Re: Bandwidth Allocations under CFQ I/O Scheduler

[Jens Axboe]

On Wed, Oct 18 2006, Jakob Oestergaard wrote:
> On Tue, Oct 17, 2006 at 03:23:13PM +0200, Jens Axboe wrote:
> > On Tue, Oct 17 2006, Arjan van de Ven wrote:
> ...
> > > Hi,
> > > 
> > > it's a nice idea in theory. However... since IO bandwidth for seeks is
> > > about 1% to 3% of that of sequential IO (on disks at least), which
> > > bandwidth do you want to allocate? "worst case" you need to use the
> > > all-seeks bandwidth, but that's so far away from "best case" that it may
> > > well not be relevant in practice. Yet there are real world cases where
> > > for a period of time you approach worst case behavior ;(
> > 
> > Bandwidth reservation would have to be confined to special cases, you
> > obviously cannot do it "in general" for the reasons Arjan lists above.
> 
> How about allocating I/O operations instead of bandwidth ?
> 
> So, any read is really a seek+read, and we count that as one I/O
> operation. Same for writes.
> 
> Since the total "capacity" of the system is typically (in real-world
> scenarios) the number of operations (seek+X) rather than the raw
> sequential bandwidth anyway, I suppose that I/O operations would be what
> you wanted to allocate anyway.
> 
> Anyway, just a thought...

While that may make some sense internally, the exported interface would
never be workable like that. It needs to be simple, "give me foo kb/sec
with max latency bar for this file", with an access pattern or assumed
sequential io.

Nobody speaks of iops/sec except some silly benchmark programs. I know
that you are describing pseudo-iops, but it still doesn't make it more
clear.
Things aren't as simple 

-- 
Jens Axboe

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