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

From: Zdenek Kabelac <zkabelac@redhat.com>
To: Martin Wilck <mwilck@suse.com>,
	LVM general discussion and development <linux-lvm@redhat.com>,
	David Teigland <teigland@redhat.com>
Cc: Oleksandr Natalenko <oleksandr@redhat.com>,
	linux-lvm@e1890.dsca.akamaiedge.net,
	Christian Hesse <list@eworm.de>,
	Heming Zhao <heming.zhao@suse.com>
Subject: Re: [linux-lvm] [PATCH 1/1] pvscan: wait for udevd
Date: Mon, 22 Feb 2021 10:57:27 +0100	[thread overview]
Message-ID: <bcb2eb82-46b3-8e6b-c9b6-56442b4f63a3@redhat.com> (raw)
In-Reply-To: <f627b3e7a0d16f31ec68cb07b3454e977e8fe1d0.camel@suse.com>

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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