dlm: overview

These are the distributed lock manager (dlm) patches against 2.6.12-rc4 that we'd like to see added to the kernel. We've made changes based on the suggestions that were sent, and are happy to get more, of course.

The original overview
http://marc.theaimsgroup.com/?l=linux-kernel&m=111444188703106&w=2

For those interested in performance, there is some information here
http://sources.redhat.com/cluster/dlm/

Dave

Squawk.

Not only do I not know whether this stuff should be merged: I don't even know how to find that out. Unless I'm prepared to become a full-on cluster/dlm person, which isn't looking likely.

The usual fallback is to identify all the stakeholders and get them to say "yes Andrew, this code is cool and we can use it", but I don't think the clustering teams have sufficent act-togetherness to be able to do that.

Am I correct in believing that this DLM is designed to be used by multiple clustering products? If so, which ones, and how far along are they?

Looking at Ted's latest Kernel Summit agenda I see

Clustering

We need to make progress on the kernel integration of things like message passing, membership, DLM etc.

We seem to have at least two comparable kernel-side offerings (OpenSSI and RHAT's), as well as a need to hash out how user-space plays into this.

(There is now a plan for a Clustering Summit prior to KS - need to validate if this will be useful, still)

So right now I'm inclined to duck any decision making and see what happens in July. Does that sounds sane? Is that Clustering Summit going to happen?

In the meanwhile I can pop this code into -mm so it gets a bit more compile testing, review and exposure, but that wouldn't signify anything further.

Correct. Red Hat has multiple clustering products that do and will use this. GFS and CLVM are two notable ones that do now. GFS kernel patches that use this will be sent in the near future; CLVM uses the dlm from user space.

Here are my impressions of where other clustering groups are at:

OpenSSI: this project is interested in integrating an existing dlm into their system. They don't have any effort under way to develop a dlm themselves. They are also interested in using gfs which is indirectly an interest in the dlm.

Linux-HA: seem to be in a similar situation as OpenSSI

Lustre: this project includes a locking system called "dlm". The api is similar, but the implementation is not distributed in the style of a traditional dlm; what they have is largely Lustre-specific [1].

OCFS2: this project includes a dlm intended for OCFS2, but there have been some steps to make it more generic. I believe their goal is still to develop their dlm primarily for ocfs2's needs, not to develop an entirely general purpose dlm such as this one. So again, what's there is limited and largely OCFS2-specific [1].

It would be very good to hear from any others who are interested in using a dlm, too.

[1] Application-specific lock managers such as Lustre's and OCFS2's can be good ideas, and I'm not criticising them. It allows you to make specializations and simplifications to suit your particular needs. So, while I'm sure it's possible for them to use this general-purpose dlm, some may still want to do their own specialized thing. We'll try to add features and options to the general-purpose dlm to meet specialized needs, but there's a limit to that.

To some extent I'm sure the different clustering meetings will happen, although I won't be at any of them. I'm forging ahead trying to work things out rather than waiting. (Frankly, I don't think waiting for a cluster summit will matter much.)

It's worth noting that most of the clustering discussions are now about user space stuff. Since the dlm is agnostic about what's in user space, the dlm discussion and other clustering topics are largely independent.

Sure, more feedback is what we're after.

Dave

My impression is that it is unlikely everybody will agree on a single cluster setup anyways, so it might be best to use a similar strategy as with file systems ("multiple implementations - standard API to the outside world")

This would mean the DLM could be merged if the other cluster people agree that the interface it presents to the outside world is good for them too. Is it? Perhaps these interfaces should be discussed first.

-Andi

It is highly unlikely that this will ever happen because of the different nature of these projects. That said getting a kernel system for dlm and message passing should be doable given the wide variety off uses they have.

Hi Andrew,

The public announcement, with venue and travel details will be tomorrow. This is a technical workshop that will take place on June 20 and 21st in Walldorf
Germany, near Heidelberg and south of Frankfurt. The goal is to either
validate, reject, modify, or combine the kernel interfaces that Red Hat has proposed, and that other groups may propose in the next few weeks. We will only be looking at code that actually exists.

For those who cannot physically attend, we will do our utmost to provide a two-way audio connection, hopefully without toll charges.

This Second Annual Cluster Summit (sorry if it sounds presumptuous) is sponsored by Red Hat, Hewlett-Packard, Fujitsu-Siemens, Dell, and Oracle. My apologies in advance for leaving only four weeks leadtime for the public announcement.

Regards,

Daniel