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[cinder] lvm_mirrors option seems to be broken


Final suggestion: on my system, lvm seems to work best with the
"--type raid10" option.

Perhaps a cinder.conf option for lvm_type=raid10 would be useful?
raid1 too?  I am seeing this warning from lvcreate quite a lot, so
something needs to change fairly soon:

WARNING: Log type "mirrored" is DEPRECATED and will be removed in the
future. Use RAID1 LV or disk log instead.

Thanks,
Mark


On Tue, Jul 30, 2019 at 10:08 AM Mark Lehrer <lehrer at gmail.com> wrote:
>
> 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