It's always possible to rebuild into a consistent configuration by assigning a precedence order; for parity RAID, the data drives take precedence over parity drives, and for RAID-1 sets it assigns an arbitrary master.
You *should* never lose a whole stripe ... for example, RAID-5 updates do "read old data / parity, write new data, write new parity" ... there is no need to touch any other data disks, so they will be preserved through the rebuild. Similarly, if only one block is being updated there is no need to update the entire stripe.
David - what caused /dev/md to decide to take an array offline?
Cheers
Dave
On Tue, Feb 23, 2010 at 3:22 PM,
<david@lang.hm> wrote:
On Tue, 23 Feb 2010, Aidan Van Dyk wrote:
* david@lang.hm <david@lang.hm> [100223 15:05]:
However, one thing that you do not get protection against with software
raid is the potential for the writes to hit some drives but not others.
If this happens the software raid cannot know what the correct contents
of the raid stripe are, and so you could loose everything in that stripe
(including contents of other files that are not being modified that
happened to be in the wrong place on the array)
That's for stripe-based raid. Mirror sets like raid-1 should give you
either the old data, or the new data, both acceptable responses since
the fsync/barreir hasn't "completed".
Or have I missed another subtle interaction?