On 22/03/17 14:53, Gandalf Corvotempesta wrote: > 2017-03-21 17:49 GMT+01:00 Phil Turmel <philip@xxxxxxxxxx>: >> The correlation is effectively immaterial in a non-degraded raid5 and >> singly-degraded raid6 because recovery will succeed as long as any two >> errors are in different 4k block/sector locations. And for non-degraded >> raid6, all three UREs must occur in the same block/sector to lose >> data. Some participants in this discussion need to read the statistical >> description of this stuff here: >> >> http://marc.info/?l=linux-raid&m=139050322510249&w=2 >> >> As long as you are 'check' scrubbing every so often (I scrub weekly), >> the odds of catastrophe on raid6 are the odds of something *else* taking >> out the machine or controller, not the odds of simultaneous drive >> failures. > > This is true but disk failures happens much more than multiple UREs on > the same stripe. > I think that in a RAID6 is much easier to loose data due to multiple > disk failures. Certainly multiple disk failures is an easy way to loose data in /any/ storage system (or at least, loose data since the last backup). The issue here is whether it is more or less likely to be a problem in RAID6 than other raid arrangements. And the answer is that complete disk failures are not more likely during a RAID6 rebuild than during other raid rebuilds, and a RAID6 will tolerate more failures than RAID1 or RAID5. Of course, multiple disk failures /do/ occur. There can be a common cause of failure. I have had a few raid systems die completely over the years. The causes I can remember include: 1. The SAS controller card died - and I didn't have a replacement. The data on the disks is probably still fine. 2. The whole computer died in some unknown way. The data on the disks was fine - I put them in another cabinet and re-assembled the md array. 3. A hardware raid card died. The data may have been on the disks, but the hardware raid was in a proprietary format. 4. I knocked a disk cabinet off its shelf. This let to multiple simultaneous drive failures. Based on these, my policy is: 1. Stick to SATA drives that are easily available, easily replaced, and easily read from any system. 2. Avoid hardware raid - use md raid and/or btrfs raid. 3. Do a lot of backups - on independent systems, and with off-site copies. Raid does not prevent loss from fire or theft, or a UPS going bananas, or a user deleting the wrong file. 4. Mount your equipment securely, and turn round slowly! > > Last years i've lose a server due to 4 (of 6) disks failures in less > than an hours during a rebuild. > > The first failure was detected in the middle of the night. It was a > disconnection/reconnaction of a single disks. > The riconnection triggered a resync. During the resync another disk > failed. RAID6 recovered even from this double failure > but at about 60% of rebuild, the third disk failed bringing the whole raid down. > > I was waked up by our monitoring system and looking at the server, > there was also a fourth disk down :) > > 4 disks down in less than a hour. All disk was enterprise: SAS 15K, > not desktop drives. > -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
