On Mon Aug 10, 2026 at 3:54 AM CEST, Qu Wenruo wrote:
>
>
> 在 2026/8/10 10:35, koraynilay 写道:
>> On Mon Aug 10, 2026 at 2:51 AM CEST, Qu Wenruo wrote:
>>>>>>> So either option 2 or 3 would be fine to me. Although I personally prefer
>>>>>>> option 3 a little more, just because it's much cleaner code wise.
>>>>>>
>>>>>> Option 2 preserves legacy behavior that is 12 years old now, and it
>>>>>> costs a single comparison in two 'if' statements.
>>>>>>
>>>>>> Option 3 makes an already confusing situation worse--it makes the
>>>>>> underspecified behavior change depending on kernel version.
>>>>>
>>>>> One should never rely on something not documented in the first place.
>>>>
>>>> Option 3 prevents existing mount-option compression level specifications
>>>> from working when the attribute agress with the mount option; otherwise,
>>>> they would be blocked by a btrfs.compression string that doesn't specify
>>>> a level.  That's a _regression_.
>>>
>>> Let me be this clear, the current one nor option 2 is not working either.
>>>
>>> If the current algo is different from the XATTR algo, it will be
>>> whatever random number clamped to the XATTR algo for the current code.
>>>
>>> This applies to the option 2 solution. When mount option changed, the
>>> level will suddenly change from whatever previous mount option to the
>>> default.
>> 
>> TBF, I can see how it could be useful (or rather, how it could be good
>> to have it as an option) to have some files with btrfs.compression="zstd"
>> and then use -o compress= to decide on the fly how much compressed the
>> new data added to them should be.
>> Both are (read: will be, after the per-inode patch) 1 command away, but
>> there *might* be use-cases where mount is more suitable.
>
> To be honest, with the proper XATTR compression level specification, I 
> think we should even deprecate compress= mount option, and make the 
> XATTR one the only recommended way to specific compression.

Ah, and in that case, to set compression on the whole fs use btrfs prop to
set it on the root?

> There are already too many corner cases with mount option.
>
> IMHO, a good design should allow and only allow the best way to do a thing.
>
> And option 3 matches perfect for the XATTR only compression future. It 
> still allows old XATTR to work, have a very sane default level, very 
> explicit and clear independent from whatever stupid mount option there 
> could be.