Re: [linux-lvm] [PATCH 1/1] pvscan: wait for udevd

[David Teigland <teigland@redhat.com>]

On Thu, Feb 18, 2021 at 04:19:01PM +0100, Martin Wilck wrote:
> > Feb 10 17:24:26 archlinux lvm[643]:   pvscan[643] VG sys run
> > autoactivation.
> > Feb 10 17:24:26 archlinux lvm[643]:   /usr/bin/dmeventd: stat failed:
> > No such file or directory
> 
> What's going on here? pvscan trying to start dmeventd ? Why ? There's a
> dedicated service for starting dmeventd (lvm2-monitor.service). I can
> see that running dmeventd makes sense as you have thin pools, but I'm
> at a loss why it has to be started at that early stage during boot
> already.
> 
> This is a curious message, it looks as if pvscan was running from an
> environment (initramfs??) where dmeventd wasn't available. The message
> is repeated, and after that, pvscan appears to hang...

I've found that when pvscan activates a VG, there's a bit of code that
attempts to monitor any LVs that are already active in the VG.  Monitoring
means interacting with dmeventd.  I don't know why it's doing that, it
seems strange, but the logic around monitoring in lvm seems ad hoc and in
need of serious reworking.  In this case I'm guessing there's already an
LV active in "sys", perhaps from direct activation in initrd, and when
pvscan activates that VG it attempts to monitor the already active LV.

Another missing piece in lvm monitoring is that we don't have a way to
start lvm2-monitor/dmeventd at the right time (I'm not sure anyone even
knows when the right time is), so we get random behavior depending on if
it's running or not at a given point.  In this case, it looks like it
happens to not be running yet.  I sometimes suggest disabling lvm2-monitor
and starting it manually once the system is up, to avoid having it
interfere during startup.

> > Feb 10 17:24:26 archlinux lvm[643]:   /usr/bin/dmeventd: stat failed:
> > No such file or directory
> > Feb 10 17:24:26 archlinux lvm[643]:   WARNING: Failed to monitor
> > sys/pool.
> > Feb 10 17:24:26 archlinux systemd[1]: Stopping LVM event activation
> > on device 9:0...

The unwanted failed monitoring seems to have caused the pvscan command to
exit with an error, which then leads to further mess and confusion where
systemd then thinks it should stop or kill the pvscan service, whatever
that means.  pvscan should probably never exit with an error.

Dave

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