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Date: Wed, 26 Aug 2009 00:08:09 -0400 From: Rik van Riel <riel@...hat.com> To: Pavel Machek <pavel@....cz> CC: Neil Brown <neilb@...e.de>, Ric Wheeler <rwheeler@...hat.com>, Theodore Tso <tytso@....edu>, Florian Weimer <fweimer@....de>, Goswin von Brederlow <goswin-v-b@....de>, Rob Landley <rob@...dley.net>, kernel list <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com, rdunlap@...otime.net, linux-doc@...r.kernel.org, linux-ext4@...r.kernel.org, corbet@....net Subject: Re: [patch] ext2/3: document conditions when reliable operation is possible 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@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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