Re: A patchset to limit the number of outstanding bios

[Joe Thornber <thornber@redhat.com>]

On Mon, Oct 13, 2014 at 11:27:20AM -0400, Mikulas Patocka wrote:
> Hi
> 
> I'm announcing the release of a patchset that limits the number of 
> outstanding bios in the device mapper.

Interesting; Mike and I were considering this for throttling io to
thinp a couple of weeks ago.  The reason we didn't go with this was,
for thinp, there are cheap bios (eg, a quick remap) and expensive bios
(eg, one that triggers a copy on write exception).  This means it's
very hard to come up with a single number that represents too many
bios.  Instead I throttle by looking at how long it's taking the
worker thread to get through it's work queue.

Could you make this limit optional please?  eg, setting the limit to 0
turns it off, and I'd prefer a default of 0/off.

- Joe