On 21/03/17 14:24, Reindl Harald wrote: > > > Am 21.03.2017 um 14:13 schrieb David Brown: >> On 21/03/17 12:03, Reindl Harald wrote: >>> >>> Am 21.03.2017 um 11:54 schrieb Adam Goryachev: >> <snip> >>> >>>> In addition, you claim that a drive larger than 2TB is almost certainly >>>> going to suffer from a URE during recovery, yet this is exactly the >>>> situation you will be in when trying to recover a RAID10 with member >>>> devices 2TB or larger. A single URE on the surviving portion of the >>>> RAID1 will cause you to lose the entire RAID10 array. On the other >>>> hand, >>>> 3 URE's on the three remaining members of the RAID6 will not cause more >>>> than a hiccup (as long as no more than one URE on the same stripe, >>>> which >>>> I would argue is ... exceptionally unlikely). >>> >>> given that when your disks have the same age errors on another disk >>> become more likely when one failed and the heavy disk IO due recovery of >>> a RAID6 with takes *many hours* where you have heavy IO on *all disks* >>> compared with a way faster restore of RAID1/10 guess in which case a URE >>> is more likely >>> >>> additionally why should the whole array fail just because a single block >>> get lost? the is no parity which needs to be calculated, you just lost a >>> single block somewhere - RAID1/10 are way easier in their implementation >> >> If you have RAID1, and you have an URE, then the data can be recovered >> from the other have of that RAID1 pair. If you have had a disk failure >> (manual for replacement, or a real failure), and you get an URE on the >> other half of that pair, then you lose data. >> >> With RAID6, you need an additional failure (either another full disk >> failure or an URE in the /same/ stripe) to lose data. RAID6 has higher >> redundancy than two-way RAID1 - of this there is /no/ doubt > > yes, but with RAID5/RAID6 *all disks* are involved in the rebuild, with > a 10 disk RAID10 only one disk needs to be read and the data written to > the new one - all other disks are not involved in the resync at all True... > > for most arrays the disks have a similar age and usage pattern, so when > the first one fails it becomes likely that it don't take too long for > another one and so load and recovery time matters False. There is no reason to suspect that - certainly not to within the hours or day it takes to rebuild your array. Disk failure pattern shows a peak within the first month or so (failures due to manufacturing or handling), then a very low error rate for a few years, then a gradually increasing rate after that. There is not a very significant correlation between drive failures within the same system, nor is there a very significant correlation between usage and failures. It might seem reasonable to suspect that a drive is more likely to fail during a rebuild since the disk is being heavily used, but that does not appear to be the practice. You will /spot/ more errors at that point - simply because you don't see errors in parts of the disk that are not read - but the rebuilding does not cause them. And even if it /were/ true, then the key point is if there is an error that causes data loss. An error during reading for a RAID1 rebuild means lost data. An error during reading for a RAID6 rebuild means you have to read an extra sector from another disk and correct the mistake. -- 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
