Re: dm kcopyd: Increase sub-job size to 512KiB

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Jul 16 2019 at  9:59am -0400,
Nikos Tsironis <ntsironis@arrikto.com> wrote:

> On 7/15/19 9:22 PM, Mike Snitzer wrote:
> > On Fri, Jul 12 2019 at  9:45am -0400,
> > Nikos Tsironis <ntsironis@arrikto.com> wrote:
> > 
> >> Hi Mike,
> >>
> >> A kind reminder about this patch. Do you require any changes or will you
> >> merge it as is?
> > 
> > I think we need changes to expose knob(s) to tune this value on a global
> > _and_ device level via sysfs.  E.g.:
> > 
> > 1) dm_mod module param for global
> > 2) but also allow a per-device override, like:
> >    echo 512 > /sys/block/dm-X/dm/kcopyd_subjob_size
> > 
> 
> Hi Mike,
> 
> Thanks for your feedback. I agree, this sounds like the best thing to do.
> 
> > 1 is super easy and is a start.  Layering in 2 is a bit more involved.
> 
> Maybe I could help with (2). We could discuss about it and how you think
> it's best to do it and then I could proceed with an implementation.
> 
> Please let me know what you think.
> 
> > 
> > In hindsight (given how risk-averse I am on changing the default) I
> > should've kept the default 128 but allowed override with modparam
> > dm_mod.kcopyd_subjob_size=1024
> > 
> > Would this be an OK first step?
> 
> Yes, this would be great.
> 
> > 
> > If so, we're still in the 5.3 merge window, I'll see what I can do.
> 
> Shall I proceed with a patch adding the dm_mod.kcopyd_subjob_size
> modparam?

Sure.  And it could be that we won't need 2.

Ideally the default would work for every setup.  Less knobs the better.
But as a stop-gap I think we need to expose a knob that allows override.

Thinking further, I don't think changing the default to 512K is too
risky (famous last words).  So please just update your original patch to
include the modparam so that users can get the old 64K back if needed.

BTW, the param name should probably be "kcopyd_subjob_size_kb" to
reflect the value is KB.

Thanks,
Mike