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Re: [Linux-cluster] Distributed RAID



Michael O'Sullivan wrote:

I have just read that GFS on mdadm does not work because mdadm is not cluster aware. I really hoped to build a n + 1 RAID of the disks I have presented to the RHCS nodes via iSCSI. I had a look at DDRAID which is old and looks like it only supports 3, 5 and 9 disks in the distributed RAID. I currently only have two (multipathed) devices, but I want them to be active-active. If I put them into a mirrored logical volume in CLVM will this do the trick? Or will I have to install DRDB? Is there any more up-to-date distributed RAID options available for when I want to make a 2 + 1, 3 +1, etc storage array? There are some posts that say this may be available in CLVM soon or that mdadm may be cluster aware soon. Any progress on either of these options?

You probably saw me asking these very same questions in the archives, without any response.

DDRAID is unmaintained, and IIRC the code was removed from the current development tree a while back. So don't count on it ever getting resurrected.

I rather doubt md will become cluster aware any time soon. CLVM doesn't yet support even more important features like snapshotting, so I wouldn't count on it supporting anything more advanced.

For straight mirroring (which is all you could sensibly do with 2 nodes anyway), I can highly recommend DRBD. It "just works" and works well. I have a number of 2-node clusters deployed with it with shared-root.

Of you really want to look into larger scale clustering with n+m redundancy, look into cleversafe.org. There's a thread I started on the forum there looking into exactly this sort of thing. I'll be testing it in the next month or so, when I get the hardware together, but it's looking plausible. It also provides proper n+m redundancy.

Another thing to note is that RAID5 is not really usable on today's big disks in arrays of more than 6. Think about the expected read failure rates on modern disks: 10^-14. That's about one uncorrectable error every 10TB. So if you have a 6x1TB disk array, and you lose a disk, you have to read 5TB of data to reconstruct onto a fresh disk. Since you get one uncorrectable error every 10TB, that means you have a 50/50 chance of another disk encountering an error and dropping out of the array, and losing all your data. These days higher RAID levels are really a necessity, not an optional extra, and at this rate, considering the read error rates have stayed constant while sizes have exploded, even RAID6 won't last long.

Gordan


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