Hi Daniel, Daniel Stodden wrote:
Hi. Can someone spare a little time to help me understand the multipath utility and daemon a little better? I read myself through the available documentation. The refbook says that it's multipath who's creating maps, and multipathd responsible for path checking.
Well, yes, in principle.
Running multipath (at least the version 0.4.7 I've got here), this actually doesn't seem to be the case. Unless I'm totally mistaken, multipathd actually does create DM maps as paths are discovered. And not just fork/exec the utility as many sources (most notably the man page) state.
Indeed, that has changed. The multipath binary is basically obsolete now, as the daemon is capable of discovering new paths itself.
No, I'm not asking for general advice on how old documentation relates to new code. :) My question here would be rather: Everywhere I've been looking at until now there's seems to be a multipath %M:%m being run from udev. In addition to the running daemon.
Not everywhere :-) What should be done is to have multipathd read uevent from a socket provided by udev. Then multipathd won't race with udev and will handle device events directly. The multipath binary itself is then just used for maintenance and manual configuration changes. Look on OpenSUSE 11.1 / SLES11 on how it's done.
Is this redundant or does the run from udevd still add some major value?
Definitely. multipathd requires a device node for path checkers etc, so it has to wait until udev has created one. Plus udev runs separate programs (vol_id etc) which will add additional information to the uevent; some of which can be used by multipathd. The end goal here is to remove the scsi_id callout from multipathd and use the udev provided information. And, indeed, have all information provided by udev so that multipathd doesn't have to look for it itself; most of which is already present, so it's only a slight change to udev to add the remaining information. Be advised that there are several bugs in the upstream code, so trying this on a recent kernel with give you an 'interesting' user experience. I have some (well, for rather large numbers of 'some') patches on my git tree at git://git.kernel.org/pub/scm/linux/kernel/git/hare/multipath-tools.git branch sles11 so you might have a look there. And yes, I'll be posting them here in the near future. I don't _want_ to keep them private, but somehow customers insist on sending me new bug descriptions thus preventing me from cleaning them up and posting them. Do feel free to comment on them. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: Markus Rex, HRB 16746 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
