Re: btrfs und lvm-cache?

[Neuer User <auslands-kv@gmx.de>]

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.

> 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?

> - 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?

> - 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.
> 
Sounds complicated, but possible. Pity that LVM does not auto-remove the
cache if it is not there...

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