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

From: Peter Rajnoha <prajnoha@redhat.com>
To: Benjamin Marzinski <bmarzins@redhat.com>
Cc: "zkabelac@redhat.com" <zkabelac@redhat.com>,
	Heming Zhao <heming.zhao@suse.com>,
	"teigland@redhat.com" <teigland@redhat.com>,
	"linux-lvm@redhat.com" <linux-lvm@redhat.com>,
	Martin Wilck <martin.wilck@suse.com>
Subject: Re: [linux-lvm] Discussion: performance issue on event activation mode
Date: Thu, 30 Sep 2021 00:06:53 +0200	[thread overview]
Message-ID: <20210929220653.p45cvgzdhlbnp4gy@alatyr-rpi.brq.redhat.com> (raw)
In-Reply-To: <20210928174246.GF3087@octiron.msp.redhat.com>

On Tue 28 Sep 2021 12:42, Benjamin Marzinski wrote:
> On Tue, Sep 28, 2021 at 03:16:08PM +0000, Martin Wilck wrote:
> > I have pondered this quite a bit, but I can't say I have a concrete
> > plan.
> > 
> > To avoid depending on "udev settle", multipathd needs to partially
> > revert to udev-independent device detection. At least during initial
> > startup, we may encounter multipath maps with members that don't exist
> > in the udev db, and we need to deal with this situation gracefully. We
> > currently don't, and it's a tough problem to solve cleanly. Not relying
> > on udev opens up a Pandora's box wrt WWID determination, for example.
> > Any such change would without doubt carry a large risk of regressions
> > in some scenarios, which we wouldn't want to happen in our large
> > customer's data centers.
> 
> I'm not actually sure that it's as bad as all that. We just may need a
> way for multipathd to detect if the coldplug has happened.  I'm sure if
> we say we need it to remove the udev settle, we can get some method to
> check this. Perhaps there is one already, that I don't know about. If

The coldplug events are synthesized and as such, they all now contain
SYNTH_UUID=<UUID> key-value pair with kernel>=4.13:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/ABI/testing/sysfs-uevent

I've already tried to proposee a patch for systemd/udev that would mark
all uevents coming from the trigger (including the one used at boot for
coldplug) with an extra key-value pair that we could easily match in rules,
but that was not accepted. So right now, we could detect that
synthesized uevent happened, though we can't be sure it was the actual
udev trigger at boot. For that, we'd need the extra marks. I can give it
another try though, maybe if there are more people asking for this
functionality, we'll be at better position for this to be accepted.

-- 
Peter

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