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.
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.
- 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.
- if your SSD dies, you may have to use vgcfgbackup, manual config edit,
then vgcfgrestore to remove the cache, because last time I checked, LVM
tools still were handling writethrough cache the same as writeback,
disallowing volume activation without the cache and removal of missing
cache.
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
