[cinder] lvm_mirrors option seems to be broken

Mark Lehrer lehrer at gmail.com
Tue Jul 30 15:01:20 UTC 2019

I have 4 fast nvme targets that I want to combine using RAID 10 and
use with cinder-lvm.

Unfortunately, lvm+mdadm is breaking all of my writes down to 4K --
without even increasing the queue depth to match.  SSDs hate this

I tried bypassing mdadm by seting up LVM RAID10 by hand and got
roughly a 10x increase in performance, so I definitely want to use
this if possible.

However, it looks like maybe the lvm_mirrors code hasn't been kept up
to date.  I ran into two main problems trying to use it:

1) The algorithm to calculate free space is broken and always returns
0.  This means that the scheduler will never even try to use this pool

2) Thin provisioning is getting in the way... with lvm_mirrors enabled
the code is apparently trying to create a new LV on top of the thin
provisioned LV... and it is using an lvcreate option that appears to
be deprecated (--mirrorlog mirrored)

I expect I can fix #1, but what is the best way to handle #2?  I was
thinking that the easiest fix would be to use the mirror options when
the thin LV is created, though I haven't tried it by hand yet to see
if it works.  The advantage here is that this is a one-time setting
and the code path to create individual volumes wouldn't need to check
lvm_mirrors at all.


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