Re: [libvirt] RFC: Introduce a dlm-corosync for Lock manager plugin
On Wed, Dec 20, 2017 at 03:11:48PM +0800, Fu.Lin wrote:
Hi,
Currently the lock manager infrastructure has implementation of nop, sanlock
and lockd. The first do nothing, while sanlock and lockd all requires the
share
stroage, and lockd is not provided fence mechanism, sanlock would force to
restart OS when stopping daemon.
The first half of 2017, QEMU introduced the `share-rw` and `file.locking` to
handle a problem: https://bugzilla.redhat.com/show_bug.cgi?id=1080152 ,
someone
said:
There is also scope for creating new plugins for libvirt to use other
mechanism if someone comes up with other ideas.
So what about dlm-corosync?
DLM(distributed lock manage) is implemented by kernel, it provides some
userspace
API by "libdlm" to lock/unlock resource, using these API need a daemon named
dlm_controld. "Corosync" is cluster messager layer using TCP or UDP,
responsible
for members management in cluster. dlm_controld and corosync are formed a
kind of
lock which could be used in cluster. And there is another component named
"sbd"
providing fence mechanism, it's not necessary, `dlm_controld -f 0` would
disable it.
I don't have any direct experiance with DLM - just aware of it as a concept.
I think it can be considered in scope for a libvirt lock manager plugin if
you want to try writnig one.
As for implement, I think we could make use of virtlockd daemon, just
add
some RPC callback in micro-on.
How are locks acquired by libdlm scoped ? The reason we have virtlockd is
that the fcntl() locks need to be held by a running process, and we wanted
them to persist across libvirtd restarts. This required holding them in a
separate process.
Are libdlm locks automatically released when the process that acquired
them dies, or is a manual release action required ? If they always require
a manual release, then there would be no need to use virtlockd - the plugin
can just take care of acquire & release, and still cope with lbivirtd
restarts.
Regards,
Daniel
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