Re: "Enhanced" MD code avaible for review

From: Jeff Garzik
Date: Thu Mar 25 2004 - 13:44:15 EST

Kevin Corry wrote:
I'm guessing you're referring to EVMS in that comment, since we have done *part* of what you just described. EVMS has always had a plugin to recognize MD devices, and has been using the MD driver for quite some time (along with using Device-Mapper for non-MD stuff). However, as of our most recent release (earlier this month), we switched to using Device-Mapper for MD RAID-linear and RAID-0 devices. Device-Mapper has always had a "linear" and a "striped" module (both required to support LVM volumes), and it was a rather trivial exercise to switch to activating these RAID devices using DM instead of MD.


This decision was not based on any real dislike of the MD driver, but rather for the benefits that are gained by using Device-Mapper. In particular, Device-Mapper provides the ability to change out the device mapping on the fly, by temporarily suspending I/O, changing the table, and resuming the I/O I'm sure many of you know this already. But I'm not sure everyone fully understands how powerful a feature this is. For instance, it means EVMS can now expand RAID-linear devices online. While that particular example may not

Sounds interesting but is mainly an implementation detail for the purposes of this discussion...

Some of this emd may want to use, for example.

As for not posting this information on lkml and/or linux-raid, I do apologize if this is something you would like to have been informed of. Most of the recent mentions of EVMS on this list seem to fall on deaf ears, so I've taken that to mean the folks on the list aren't terribly interested in EVMS developments. And since EVMS is a completely user-space tool and this decision didn't affect any kernel components, I didn't think it was really relevent to mention here. We usually discuss such things on evms-devel@xxxxxxxxxxxx or dm-devel@xxxxxxxxxx, but I'll be happy to cross-post to lkml more often if it's something that might be pertinent.

Understandable... for the stuff that impacts MD some mention of the work, on occasion, to linux-raid and/or linux-kernel would be useful.

I'm mainly looking at it from a standpoint of making sure that all the various RAID efforts are not independent of each other.

We're obviously pretty keen on seeing MD and Device-Mapper "merge" at some point in the future, primarily for some of the reasons I mentioned above. Obviously linear.c and raid0.c don't really need to be ported. DM provides equivalent functionality, the discovery/activation can be driven from user-space, and no in-kernel status updating is necessary (unlike RAID-1 and -5). And we've talked for a long time about wanting to port RAID-1 and RAID-5 (and now RAID-6) to Device-Mapper targets, but we haven't started on any such work, or even had any significant discussions about *how* to do it. I can't

let's have that discussion :)

imagine we would try this without at least involving Neil and other folks from linux-raid, since it would be nice to actually reuse as much of the existing MD code as possible (especially for RAID-5 and -6). I have no desire to try to rewrite those from scratch.


Device-Mapper does currently contain a mirroring module (still just in Joe's -udm tree), which has primarily been used to provide online-move functionality in LVM2 and EVMS. They've recently added support for persistent logs, so it's possible for a mirror to survive a reboot. Of course, MD RAID-1 has some additional requirements for updating status in its superblock at runtime. I'd hope that in porting RAID-1 to DM, the core of the DM mirroring module could still be used, with the possibility of either adding MD-RAID-1-specific information to the persistent-log module, or simply as an additional log type.

WRT specific implementation, I would hope for the reverse -- that the existing, known, well-tested MD raid1 code would be used. But perhaps that's a naive impression... Folks with more knowledge of the implementation can make that call better than I.

I'd like to focus on the "additional requirements" you mention, as I think that is a key area for consideration.

There is a certain amount of metadata that -must- be updated at runtime, as you recognize. Over and above what MD already cares about, DDF and its cousins introduce more items along those lines: event logs, bad sector logs, controller-level metadata... these are some of the areas I think Justin/Scott are concerned about.

My take on things... the configuration of RAID arrays got a lot more complex with DDF and "host RAID" in general. Association of RAID arrays based on specific hardware controllers. Silently building RAID0+1 stacked arrays out of non-RAID block devices the kernel presents. Failing over when one of the drives the kernel presents does not respond.

All that just screams "do it in userland".

OTOH, once the devices are up and running, kernel needs update some of that configuration itself. Hot spare lists are an easy example, but any time the state of the overall RAID array changes, some host RAID formats, more closely tied to hardware than MD, may require configuration metadata changes when some hardware condition(s) change.

I respectfully disagree with the EMD folks that a userland approach is impossible, given all the failure scenarios. In a userland approach, there -will- be some duplicated metadata-management code between userland and the kernel. But for configuration _above_ the single-raid-array level, I think that's best left to userspace.

There will certainly be a bit of intra-raid-array management code in the kernel, including configuration updating. I agree to its necessity... but that doesn't mean that -all- configuration/autorun stuff needs to be in the kernel.


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