Pavel Machek wrote:
Ok, can you help? Having a piece of MD documentation explaining the "powerfail nukes entire stripe" and how current filesystems do not deal with that would be nice, along with description when exactly that happens.
Except of course for the inconvenient detail that a power failure on a degraded RAID 5 array does *NOT* nuke the entire stripe. A 5-disk RAID 5 array will have 4 data blocks and 1 parity block in each stripe. A degraded array will have either 4 data blocks or 3 data blocks and 1 parity block in the stripe. If we are dealing with a parity-less stripe, we cannot lose any data due to RAID 5, because each of the 4 data blocks has a disk block available. We could still lose a data write due to a power failure, but this could also happen with the RAID 5 array still intact. If we are dealing with a 3-data, 1-parity stripe, then 3 of the 4 data blocks have an available disk block and will not be lost (if they make it to disk). The only block that maintains on all 3 data blocks and the parity block being correct is the block that does not currently have a disk to be written to. In short, if a stripe is not written completely on a degraded RAID 5 array, you can lose: 1) the blocks that were not written (duh) 2) the block that doesn't have a disk The first part of this loss is also true in a non-degraded RAID 5 array. The fact that the array is degraded really does not add much additional data loss here and you certainly will not lose the entire stripe like you suggest. -- All rights reversed. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
