On Wed, Jun 29, 2011 at 3:47 AM, A. James Lewis <ja...@fsck.co.uk> wrote: > Is there a possibility that one could have a 3 disk RAID5 array, and > then add a 4th disk and then do a balance, growing the RAID5 onto 4 > disks and gaining the space still with RAID5? It seems that to be > consistent, BTRFS would have to do this. > > If this is the case, then I think that the BTRFS implementation of RAID5 > would have to be quite different to the MD implementation. > > James.
My understanding, gleaned from IRC several months ago, is that Btrfs would use the new drive, but not change the stripe size. Each allocation would then be written across some selection of three of the four drives. In other words, if you started with four stripes across three drives: AAA BBB CCC DDD and then added a drive and balanced, you might get something like: AAAB BBCC CDDD which would give you more space, but still use ⅓ of the space for parity. Trying to remove a drive from the original three-drive configuration would be an error, similarly to trying to remove the second to last drive in RAID 1, currently. Actually changing the stripe size would be done using the same mechanism as changing RAID levels. Again, this is just an interested but uninvolved person's understanding based on an IRC conversation, so please salt to taste. -- Erik -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
