Re: Triple parity and beyond

[NeilBrown <neilb@suse.de>]

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On Fri, 22 Nov 2013 10:07:09 -0600 Stan Hoeppner <stan@hardwarefreak.com>
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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