Re: apply different compress/compress-force settings on different subvolumes ?

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2/21/24 08:15, Roland wrote:
> Am 21.02.24 um 10:33 schrieb Qu Wenruo:
>
>> 在 2024/2/21 19:19, Roland 写道:
>>> and why is compress-force silently ignored? (see below)
>> Compress-force has no coresponding per-inode prop option though.
>>
>> But I don't think compress-force would cause much difference against
>> regular compress.
>
> apparently, force-compress can make a big difference (and compress only
> can lead to no compression at all):
>
> https://bugzilla.proxmox.com/show_bug.cgi?id=5250#c6
I can attest that this is indeed the case, though it’s very dependent on 
the exact usage. For the case of large files with highly varied data 
(think filesystem images, VM disk images, large databases, etc) that 
don’t include inherent compression, it’s very normal for compression to 
actually be a huge help, but the early parts of the file to have very 
high entropy and thus not compress well, resulting in regular `compress` 
not actually helping at all (because BTRFS quickly decides it’s not 
worth it for that file and chooses to not compress it at all).

As an example, the VM disk images I’m storing on my home server take up 
roughly 40% less space with `compress-force=zstd:1` compared to just 
`compress=zstd:1`.

In comparison, I see no effective difference when doing it on the root 
filesystem of said home server, so like I said above, it’s rather 
dependent on exact usage.

There is of course some extra overhead to attempting compression n every 
write, but at least for me, on the lower compression levels when dealing 
with ‘slow’ storage, it’s not enough to cause problems.