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

[Benjamin Marzinski]

On Fri, Apr 13, 2018 at 12:19:12AM +0200, Martin Wilck wrote:
> 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.

We can safely set SYSTEMD_READY=0 if we know that the device is now part
of an existing multipath device, but yeah.
 
> >  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. 

That's a loophole. And because I accidentally forgot to add it before

Reviewed-by: Benjamin Marzinski <bmarzins at redhat.com>

> 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)