Re: Call for RAID-6 users

[hpa@zytor.com (H. Peter Anvin)]

Followup to:  <200407302338.33823.maarten@ultratux.net>
By author:    maarten van den Berg <maarten@ultratux.net>
In newsgroup: linux.dev.raid
>
> On Friday 30 July 2004 23:11, maarten van den Berg wrote:
> > On Saturday 24 July 2004 01:32, H. Peter Anvin wrote:
> 
> > I'm still early in the testing phase, so nothing to report as yet.
> > But I have a question:  I tried to reproduce a reported issue when creating
> > a degraded raid6 array.  But when I created a raid6 array with one disk
> > missing, /proc/mdstat reported no resync going on.  Am I not correct in
> > assuming that raid6 with 1 missing drive should at least start resyncing
> > the other drive(s) ?  It would only be really degraded with two missing
> > drives...
> >
> > So instead, I defined a full raid6 array which it is now resyncing...
> > My resync speed is rather slow (6000K/sec). I'll have to compare it to
> > resyncing a raid5 array though before concluding anything from that.  Cause
> > this system is somewhat CPU challenged indeed: a lowly celeron 500.
> 
> To confirm, after stopping the raid6 array (didn't want to wait this long) I 
> created a raid5 array on the same machine and it resyncs at 14000K/sec.
> Is this expected behaviour, the 6M/sec for raid6 vs 14M/sec for raid5 ?
> I suppose raid6 has to sync two drives, which would maybe explain the speed 
> difference(?)   In any case, hdparm -tT report 50M/sec on each single drive. 
> Is this discrepancy in speed normal ?
> (yes yes, I played with the /proc/sys/dev/raid/ speed settings (to no avail))
> 

A newly created RAID-5 array uses a special trick to do the initial
sync faster.  Unfortunately that trick is not possible for RAID-6.

	-hpa