Re: CEPH to BTRFS over NFS results in no compression

["Konstantin V. Gavrilenko" <k.gavrilenko@arhont.com>]

Thanks for clarification Lukas. I didn't know that. 

I guess the easiest option for me is to recompress snapshots on a daily basis after they get uploaded.

yours,
kos


----- Original Message -----
> From: "Lukas Straub" <lukasstraub2@web.de>
> To: "Konstantin V Gavrilenko" <k.gavrilenko@arhont.com>
> Cc: "linux-btrfs" <linux-btrfs@vger.kernel.org>
> Sent: Sunday, 2 January, 2022 15:08:33
> Subject: Re: CEPH to BTRFS over NFS results in no compression

> On Sun, 2 Jan 2022 11:32:53 +0000 (UTC)
> "Konstantin V. Gavrilenko" <k.gavrilenko@arhont.com> wrote:
> 
>> Hi list,
>> 
>> I have noticed an interesting and surprising behaviour of my BTRFS with regards
>> to compression of the files and NFS.
>> 
>> I have BTRFS RAID10 with 8 disks , that is mounted with the "
>> nofail,noatime,space_cache=v2,compress-force=zstd:9,subvol=@cloudstack-secondary"
>> flags and is exported via NFS.
>> 
>> When I create a snapshot of a disk in Cloudstack from CEPH and save it to a
>> secondary storage to this BTRFS RAID10 over NFS, the file does not compress,
>> despite the compress-force mount option being set on FS
>> So in the below example, the file eeceaf0e-9780-408b-a748-1495d517a9b6 was
>> copied over NFS and is not compressed. When I copy the same file directly on a
>> host, it does get compressed pretty well, as per example below.
>> 
>> [...]
>> 
>> 
>> So what I have checked so far what works
>> - after the original files is copied over NFS, the copy of the file using #cp
>> gets compressed.
>> - after the original files is copied over NFS, the original file can be
>> compressed using #btrfs defrag -czstd option
>> - If I copy the original file to some other host, and copy it back via NFS using
>> cp, it does get compressed.
>> 
>> So the problem seems to appear only when the file is exported from Ceph and
>> copied to NFS.
>> 
>> Any hints what could be causing such a behaviour?
>> 
>> 
>> 
>> Yours sincerely,
>> Kos
>> 
> 
> Btrfs doesn't compress direct-io writes. This suggests that whatever you use for
> "I create a snapshot of a disk in Cloudstack from CEPH and save it to a
> secondary storage to this BTRFS RAID10 over NFS", it writes with O_DIRECT.
> 
> Regards,
> Lukas Straub
> 
> --