Re: [PATCH 07/13] writeback: explicit low bound for vm.dirty_ratio

[Wu Fengguang <fengguang.wu@intel.com>]

On Tue, Aug 10, 2010 at 11:57:12AM +0800, Neil Brown wrote:
> On Tue, 10 Aug 2010 12:12:06 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> 
> > > Subject: writeback: explicit low bound for vm.dirty_ratio
> > > From: Wu Fengguang <fengguang.wu@intel.com>
> > > Date: Thu Jul 15 10:28:57 CST 2010
> > > 
> > > Force a user visible low bound of 5% for the vm.dirty_ratio interface.
> > > 
> > > This is an interface change. When doing
> > > 
> > > 	echo N > /proc/sys/vm/dirty_ratio
> > > 
> > > where N < 5, the old behavior is pretend to accept the value, while
> > > the new behavior is to reject it explicitly with -EINVAL.  This will
> > > possibly break user space if they checks the return value.
> > 
> > Umm.. I dislike this change. Is there any good reason to refuse explicit 
> > admin's will? Why 1-4% is so bad? Internal clipping can be changed later
> > but explicit error behavior is hard to change later.
> 
> As a data-point, I had a situation a while back where I needed a value below
> 1 to get desired behaviour.  The system had lots of RAM and fairly slow
> write-back (over NFS) so a 'sync' could take minutes.

Jan, here is a use case to limit dirty pages on slow devices :)

> So I would much prefer allowing not only 1-4, but also fraction values!!!
> 
> I can see no justification at all for setting a lower bound of 5.  Even zero
> can be useful - for testing purposes mostly.

Neil, that's perfectly legitimate need which I overlooked.
It seems that the vm.dirty_bytes parameter will work for your case.

Thanks,
Fengguang