Fwd: Disk scheduling activity...

[Bob Friesenhahn]

On Wed, 14 Aug 2013, Willem Jan Withagen wrote:
>
> Just a point of information or curiosity, and I don't think/know if it
> is any problem...
>
> I have the raidz array with 8 disks, which I'm using to backup to.
> It is configured
> 	4 disks on a mvs controller
> 	4 disks on an Areca controller (JBODs with battery)
> 	Both controllers are on a PCI-E slot
>
> Most of the time the source just fully loads the pipe and sends 1Gbit/s.
>
> When that happens I see this alternating pattern of writing either to
> the 4 mvs disks, or writing to the Areca disks.
> But almost never are all disk accesses at the same time.
> And really never, never is there a mix of writing between the controller
> sets.

Are all 8 disks in the same raidz vdev?  Are you basing write activity 
on the drive LEDs?

The Areca controller may be caching the writes in its battery-backed 
cache and deferring the writes to when zfs tells it to flush its 
cache.  The other controller may be issuing the writes right away. 
This would explain apparent 'split' writing behavior.

There is even the possibilty that one of the controllers ignores the 
cache flush request and performs the writes later when it feels like 
it.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/