On Sat, 07 Aug 2010 14:28:55 +0200 Stefan /*St0fF*/ Hübner <stefan.huebner@xxxxxxxxxxxxxxxxxx> wrote: > Hi Brian, > > --assume-clean skips over the initial resync. Which - if you will > create a filesystem after creating the array - is a time-saving idea. > But keep in mind: even if the disks are brand new and contain only > zeros, the parity would probably look not all zeros. So reading from > such an array would be a bad idea. > But if the next thing you do is create LVM/filesystem etc., then all bit > read from the array will have been written to before (and by that are in > sync). There is an important point that this misses. When md updates a block on a RAID5 it will sometimes use a read-modify-write cycle which reads the old block and old parity, subtracts the old block from the parity block and then added the new block to the parity block. Then it writes the new data block and the new parity block. If the old parity was correct for the old stripe, then the new parity will be correct for the new stripe. But if the old was wrong then the new will be wrong. So if you use assume-clean then the parity may well be wrong and could remain wrong even when you write new data. If you then lose a device, the data for that device will be computed using wrong parity and you will get wrong data - hence data corruption. So you should only use --assume-clean if you know the array really is 'clean'. RAID1/RAID10 cannot suffer from this so --assume-clean is quite safe with those array types. The current implementation of RAID6 never does read-modify-write so --assume-clean is currently safe with RAID6 too. However I do not promise that RAID6 might not change to use read-modify-write cycles in some future implementation. So I would not recommend using --assume-clean on RAID6 just to avoid the resync cost. NeilBrown > > Stefan > > Am 06.08.2010 03:19, schrieb brian.foster@xxxxxxx: > > Hi all, > > > > I've read in the list archives that use of --assume-clean on raid5 > > (raid6?) is not safe assuming the member drives are not sync, but it's > > not clear to me as to why. I can see the content of an written raid5 > > array change if I fail a drive out of the array (created w/ > > --assume-clean), but data that I write prior to failing a drive remains > > intact. Perhaps I'm missing something. Could somebody elaborate on the > > danger/risk of using --assume-clean? Thanks in advance. > > > > Brian > > -- > > 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 > > -- > 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 -- 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
