Ext3 and RAID: silent data killers?

Posted Sep 1, 2009 7:46 UTC (Tue) by job (guest, #670)
In reply to: Ext3 and RAID: silent data killers? by k8to
Parent article: Ext3 and RAID: silent data killers?

I would expect the same problem to affect RAID10, as a double fault can kill them too if you're very unlucky.

Ext3 and RAID: silent data killers?

Posted Sep 1, 2009 8:05 UTC (Tue) by drag (guest, #31333) [Link] (1 responses)

Well as single fault can destroy any data if you want to look at it that way... But generally with one drive gone either Raid 6 or Raid 10 should still be adequate.

With RAID 5 the amount of time it takes to recover is so long nowadays that the chances of having a double fault is pretty good. It was one thing to have 20GB with 30MB/s performance, but it's quite another to have 1000GB with 50MB/s performance...

Ext3 and RAID: silent data killers?

Posted Sep 11, 2009 1:18 UTC (Fri) by Pc5Y9sbv (guest, #41328) [Link]

I agree you cannot blindly use RAID5 without considering the sizing, but what do you consider an acceptable recovery time?

My cheap MD RAID5 with three 500 GB SATA drives allows me to have 1TB and approximately 100 MB/s per drive throughput, which implies a full scan to re-add a replacement drive might take 2 hours or so (reading all 500 GB from 2 drives and writing 500 GB to the third at 75% of full speed). I have never been in a position where this I/O time was worrisome as far as a double fault hazard. Having a commodity box running degraded for several days until replacement parts are delivered is a more common consumer-level concern, which has not changed with drive sizes.

Ext3 and RAID: silent data killers?

Posted Sep 3, 2009 5:05 UTC (Thu) by k8to (guest, #15413) [Link] (1 responses)

Double fault can kill raid 10 also, but you're much less likely to have the fault propogate as discussed in the article, and the downtime for bringing in a standby is much smaller, so standby drives are more effective.

Meanwhile, you also get vastly better performance, and higher reliability of implementation.

It's really a no brainer unless you're poor.

Ext3 and RAID: silent data killers?

Posted Sep 3, 2009 5:26 UTC (Thu) by dlang (guest, #313) [Link]

actually, if you have a read-mostly workload raid 5/6 can end up being as fast as raid 10. I couldn't believe this myself when I first ran into it, but I have a large (multiple TB) database used for archiving log data and discovered that read/search performance was the same with raid 6 as with raid 10.

in digging further I discovered that they key to performance was to have enough queries in flight to keep all disk heads fully occupied (one outstanding query per drive spindle), and you can do this with both raid 6 and raid 10.