Re: compsize reports that filesystem uses zlib compression, while I set zstd compression everywhere

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

On Thu, Oct 27, 2022 at 01:29:35AM +0300, Nemcev Aleksey wrote:
> 26.10.2022 22:07, Chris Murphy пишет:
> > 
> > On Wed, Oct 26, 2022, at 6:21 AM, Nemcev Aleksey wrote:
> > > Hello.
> > > 
> > > Recently I decided to compress the existing Btrfs volume with zstd.
> > > 
> > > To do this, I set property `compression=zstd` for all subvolumes to
> > > compress new files with the default zstd:3 compression level.
> > > 
> > > Then I run `btrfs filesystem defragment -c zstd:12 -v -r` on all
> > > subvolumes to compress existing files using zstd:12 compression level
> > > (to spend more time now and save more space later, but don't want to
> > > keep slow zstd:12 level all the time).
> > > 
> > > After defragment, I run `btrfs filesystem balance` on all subvolumes to
> > > make Btrfs happy.
> > > 
> > > And after all, I run `compsize` on the root subvolume to check the
> > > compression ratio, and compsize shows me the following:
> > > Processed 1100578 files, 1393995 regular extents (1649430 refs), 666312
> > > inline.
> > > Type      Perc    Disk Usage  Uncompressed Referenced
> > > TOTAL      77%     169G        217G        226G
> > > none      100%      99G         99G        100G
> > > zlib       52%      54G        102G        109G
> > > zstd       19%      12M         65M         66M
> > > prealloc  100%      15G         15G         16G
> > > 
> > > I'm very confused why almost all compressed data is compressed with zlib
> > > while I haven't used zlib at any step.
> > > Why do compsize reports this?
> > > Should I worry about this? It seems zstd offers a much faster
> > > decompression speed than zlib.
> > > Thank you.
> > 
> > If you use chattr +c anywhere, it uses the btrfs default compression algo which is zlib. There is a way to set a compression algo property, but I'm uncertain if the kernel honors it.
> > 
> > So you'll want to recursively remove the compression file attribute. I'm not sure it's worth recompressing but you can use defrag -z for that.
> > 
> Thank you for the idea, I understood where I failed.
> 
> `btrfs filesystem defragment -c zstd -r /` is not correct command, but
> `btrfs filesystem defragment -czstd -r /` is correct one (note the space).
> 
> And the first command will try to defragment the file named zstd using
> default compression (zlib), not the compression from the `compression`
> subvolume property.
> 
> And also it's not possible to specify compression level in defragment - so,
> I'll change the level in subvolume properties, defragment with -czstd, and
> change the level back.

It's not possible to set the level in subvol properties either.  Only the
compress= mount option can set the level.