[dm-devel] Improve processing efficiency for addition and deletion of multipath devices

[Benjamin Marzinski]

On Mon, Nov 28, 2016 at 11:28:56AM +0100, Martin Wilck wrote:
> On Mon, 2016-11-28 at 10:19 +0800, tang.junhui at zte.com.cn wrote:
> > 
> > 4.        Proposal 
> > Other than processing uevents one by one, uevents which coming from
> > the 
> > same LUN devices can be mergered to one, and then uevent processing 
> > thread only needs to process it once, and it only produces one DM
> > addition 
> > uevent which could reduce system resource consumption. 
> > 
> 
> Here comes an idea how to achieve this without a lot of additional
> code:
> 
> libmultipath already has code to check whether any maps need to be
> updated (in coalesce_paths()). Instead of recording uevents, merging
> them, and calling ev_add_path() for every affected WWID, it might be
> sufficient to set daemon state to DAEMON_CONFIGURE and wake up the main
> multipathd thread to call reconfigure(). Then we only need to make sure
> that coalesce_paths() really reloads or creates maps only when
> necessary. I have some patches here that I made for that purpose, for a
> different scenario (multipathd to avoid RELOAD ioctls when it's started
> in a scenario where most paths are already set up by udev).

There is a long standing multipath issue that this will likely make a
lot worse.  Currently, multipathd can drop paths that it doesn't have
access to on reconfigure. This is wrong. When the path comes back, it
won't issue another add uevent. This means that multipathd won't be able
to re-enable it (since it stopped monitoring it during the reconfigure).
In fact the only way to get the path back is to either manually
intervene or to have the path actually get removed from the system and
come back.

The correct thing for multipath to do is to only remove paths when it is
manually instructed to, when a reconfigure blacklists the path, or when
it gets a remove uevent.  Now currently this isn't a huge issue since
people don't commonly reconfigure their multipath device, but it we
start running reconfigure all the time, it certainly will become one.

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