[gentoo-user] btrfs: subvol without compression

["Stefan G. Weichinger" <lists@xunil.at>]

... I am quite happy now with the performance of that new server I am
preparing.

See thread "Intel(R) C600 SAS Controller" for that story:

https://www.mail-archive.com/gentoo-user@lists.gentoo.org/msg146119.html

Right now I get quite good results when doing backups of the 2 existing
VMs (which have their virtio-disks on LVM-LVs on the host) ... up to
200MB/s ... I can show for reference, if someone is interested.

I think that is around the possible maximum.

--->

The issue I want to share with you is related to a btrfs subvol I have here.

Block device sda builds the btrfs-pool containing the root-fs:

# btrfs fi show
Label: ROOT  uuid: 9133c469-df1e-45f5-a09f-d1b9c75c69da
	Total devices 1 FS bytes used 29.47GiB
	devid    1 size 500.00GiB used 278.04GiB path /dev/sda

Btrfs v3.12

These are the subvolumes (I could/should rm some, but it doesn't matter
for this issue, afaik):

# btrfs su list /
ID 257 gen 4282 top level 5 path __active
ID 258 gen 4874 top level 5 path __active/root
ID 266 gen 4772 top level 258 path images
ID 267 gen 838 top level 258 path images/otrs
ID 289 gen 4285 top level 258 path images/windows
ID 538 gen 4874 top level 5 path __active/virt-backup


fstab has:

# grep btrfs /etc/fstab
LABEL=ROOT	/		btrfs	defaults,noatime,compress=lzo	0 0
LABEL=ROOT	/mnt/virt-backup	btrfs	compress=no,noatime,subvolid=538 0 0


... so I want to mount subvolid 538 with disabled compression (to speed
up backups as the files written to it are compressed on the fly via pigz
already).


But after booting I get that dir mounted with compress=lzo (which is
default).

# mount | grep btrfs
/dev/sda on / type btrfs (rw,noatime,compress=lzo,space_cache)
/dev/sda on /mnt/virt-backup type btrfs
(rw,noatime,compress=lzo,space_cache)

remounting works, though:

booze ~ # mount -o remount,compress=no /mnt/virt-backup/

booze ~ # mount | grep btrfs
/dev/sda on / type btrfs (rw,noatime,space_cache)
/dev/sda on /mnt/virt-backup type btrfs (rw,noatime,space_cache)

BUT it remounts / without compression as well ... !

Is it a bug? A mistake or misunderstanding?

Could someone test this on his gentoo-btrfs-box?

--

Additional info:

# cat /proc/version
Linux version 3.12.21-gentoo-r1 ...

sys-fs/btrfs-progs-3.12-r1

# btrfs su get-default /
ID 258 gen 4886 top level 5 path __active/root

# line in grub.cfg mounts default subvol (because no specific subvol is
specified)

linux	/boot/vmlinuz-3.12.21-gentoo-r1
root=UUID=9133c469-df1e-45f5-a09f-d1b9c75c69da ro

Thanks, regards, Stefan