Re: How to do strict synchronous i/o on Windows?

[Martin Steigerwald <Martin@lichtvoll.de>]

Am Dienstag, 14. August 2012 schrieb Greg Sullivan:
> On 15/08/2012, Martin Steigerwald <Martin@lichtvoll.de> wrote:
> > Am Dienstag, 14. August 2012 schrieb Greg Sullivan:
> >> On 15 August 2012 03:36, Martin Steigerwald <Martin@lichtvoll.de> wrote:
> >> > Hi Greg,
> > […]
> >> > Am Dienstag, 14. August 2012 schrieb Greg Sullivan:
> >> >
> >> >> On Aug 14, 2012 11:06 PM, "Jens Axboe" <axboe@kernel.dk> wrote:
> >> >> >
> >> >> > On 08/14/2012 08:24 AM, Greg Sullivan wrote:
> >> >> > > I need to simulate strict synchronous, round robin i/o to a group
> >> >> > > of
> >> >> > > files. I am on Windows 7 32-bit.
> >> >> > > fio is very nearly working, except that even with a queue depth of
> >> >> > > 1,
> >> >> > > it is still resulting in a disk queue that is > 1, because the
> >> >> > > "iodepth" parameter is not global - it is per thread. (correct?)
> >> >> > >
> >> >> > > I've tried using the "sync" engine, however that doesn't work at
> >> >> > > all -
> >> >> > > just spews out errors.
> >> >> >
> >> >> > That'll be the case for ANY platform and IO engine. If you have more
> >> >> > than 1 thread or process going, you can have > 1 depth at the device
> >> >> > side. The definition of a sync IO call is that the call doesn't
> >> >> > return
> >> >> > until the IO is done. If you have overlapped calls due to more than
> >> >> > 1
> >> >> > thread, then that is no longer true.
> >> >> >
> >> >> > What you are looking for is outside the scope of an application. You
> >> >> > would have to limit the queue depth on the operating system side to
> >> >> > achieve that. Or artificially limit fio in some way, which would not
> >> >> > make a lot of sense imho.
> >> >
> >> >> Thanks Jens. I do in fact have an application that reads in exactly
> >> >> the
> >> >> manner I described. I have monitored the queue depth - it does not
> >> >> rise
> >> >> above 1.  It is a real time musical sample streamer.
> >> >>
> >> >> Please consider this a new feature request for fio - thankyou.
> >> >
> >> > Is this application multithreaded? If so, are mutiple threads doing I/O
> >> > at the same time? If not I´d suggest just testing with one job.
> >>
> >> I don't know whether it is multithreaded or not. All I know is that it
> >> reads many files sequentially and in a round-robin fashion, without
> >> causing any disk queuing.
> >>
> >> Is it possible to read from more than file in a single job, in a
> >> round-robin fashion? I tried putting more than one file in a single
> >> job, but it only opened one file. If you mean to just do random reads
> >> in a single file - I've tried that, and the throughput is
> >> unrealistically low. I suspect it's because the read-ahead buffer
> >> cannot be effective for random accesses.  Of course, reading
> >> sequentially from a single file will result in a throughput that is
> >> far too high to simulate the application.
> >
> > Have you tried
> >
> >        nrfiles=int
> >               Number of files to use for this job.  Default: 1.
> >
> >        openfiles=int
> >               Number of files to keep open at the same time.  Default:
> >               nrfiles.
> >
> >        file_service_type=str
[…]
> > ? (see fio manpage).
> >
> > It seems to me that all you need is nrfiles. I´d bet that fio distributes
> > the I/O size given among those files, but AFAIR there is something about
> > that in fio documentation as well.
> >
> > Use the doc! ;)
[…]
> Yes, I have tried all that, and it works, except that it causes disk
> queuing, as I stated in my first post. I thought you meant to put all
> the files into a single [job name] section of the ini file, to enforce
> single threaded io.

With just one job running at once?

Can you post an example job file?

Did you try the sync=1 / direct=1 suggestion from Bruce Chan?

I only know the behaviour of fio on Linux where I/O depth of greater
than one is only possible with libaio and direct=1. The manpage hints
at I/O depth is one for all synchronous I/O engines, so I´d bet that
refers to Windows as well.

Other than that I have no idea.

-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
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