Re: [PATCH 0/7] dlm: overview

From: David Teigland
Date: Tue Apr 26 2005 - 00:38:15 EST

On Mon, Apr 25, 2005 at 01:39:52PM -0700, Wim Coekaerts wrote:
> > This is a distributed lock manager (dlm) that we'd like to see added to
> > the kernel. The dlm programming api is very similar to that found on
> > other operating systems, but this is modeled most closely after that in
> > VMS.
> do you have any performance data at all on this ? I like to see a dlm
> but I like to see something that will also perform well.

No. What kind of performance measurements do you have in mind? Most dlm
lock requests involve sending a message to a remote machine and waiting
for a reply. I expect this network round-trip is the bulk of the time for
a request, which is why I'm a bit confused by your question.

Now, sometimes there are two remote messages (when a resource directory
lookup is needed). You can eliminate that by not using a resource
directory, which will soon be a configurable option.

> My main concern is that I have not seen anything relying on this code do
> "reasonably well". eg can you show gfs numbers w/ number of nodes and
> scalability ?

I'd suggest that if some cluster application is using the dlm and has poor
performance or scalability, the reason and solution lies mostly in the
app, not in the dlm. That's assuming we're not doing anything blatantly
dumb in the dlm, butI think you may be placing too much emphasis on the
role of the dlm here.

> I think it's time we submit ocfs2 w/ it's cluster stack so that folks
> can compare (including actual data/numbers), we have been waiting to
> stabilize everything but I guess there is this preemptive strike going
> on so we might just as well. at least we have had hch and folks comment,
> before sending to submit code.

Strike? Preemption? That sounds frightfully conspiratorial and
contentious; who have you been talking to? It's obvious to me that ocfs2
and gfs each have their own happy niche; they're hardly equivalent (more
so considering all the flavors of local file systems.) This is surely a
case of "different", not "conflict"!

> Andrew - we will submit ocfs2 so you can have a look, compare and move
> on. we will work with any stack that eventuslly gets accepted, just want
> to see the choice out there and an educated decision.
> hopefully tomorrow, including data comparing single node and multinode
> performance.

I'd really like to see ocfs succeed, but good heavens, why do we need to
study an entire cluster fs when looking at a dlm!? A cluster fs may use a
dlm, but a dlm is surely a stand-alone entity with _many_ applications
beyond a cluster fs (which is frankly a rather obscure app.)

We've made great effort to make the dlm broadly useful beyond the realm of
gfs or cluster file systems. In the long run I expect other cluster apps
will out-use the dlm by far.


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