[linux-lvm] [PATCH 1/1] pvscan: wait for udevd
Zdenek Kabelac
zkabelac at redhat.com
Mon Feb 22 09:57:27 UTC 2021
Dne 21. 02. 21 v 21:23 Martin Wilck napsal(a):
> On Fri, 2021-02-19 at 23:47 +0100, Zdenek Kabelac wrote:
>>
>> Right time is when switch is finished and we have rootfs with /usr
>> available - should be ensured by lvm2-monitor.service and it
>> dependencies.
>
> While we're at it - I'm wondering why dmeventd is started so early. dm-
> event.service on recent installments has only "Requires=dm-
> event.socket", so it'll be started almost immediately after switching
> root. In particular, it doesn't wait for any sort of device
> initialization or udev initialization.
Hi
Dmeventd alone does not depend on lvm2 in any way - it's the lvm2 plugin which
then does all the 'scanning' for VGs/LVs and gets loaded when lvm2 connects to
monitoring socket. That's also why dmeventd belongs to dm subsystem.
Dmeventd is nothing else then a process to check DM devices periodically - and
can be used by i.e. dmraid or others...
So as such it doesn't need any devices - but it needs to be initialized early
so it can accept connections from tools like lvm2 and starts to monitor a
device without delaying command (as lvm2 wait for confirmation device is
monitored).
> I've gone through the various tasks that dmeventd is responsible for,
> and I couldn't see anything that'd be strictly necessary during early
> boot. I may be overlooking something of course. Couldn't the monitoring
As said - during ramdisk boot - monitor shall not be used (AFAIK - dracut is
supposed to use disabled monitoring in it's modified copy of lvm.conf within
ramdisk)
But we want to switch to monitoring ASAP when we switch to rootfs - so the
'unmonitored' window is as small as possible - there are still same 'grey'
areas in the correct logic thought...
Zdenek
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