus/metrics2/wal# filefrag * | grep -v ' 0 extents
>> found'
>> 00001030: 82 extents found
>> 00001031: 81 extents found
>> 00001032: 79 extents found
>> 00001033: 82 extents found
>> 00001034: 78 extents found
>> 00001035: 78 extents found
>> 00001036: 81 extents found
>> 00001037: 79 extents found
>> 00001038: 79 extents found
>> 00001039: 89 extents found
>> 00001040: 80 extents found
>> 00001041: 74 extents found
>> 00001042: 81 extents found
>> 00001043: 97 extents found
>> 00001044: 101 extents found
>> 00001045: 316 extents found
>> checkpoint.00001029: FIBMAP/FIEMAP unsupported
>>
>> (I did the grep -v, because there were a gazillion of empty wal files,
>> presumably created when the fs was already full).
>>
>> The above numbers though still don't look to bad, do they?
>
> Depends, in my previous 16M case. you only got 2 extents, but still
> wasted 8M (33.3% space wasted).
>
> But WAL indeeds looks like a bad patter for btrfs.
>
>>
>> And checking all:
>> # find /data/main/ -type f -execdir filefrag {} \; | cut -d : -f 2 |
>> sort | uniq -c | sort -V
>>     3706  0 extents found
>>      450  1 extent found
>>       25  3 extents found
>>       62  2 extents found
>>        1  8 extents found
>>        1  9 extents found
>>        1  10 extents found
>>        1  11 extents found
>>        1  32 extents found
>>        1  74 extents found
>>        1  80 extents found
>>        1  89 extents found
>>        1  97 extents found
>>        1  101 extents found
>>        1  316 extents found
>>        2  78 extents found
>>        2  82 extents found
>>        3  5 extents found
>>        3  79 extents found
>>        3  81 extents found
>>        6  4 extents found
>>
>>
>>
>>> E.g. You write a 16M data extents, then over-write the tailing 8M,
>>> now
>>> we have two data extents, the old 16M and the new 8M, wasting 8M
>>> space.
>>>
>>> In that case, you can try defrag, but you still need to delete some
>>> data
>>> first so that you can do defrag...
>>
>>
>> Well my main concern is rather how to prevent this from happening in
>> the first place... the data is already all backuped into Thanos, so I
>> could also just wipe the fs.
>> But this seems to occur repeatedly (well, okay only twice so far O:-)
>> ).
>> So that would mean we have some IO pattern that "kills" btrfs.
>
> Thus we have "autodefrag" mount option for such use case.
>
> Thanks,
> Qu
>>
>>
>> Cheers,
>> Chris.
>