W dniu 24.12.2015 o 16:29, Neuer User pisze:
Am 24.12.2015 um 15:56 schrieb Piotr Pawłow:
Hello,
- both hdd and ssd in one LVM VG
- one LV on each hdd, containing a btrfs filesystem
- both btrfs LV configured as RAID1
- the single SDD used as a LVM cache device for both HDD LVs to speed up
random access, where possible
I have a setup like this for my /home. It works but it's a crappy solution.
Indeed? Exactly like this? Great to hear. But sad to hear it is not a
good solution.
Exactly. Single SSD caching 2 LVs used for btrfs RAID1. Don't get me
wrong, it's still a lot better than without caching, but not optimal. An
optimal solution would have to be integrated with the FS like in ZFS.
The effective capacity for caching is halved, and it takes twice as much
time to fully cache your working set, because you get a cache miss at
least once for each mirror.
There are also some gotchas:
- you should use "device=" mount options, or else there is a danger of
btrfs mounting origin devices and even mixing cached and origin in one
FS. I completely broke my FS before realizing what's going on.
Hmm, strange. I thought btrfs should not even know of the lvmcache. It
would just try to mount the HDD LVs and the caching is done
automatically via lvmcache?
Unfortunately, at least on my system, there are device files for origin LVs:
# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log
Cpy%Sync Convert
home1 pp Cwi-aoC--- 1,42t [cache0] [home1_corig] 100,00
11,02 0,00
home2 pp Cwi-aoC--- 1,42t [cache1] [home2_corig] 100,00
11,02 0,00
[...]
# ls -1 /dev/mapper/
[...]
pp-home1
pp-home1_corig
pp-home2
pp-home2_corig
[...]
... which btrfs would detect, pick up at random and assemble into the
RAID set. I had to do this in fstab to force only specified devices:
UUID=[...] /home btrfs
noatime,autodefrag,subvol=@home,device=/dev/mapper/pp-home1,device=/dev/mapper/pp-home2
- you should use writethrough mode if you only have one SSD. There was a
bug in LVM where it wouldn't save the caching mode and revert to
writeback after restart, so make sure you use the latest version of LVM
tools.
Do you know, which version is good?
I know it was buggy in Ubuntu Vivid (version 2.02.111 I think), and in
Wily it's OK (curently 2.02.122).
Looking at the changelog, it may have been fixed in 2.02.112 by commit
9d57aa9a0fe00322cb188ad1f3103d57392546e7:
"cache-pool: Fix specification of cachemode when converting to cache-pool
Failure to copy the 'feature_flags' lvconvert_param to the matching
lv_segment field meant that when a user specified the cachemode argument,
the request was not honored."
Of cource I may be wrong, I haven't bisected it.
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