cap on writeback?

[Raymond Jennings]

On Mon, Mar 25, 2013 at 5:06 PM,  <Valdis.Kletnieks at vt.edu> wrote:
> On Mon, 25 Mar 2013 16:33:48 -0700, Raymond Jennings said:
>> Just curious, is there a cap on how much data can be in "writeback" at
>> the same time?
>>
>> I'm asking because I have over a gigabyte of data in "dirty", but
>> during flush, only about 60k or so is in writeback at any one time.
>
> Only a gigabyte? :)

Well, yes.  Considering I'm on a piece of crap that only has 2G total
ram on a stick, part of which is reserved for video.  It's a budget
computer I bought in a pinch, sue me :P

>  (I've got a box across the hall that has 2.6T of RAM,
> and yes, it's pretty sad when it decides it's time for writeback across
> an NFS or GPFS mount, even though it's a 10GE connection.)
>
> For the record, writeback is one of those things that's really hard to
> get right, because there's always corner cases.  Probably why we seem to
> end up screwing around with it every 2-3 releases. :)

But anyway, I'm appreciative (but already aware) of the complications,
but this still doesn't answer my question.

Is there some sort of mechanism that throttles the size of the writeback pool?

I would like to remove that cap if it exists, or at least make it a
proc/sys tunable of sorts.

it's somewhat related to my brainfuck queue, since I would like to
stress test it digesting a huge pile of outbound data and seeing if it
can make writeback less seeky.