Neil Brown [off-list ref] wrote:

 You can get better speed on creation with
   mdadm -C -l6 -n4 -x2 /dev/sd[ab] missing missing /dev/sd[cd]

 i.e. create with 2 missing devices and 2 spares.  Recovery onto the
 spares will do purely sequential IO on all devices and so will go
 much faster.
 Doing this requires regenerating datablocks which is more cpu
 intensive that generating P and Q, so there might be some CPU
 overhead, but the disk throughput is still much faster.

 In 2.6.16, raid6 resync has been improved somewhat.  This will not
 affect the initial sync, but resync after a crash will only write
 P/Q blocks which are wrong and there are usually very few of those.
 This means that it will mostly to sequential reads on all drives, so
 you get full device speed.

Thanks!!


-- 
Sebastian Kuzminsky