New RAID-6 snapshot

Well, I implemented read-modify-write for RAID-6, and at least on my 6-disk system it was a small but measurable performance *loss*. Therefore, I have cleaned up the patches slightly, but basically RAID-6 seems to work and it now needs people to test it out and let me know what bugs is there.

Additionally, the previously mentioned generic md bug relating to recovery onto multiple disks should be fixed in the main md code (it's also needed for RAID-1.)

The new snapshot is at:

    http://www.kernel.org/pub/linux/kernel/people/hpa/raid6-20031230.tar.gz

... although I reserve the right to upload newer snapshots on an ongoing basis :)

-hpa

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What was your previous patch doing instead?

The published snapshots (including the latest, since I backed out read-modify-write) all do read-reconstruct-write.

-hpa

Does that mean that RRW (as the name seems to imply) would need to read the entire stripe before it could write back the updated stripe compared to a partial read with RMW?

Sort of. If you have, say, seven drives total: D D D D D P Q, the access pattern look like:

D D D D D P Q
^
Write this drive

... then RCW needs to read:

D D - D D - -

... and RMW needs to read:

- - D - - P Q

Now, if this is a "small" write (smaller than a page) then the stripe being modified always needs to be written, which helps RMW. On the other hands, large writes really help RCW:

D D D D D P Q
^^^^^^^
Write these four drives

... then RCW only needs to read one drive, whereas RMW needs to read six.

Now, you'd never use RMW only; it's only used when it should be faster than RCW, which is why I'm a bit surprised about the performance degradation. With only six drives in my test system it's pretty clear that RMW should very rarely be invoked, however, the performance ought to have stayed the same rather than slip.

-hpa

You mentioned that none of your snapshots have RMW in them. Can you put up a patch that adds RMW to the snapshots? Maybe someone will notice something...

Did you get any data corruption with the RMW patch applied?

It would take considerable amounts of work to get it into releasable shape, because I'd have to add RMW functionality to all the implemented algorithms -- not just the one my machine uses. However, if someone is interested in vgrepping through the source I can post the part of the diff patch that applies to raid6main.c.

I doubt it's actually wrong, though. It probably just causes worse access patterns in my particular configuration.

No.

-hpa