On Fri, 2010-03-12 at 07:06 +0000, Jefferson Ogata wrote: > On 2010-03-12 04:26, Craig White wrote: > > On Fri, 2010-03-12 at 02:23 +0000, Jefferson Ogata wrote: > >> On 2010-03-11 22:23, Matthew Geier wrote: > >>> I've had a disk fail in such a way on a SCSI array that all disks on > >>> that SCSI bus became unavailable simultaneously. When half the disks > >>> dropped of the array at the same time, it gave up and corrupted the RAID > >>> 5 meta data so that even after removing the offending drive, the array > >>> didn't recover. > >> I also should point out (in case it isn't obvious), that that sort of > >> failure would take out the typical RAID 10 as well. > > ---- > > ignoring that a 2nd failed disk on RAID 5 is always fatal and only 50% > > fatal on RAID 10, I suppose that would be true. > > The poster wrote that all of the disks on a bus failed, not just a > second one. Depending on the RAID structure, this could take out a RAID > 10 100% of the time. ---- actually, this is what he wrote...
"When half the disks dropped of the array at the same time, it gave up and corrupted the RAID 5 meta data so that even after removing the offending drive, the array didn't recover." Half != all I had a 5 disk RAID 5 array fail the wrong disk and thus had 2 drives go offline and had a catastophic failure and thus had to re-install and recover from backup once (PERC 3/di & SCSI disks). Not something I wish to do again. ---- > In your "second disk" scenario, comparing RAID 5 with RAID 10 in terms > of failure likelihood isn't fair; you need to compare RAID 50 with RAID > 10. And the odd depend on the number of disks and the RAID structure. > > Suppose you have 12 disks arranged as a 6x2 RAID 10, and the same number > of disks as a 2x6 RAID 50. When the second disk fails the odds of loss are: > > - RAID 50: 5/11. > - RAID 10: 1/11. > > If instead we have the 12 disks as a 3x4 RAID 50, then the odds of loss > when the second disk fails are: > > - RAID 50: 3/11. > - RAID 10: 1/11. > > We can now tolerate a third disk failure with our RAID 50 with the odds > of loss: > > - RAID 50: 6/10. > - RAID 10: 2/10. > > How often does this happen? It hasn't happened to me, and it hasn't > happened to anyone I know. ---- I don't think I understand your 'odds' model. I interpret the first example as RAID 50 having 5 times more likelihood of loss than RAID 10 and I presume that isn't what you were after ---- > > In the alternative fair comparison, RAID 5 vs. RAID 1, the second > failure kills both RAIDs 100% of the time. ---- actually, I didn't raise the RAID 5 vs RAID 10 comparison, I only amplified with my experiences ---- > It's pretty clear you don't speak from any recent experience as far as > RAID 5 performance goes, and you yourself say as much when you say you > "had already forsaken RAID 5". Like Oracle, you're living in the past. > You should do some of your own benchmarks. ---- I'd agree with that assessment... I gave up on RAID 5 a few years ago. In addition, reading the previously linked article in enterprisestorage.com tells me that when I use SATA drives, I should avoid RAID 5... good enough for me. ---- > In any case, the argument in that article applies to RAID 10 as well; it > gives you better probabilities but eventually it will take too long to > rebuild mirrors and failure will be just as inevitable as with RAID 5. > Error rates will have to drop to prevent this, and no doubt they will, > sufficiently that the article's argument is moot. Eventually they will > drop to the point where we will be using RAID 0. > > > On top of that, > > it seems to me that RAID 10 smokes RAID 5 on every performance > > characteristic my clients are likely to use (and yes, that means > > databases). RAID 5 primarily satisfies the needs for maximum storage for > > the least amount of money and that was rarely what I need in a storage > > system for a server. > > For a lot of access patterns, RAID 5 yields much better write bandwidth > than RAID 10. I don't know why you think RAID 10 "smokes" RAID 5. You > should grab a PERC 6 and a couple of MD1000s and try some different > configurations. I don't think you'll see any smoke in the margins, even > over the oddly limited gamut of access patterns your clients use. ---- the last time I bought an MD-1000, Dell would only sell me the PERC-5e, I don't know why. I could see possibly using RAID 50 but RAID 5 is just not a path I want to venture any more. Craig -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. _______________________________________________ Linux-PowerEdge mailing list [email protected] https://lists.us.dell.com/mailman/listinfo/linux-poweredge Please read the FAQ at http://lists.us.dell.com/faq
