Fwd: Disk scheduling activity...

[Willem Jan Withagen]

On 2013-08-14 16:03, Bob Friesenhahn wrote:
> 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?

Yes is a raidz1 with 8 disks. I know it is not optimal in performance, 
but I needed the amount of remaining diskspace.

> Are you basing write activity on the drive LEDs?

Yup.

>
> 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.

Sounds like a fair assumption. Could remove the battery and see what 
happens then. The mvs device is relatively "simple" and has no 
significant memory on board.

> 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.

That would then be the Areca controller, bacause I have the feeling that 
it always writes later.

--WjW