[dm-devel] [PATCH v4 18/20] multipath -u: quick check if path is multipathed

[Martin Wilck]

On Thu, 2018-04-12 at 13:46 -0500, Benjamin Marzinski wrote:
> On Wed, Apr 04, 2018 at 06:16:25PM +0200, Martin Wilck wrote:
> > With "find_multipaths smart", we accept paths as valid if they are
> > already part of a multipath map. This patch avoids doing a full
> > path
> > and device-mapper map scan for this case, speeding up "multipath
> > -u"
> > considerably.
> 
> I feel like this supports my idea from 17/20. I really think that in
> smart mode, the only time we should be claiming a device as multipath
> is
> on an add event, if it has always been claimed (or pretend claimed)
> as a
> multipath path, or if it is currently a multipath path. Once we have
> let
> a uevent go by for a path without setting SYSTEMD_READY=0, anything
> else
> is free to use it, and we simply can't safely set SYSTEMD_READY=0,
> unless we know that multipathd has already grabbed the device.

We have to remember that in the udev db then. That doesn't work for
coldplug aside, but we agree that's not an issue. By checking a
suitable udev variable, we can make sure that we never set
SYSTEMD_READY=0 after having released the device to the system.

>  I feel
> like checking the wwids file should give you confidence that a device
> either currently is a multipath path, or has always been claimed as
> one.
> However, this patch can fix any loop holes where a device isn't in
> the
> wwids file, but it multipathed (I don't know of any offhand).

multipath -u can't assume that multipathd is running. The map may have
been set up in the initrd, and thus would not necessarily be in the
WWIDs file in the root FS. 

Currently multipath -u scans all paths and maps in this case, just to
figure out if the given path is maybe already part of a map. My patch
was just meant as a shortcut for this case. If we could rely on the
WWIDs file that would be even easier, but I don't think we can.

Martin

-- 
Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)