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[cinder] lvm_mirrors option seems to be broken


OK things aren't quite as broken as I thought at first.

It looks like lvm_type needs to be set to default (instead of thin),
otherwise the lvm_mirrors option causes havoc.  Maybe a warning in the
logs about incompatible options would save the next person some time.

Thanks!
Mark


On Tue, Jul 30, 2019 at 9:01 AM Mark Lehrer <lehrer at gmail.com> wrote:
>
> 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
> workload.
>
> 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.
>
> Thanks,
> Mark