Tuomas, It doesn't quite work that way. IN RAID5 implementations the parity is distributed across disks in a pattern such as this (3 disk case shown): Disk 1 Disk 2 Disk 3 ------ ------ ------ Data Data Parity Data Parity Data Parity Data Data If you lose a disk (or start in degraded mode) you will get the following pattern (disk 3 shown missing): Disk 1 Disk 2 ------ ------ Data Data Data Parity Parity Data If data needs to be read that was on disk 3, then the data and parity from disk 1 and 2 are read and the data is reconstructed from that information. A similar strategy is used for writing. As a result there is a performance hit for reading/writing data that resides on disk 3. Note that the loss of 1 more disk renders the RAID 5 set inoperable, as it will no longer be possible to reconstruct data that was on either of the missing disks. Most raid controllers will give up and assume that you rather treat this as a dead device than take into account the possibility that you might want some of the data from the remaining disks. Thus the real risk of running in degraded mode is that all data can be lost. It is not possible to flip a bit to change a set of disks from RAID 1 to RAID 5, as the physical layout is different. Cheers, Frank -----Original Message----- From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid-owner@xxxxxxxxxxxxxxx] On Behalf Of Tuomas Leikola Sent: Wednesday, April 26, 2006 2:46 PM To: Jon Lewis Cc: John Rowe; linux-raid@xxxxxxxxxxxxxxx Subject: Re: Two-disk RAID5? > No. When one of the 2 drives in your RAID5 dies, and all you have for > some blocks is parity info, how will the missing data be reconstructed? > > You could [I suspect] create a 2 disk RAID5 in degraded mode (3rd member > missing), but it'll obviously lack redundancy until you add a 3rd disk, > which won't add anything to your RAID5 storage capacity. IMO if you have a 2-disk raid5, the parity for each block is the same as the data. There is performance drop as I suspect md isn't smart enough to read data from both disks, but that's all. When one disk fails, the (lone) parity block is quite enough to reconstruct. With XOR parity, you can always assume any amount of additional disks full of zero, it doesn't really change the algorithm. (maybe mdadm could/can change a raid-1 into raid5 by just changing the superblocks, for the purpose of expanding into more disks..) - tuomas - 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
