Re: BTRFS doesn't compress on the fly

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

On 2023/12/22 16:28, Gerhard Wiesinger wrote:
> On 03.12.2023 11:19, Qu Wenruo wrote:
>>
>>
>>> BTW:
>>>
>>> if compression is forced, should be then just any "block" be compressed?
>>
>> There is a long existing problem with compression with preallocation.
>>
>> One easy example is, if we go compression for the preallocated range,
>> what we do with the gap (compressed size is always smaller than the real
>> size).
>>
>> If we leave the gap, then the read performance can be even worse, as now
>> we have to read several small extents with gaps between them, vs a large
>> contig read.
>>
>> IIRC years ago when I was a btrfs newbie, that's the direction I tried
>> to go, but never reached upstream.
>>
>> Thus you can see some of the reason why we do not go compression for
>> preallocated range.
>>
>> But I still don't believe we should go as the current behavior.
>> We should still try to go compression as long as we know the write still
>> needs COW, thus we should fix it.
>
>
> Any progress with the fix?

Tried several solution, the best one would still lead to reserved space
underflow.

The proper fix would introduce some larger changes to the whole delalloc
mechanism.

Thus it's not something can be easily fixed yet.

Thanks,
Qu
>
> Thnx.
>
> Ciao,
>
> Gerhard
>