[DRBD-user] Configuring the scheduler for a DRBD backend

Lars Ellenberg lars.ellenberg at linbit.com
Mon Jul 15 17:56:51 CEST 2013

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On Mon, Jul 15, 2013 at 04:59:42PM +0200, Wiebe Cazemier wrote:
> Hi, 
> As specified here: 
> http://www.drbd.org/users-guide-8.3/s-latency-tuning.html
> I'd like to tune some params. But, in:
> echo deadline > /sys/block/device/queue/scheduler
> what is 'device'? I have an LV on RAID as a DRBD back-end. Which of these devices do I change the scheduler of:

The "real" device, the "request based" device below drbd,
typically the "/dev/sdX".

> /dev/sda and /dev/sdb
> /dev/md1
> /dev/dm-3 (being a link to the LV in question.

MD raid, and devicemapper stuff are typically bio based,
so they don't have no queue nor elevator/scheduler.

> And, do they interfere when one is CFQ and another deadline?

If you had stacked request based devices, they probably would interfere,
and I'd suggest to set some levels of such a stack to no-op.

> Another point is, are those /dev/dm-x devices consistent on reboots?

Most of the time, unless you changed "something", but not "reliably".
But they are not relevant.

Why not just use deadline as default scheduler, and be done with it ;-)
(kernel boot parameter elevator=deadline)

: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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