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RAID and unrecoverable read errors

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October 7, 2010 3:39:25 AM

I was all ready to make a nice 6 TB RAID 5 array (5 x 1.5 TB HDs) when i started reading about the dangers of RAID 5 due to unrecoverable read errors. It makes sense that if you have an unrecoverable error on a parody portion of the data, it can't rebuild a file w/o the parody data. But my question is as follows:

1) Will 1 unrecoverable read error mean your entire array is doomed and unrecoverable, or will just a portion of the data, say whatever few files the unreadable parody data was for, be unrecoverable and if those files just happen to be critical files you are SOL?

2) Is RAID 10 affected by this same problem? I realize there is no parody data for RAID 10, but if when rebuilding a RAID 10 array there is an unrecoverable read error, will you loose the entire array (like with RAID 5) or just the file associated with the unrecoverable data (which is what I would guess would happen)?

Although I'm not personally concerned with if this would happen for RAID 0 or RAID 1, if you happen to know, please include that in your reply.

Thanks.
a c 126 G Storage
October 7, 2010 11:07:07 PM

Applies to any RAID really. If you lose redundancy, and want to rebuild with a new disks, then if one of the remaining disks encounter BER; then the rebuild process will stop and the array will fail and the data inaccessible, at least with most controllers/engines.

The data is not doomed; only 512-bytes or possibly more (in case of parity RAID). But it requires specific user interaction and poor interaction can permanently destroy data. So at the very least you could say your data is in jeopardy.

If you don't have a disk failure but only BER, then the problem is less severe. But without TLER disks you will have array splits/dropouts.

If you want a good protection against BER, then going to something like ZFS is one good option. It doesn't run on Windows and not even on Linux (not well at least) but it does run on OpenSolaris and FreeBSD. Many derivatives have been developed to suit as a NAS OS. I'm currently in the process of bulding a ZFS web-interface on FreeBSD. Check the link in my sig if you're interested.

Regardless, don't think RAID5 will protect your data. Backups protect your data! Think about having two boxes like a clone, where one act as backups. Or external HDDs which act as backup or off-site backups; etc. I just have two ZFS boxes where one is nightly backup with snapshots. Formidable protection against data-loss!

Another advantage is that you don't need (nor should use) expensive RAID cards or TLER-capable disks. You can use normal SATA controllers HBA, including your onboard ports, and normal consumer-class HDDs.

If you want to stay on the Windows route then at least make sure you have a backup since RAID on Windows in my opinion is not reliable enough to protect your data. The simplistic NTFS filesystem is not robust enough to protect your data to such a point where you don't need a backup. Not even ZFS is i think, though it sure adds several layers of protection to your data. The biggest of which are checksums; essential to data security. Essentially, ZFS is your best choice to guard against data corruption.

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