Re: [PATCH RFC 6/6] btrfs: Add roundrobin raid1 read policy

[Michal Rostecki <mrostecki@suse.de>]

On Thu, Feb 11, 2021 at 03:27:38AM +0100, Michał Mirosław wrote:
> On Wed, Feb 10, 2021 at 07:23:04PM +0000, Michal Rostecki wrote:
> > On Wed, Feb 10, 2021 at 01:58:15PM +0100, Michał Mirosław wrote:
> > > On Wed, Feb 10, 2021 at 12:29:25PM +0000, Michal Rostecki wrote:
> > > > On Wed, Feb 10, 2021 at 05:24:28AM +0100, Michał Mirosław wrote:
> > > > > This looks like it effectively decreases queue depth for non-last
> > > > > device. After all devices are filled to queue_depth-penalty, only
> > > > > a single mirror will be selected for next reads (until a read on
> > > > > some other one completes).
> > > > > 
> > > > 
> > > > Good point. And if all devices are going to be filled for longer time,
> > > > this function will keep selecting the last one. Maybe I should select
> > > > last+1 in that case. Would that address your concern or did you have any
> > > > other solution in mind?
> > > 
> > > The best would be to postpone the selection until one device becomes free
> > > again. But if that's not doable, then yes, you could make sure it stays
> > > round-robin after filling the queues (the scheduling will loose the
> > > "penalty"-driven adjustment though).
> > 
> > Or another idea - when all the queues are filled, return the mirror
> > which has the lowest load (inflight + penalty), even though it's greater
> > than queue depth. In that case the scheduling will not lose the penalty
> > adjustment and the load is going to be spreaded more fair.
> > 
> > I'm not sure if postponing the selection is that good idea. I think it's
> > better if the request is added to the iosched queue anyway, even if the
> > disks' queues are filled, and let the I/O scheduler handle that. The
> > length of the iosched queue (nr_requests, attribute of the iosched) is
> > usually greater than queue depth (attribute of the devide), which means
> > that it's fine to schedule more requests for iosched to handle.
> > 
> > IMO btrfs should use the information given by iosched only for heuristic
> > mirror selection, rather than implement its own throttling logic.
> > 
> > Does it make sense to you?
> > 
> > An another idea could be an additional iteration in regard to
> > nr_requests, if all load values are greater than queue depths, though it
> > might be an overkill. I would prefer to stick to my first idea if
> > everyone agrees.
> 
> What if iosched could provide an estimate of request's latency? Then
> btrfs could always select the lowest. For reads from NVME/SSD I would
> normally expect something simple: speed_factor * (pending_bytes + req_bytes).
> For HDDs this could do more computation like looking into what is there
> in the queue already.
> 
> This would deviate from simple round-robin scheme, though.

I think that idea is close to what Anand Jain did in his latency
policy, though instead of trying to predict the latency of the currently
scheduled request, it uses stats about previous read operations:

https://patchwork.kernel.org/project/linux-btrfs/patch/63f6f00e2ecc741efd2200c3c87b5db52c6be2fd.1611114341.git.anand.jain@oracle.com/

Indeed your idea would deviate from the simple round-robin, so maybe it
would be good to try it, but as a separate policy in a separate
patch(set).

Regards,
Michal