Re: BTRFS doesn't compress on the fly

[Gerhard Wiesinger <lists@wiesinger.com>]

On 22.12.2023 07:13, Qu Wenruo wrote:
>
>
> 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

Any update on the issue or plans to fix it?

Thanks.

Ciao,

Gerhard