Re: Triple parity and beyond

On 24/11/13 22:13, Stan Hoeppner wrote:

On 11/23/2013 11:14 PM, John Williams wrote:

quoted

On Sat, Nov 23, 2013 at 8:03 PM, Stan Hoeppner [off-list ref] wrote:

quoted

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.

quoted

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.