Re: [PATCH 0/4] btrfs: add per-inode compression levels in xattrs

在 2026/8/10 11:36, Zygo Blaxell 写道:
> On Mon, Aug 10, 2026 at 11:27:49AM +0930, Qu Wenruo wrote:
>>
>>
>> 在 2026/8/10 11:27, koraynilay 写道:
>>> 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?
>>
>> Yep.
> 
> I am vehemently opposed to deprecation of a feature that will require
> updating _billions_ of inodes per server to get the same effect, when
> the filesystem was previously able to handle a 4-level hierarchy of
> compression options with "defer to next level" since the beginning.
> 
> I will maintain a fork if I have to.  Hard NAK.

Do whatever you want.

> 
> We can have clearer documentation about how options are processed,
> and clearly what options mean "look up to the next level" vs "use the
> default" or "use the locally defined value."
> 
>>>> 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.
>>
>>
>>
>