[linux-lvm] Discussion: performance issue on event activation mode

[Peter Rajnoha]

On Tue 08 Jun 2021 13:23, Martin Wilck wrote:
> On Di, 2021-06-08 at 14:29 +0200, Peter Rajnoha wrote:
> > On Mon 07 Jun 2021 16:48, David Teigland wrote:
> > > 
> > > If there are say 1000 PVs already present on the system, there
> > > could be
> > > real savings in having one lvm command process all 1000, and then
> > > switch
> > > over to processing uevents for any further devices afterward.  The
> > > switch
> > > over would be delicate because of the obvious races involved with
> > > new devs
> > > appearing, but probably feasible.
> > 
> > Maybe to avoid the race, we could possibly write the proposed
> > "/run/lvm2/boot-finished" right before we initiate scanning in
> > "vgchange
> > -aay" that is a part of the lvm2-activation-net.service (the last
> > service to do the direct activation).
> > 
> > A few event-based pvscans could fire during the window between
> > "scan initiated phase" in lvm2-activation-net.service's
> > "ExecStart=vgchange -aay..."
> > and the originally proposed "ExecStartPost=/bin/touch /run/lvm2/boot-
> > finished",
> > but I think still better than missing important uevents completely in
> > this window.
> 
> That sounds reasonable. I was thinking along similar lines. Note that
> in the case where we had problems lately, all actual activation (and
> slowness) happened in lvm2-activation-early.service.
> 

Yes, I think most of the activations are covered with the first service
where most of the devices are already present, then the rest is covered
by the other two services.

Anyway, I'd still like to know why exactly
obtain_device_list_from_udev=1 is so slow. The only thing that it does
is that it calls libudev's enumeration for "block" subsystem devs. We
don't even check if the device is intialized in udev in this case if I
remember correctly, so if there's any udev processing in parallel hapenning,
it shouldn't be slowing down. BUT we're waiting for udev records to
get initialized for filtering reasons, like mpath and MD component detection.
We should probably inspect this in detail and see where the time is really
taken underneath before we do any futher changes...

Peter