On Fri, 22 Nov 2013 10:07:09 -0600 Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> wrote: > On 11/22/2013 2:13 AM, Stan Hoeppner wrote: > > Hi David, > > > > On 11/21/2013 3:07 AM, David Brown wrote: > ... > >> I don't see that there needs to be any changes to the existing md code > >> to make raid15 work - it is merely a raid 5 made from a set of raid1 > >> pairs. > > > > The sole purpose of the parity layer of the proposed RAID 15 is to > > replace sectors lost due to UREs during rebuild. AFAIK the current RAID > > 5 and RAID 1 drivers have no code to support each other in this manner. > > Minor self correction here-- obviously this isn't the 'sole' purpose of > the parity layer. It also allows us to recover from losing an entire > mirror, which is a big upshot of the proposed RAID 15. Thinking this > through a little further, more code modification would be needed for > this scenario. > > In the event of a double drive failure in one mirror, the RAID 1 code > will need to be modified in such a way as to allow the RAID 5 code to > rebuild the first replacement disk, because the RAID 1 device is still > in a failed state. Once this rebuild is complete, the RAID 1 code will > need to switch the state to degraded, and then do its standard rebuild > routine for the 2nd replacement drive. > > Or, with some (likely major) hacking it should be possible to rebuild > both drives simultaneously for no loss of throughput or additional > elapsed time on the RAID 5 rebuild. Nah, that would be minor hacking. Just recreate the RAID1 in a state that is not-insync, but with automatic-resync disabled. Then as continuous writes arrive, move the "recovery_cp" variable forward towards the end of the array. When it reaches the end we can safely mark the whole array as 'in-sync' and forget about diabling auto-resync. NeilBrown In the 20TB drive case, this would > shave 18 hours off the total rebuild operation elapsed time. With > current 4TB drives it would still save 6.5 hours. Losing both drives in > one mirror set of a striped array is rare, but given the rebuild time > saved it may be worth investigating during any development of this RAID > 15 idea. >
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