Re: Triple parity and beyond

[David Brown <david.brown@hesbynett.no>]

On 24/11/13 22:13, Stan Hoeppner wrote:
> On 11/23/2013 11:14 PM, John Williams wrote:
>> On Sat, Nov 23, 2013 at 8:03 PM, Stan Hoeppner <stan@hardwarefreak.com> wrote:
>>
>>> Parity array rebuilds are read-modify-write operations.  The main
>>> difference from normal operation RMWs is that the write is always to the
>>> same disk.  As long as the stripe reads and chunk reconstruction outrun
>>> the write throughput then the rebuild speed should be as fast as a
>>> mirror rebuild.  But this doesn't appear to be what people are
>>> experiencing.  Parity rebuilds would seem to take much longer.
>>
>> "This" doesn't appear to be what SOME people, who have reported
>> issues, are experiencing. Their issues must be examined on a case by
>> case basis.
> 
> Given what you state below this may very well be the case.
> 
>> But I, and a number of other people I have talked to or corresponded
>> with, have had mdadm RAID 5 or RAID 6 rebuilds of one drive run at
>> approximately the optimal sequential write speed of the replacement
>> drive. It is not unusual on a reasonably configured system.
> 
> I freely admit I may have drawn an incorrect conclusion about md parity
> rebuild performance based on incomplete data.  I simply don't recall
> anyone stating here in ~3 years that their parity rebuilds were speedy,
> but quite the opposite.  I guess it's possible that each one of those
> cases was due to another factor, such as user load, slow CPU, bus
> bottleneck, wonky disk firmware, backplane issues, etc.
> 

Maybe this is just reporting bias - people are quick to post about
problems such as slow rebuilds, but very seldom send a message saying
everything worked perfectly!

There /are/ reasons why parity raid rebuilds are going to be slower than
mirror rebuilds - delays on one disk reading is one issue, and I expect
that simultaneous use of the array for normal work will have more impact
on parity raid rebuild times than on a mirror array (certainly compared
to raid10 with multiple pairs).  I just don't think it is quite as bad
as you think.