[dm-devel] [PATCH 14/35] multipathd: add "force_reconfigure" option

[Martin Wilck]

On Wed, 2021-09-15 at 19:13 -0500, Benjamin Marzinski wrote:
> On Fri, Sep 10, 2021 at 01:40:59PM +0200, mwilck at suse.com wrote:
> > From: Martin Wilck <mwilck at suse.com>
> > 
> > Since e3270f7 ("multipathd: use weaker "force_reload" at startup"),
> > (multipath-tools 0.7.0), we only reload those maps that must be
> > reloaded during startup. "multipath reconfigure", OTOH, reloads
> > every map, which may take a long time on systems with lots of
> > storage devices, as every DM_DEVICE_RELOAD ioctl involves a
> > suspend/resume cycle.
> > 
> > The logic we use during startup is actually very robust and catches
> > all cases in which a reload is necessary. "reconfigure" operations
> > are often done because of configuration changes, and usually don't
> > require a full reload of every map.
> > 
> > This patch changes the default behavior of "multipath reconfigure"
> > to "weak" reload, like we do on startup since e3270f7. The behavior
> > can be changed by setting the configuration option
> > "force_reconfigure yes" before starting the reconfigure operation.
> > "multipath -r" is also affected, but "multipath -D -r" is not.
> > 
> > It would have been nice to have introduced a new cli command
> > "reconfigure force" instead, but the way "reconfigure" is
> > implemented, that would have required a major rewrite of the code.
> 
> This looks o.k. But I don't think it would be that hard to add a new
> multipathd command to reconfigure all the devices.  My personal
> preference would be to leave force_reconfigure off by default, so that
> we keep the same behavior, and add a command to force a full reconfig.
> I'll try to work up a patch with my idea that can apply on top of this.

The problem I see is with the "delayed reconfigure" approach. We
notify child() about a pending reconfiguration by setting
DAEMON_RECONFIGURE. If clients can request different "types" of
reconfiguration, we would either need an additional flag, or a new
state (e.g. DAEMON_RECONFIGURE_FORCE) to indicate which type of
reconfiguration was requested. And then what to we do if while one
reconfigure is running, the admin runs both "reconfigure force" and
"reconfigure"? We can only schedule only one reconfigure operation. We
should probably assume "force" in that rather unlikely case.

IMO we should not implement _both_ a configuration option
"force_reconfigure" _and_ an additional command "reconfigure force". We
should decide which one we want. If you can come up with a reasonable
logic for the latter, I agree it's better.

Regards,
Martin