Re: Triple parity and beyond

[Stan Hoeppner <stan@hardwarefreak.com>]

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.

-- 
Stan