Re: [PATCH] btrfs: do not allow -o compress-force to override per-inode settings

[sys <system@lechevalier.se>]

---- From: Hugo Mills <hugo@carfax.org.uk> -- Sent: 2020-11-30 - 16:04 ----

> On Mon, Nov 30, 2020 at 09:50:13AM -0500, Josef Bacik wrote:
>> On 11/30/20 9:08 AM, Roman Mamedov wrote:
>> > On Mon, 30 Nov 2020 08:46:21 -0500
>> > Josef Bacik <josef@toxicpanda.com> wrote:
>> > 
>> > > However some time later we got chattr -c, which is a user way of
>> > > indicating that the file shouldn't be compressed.  This is at odds with
>> > > -o compress-force.  We should be honoring what the user wants, which is
>> > > to disable compression.
>> > 
>> > But chattr -c only removes the previously set chattr +c. There's no
>> > "negative-c" to be forced by user in attributes. And +c is already unset on all
>> > files by default. Unless I'm missing something? Thanks
>> > 
>> 
>> The thing you're missing is that when we do chattr -c we're setting
>> NOCOMPRESS on the file.  The thing that I'm missing is what exactly we're
>> trying to allow.  If chattr -c is supposed to just be the removal of +c,
>> then btrfs is doing the wrong thing by setting NOCOMPRESS.  We also do the
>> same thing when we clear a btrfs.compression property.
> 
>    If I'm understanding this right, there's more than two states
> here. There's a "default", and there's two "force" options -- forcing
> nocompress, and forcing/allowing compression. If there's no c flag
> set, the file could be in one of two states: default behaviour
> (presumably defined by the value of the mount options), and
> NOCOMPRESS. How do I tell which it is?

 Fsattr (chattr/lsattr) seem to have been a simplified way of exposing the compression xattr to users, wasn't it? Could it not be better to promote the use of btrfs.compression xattrs instead? 

> 
>> I guess the question is what do we want?  Do we want to only allow the user
>> to indicate we want compression, or do we want to allow them to also
>> indicate that they don't want compression? 

I think it is a good thing to have the three options, but we may need to drop the use of chattr/lsattr and go with only xattrs. 

Thanks, 
Forza 

 If we don't want to enable them
>> to disable compression for a file, then this patch needs to be thrown away,
>> but then we also need to fix up all the places we set NOCOMPRESS when we
>> clear these flags.  Thanks,
> 
>    Hugo.
> 
> -- 
> Hugo Mills             | The last man on Earth sat in a room. Suddenly, there
> hugo@... carfax.org.uk | was a knock at the door.
> http://carfax.org.uk/  |
> PGP: E2AB1DE4          |                                        Frederic Brown