Re: Max IO size

[Mukul Kotwani <mkotwani@yahoo.com>]

In fact, even with a max_sectors size set to 1, I
still get a 1k IO doing 512 byte reads with IOmeter!

Can someone please clarify?

m


--- Mukul Kotwani <mkotwani@yahoo.com> wrote:
> 
> Thanks for the reply!
> 
> All my values are set high. The sg_tablesize value
> is
> set to 249, max_sectors set to 1024, and I dont
> think
> there is any other value which affects IO size as
> such. I didnt see anything called max segment
> size/max
> size in the host template. Any other things I am
> missing?
> 
> Thanks!
> Mukul
> 
> --- Jens Axboe <axboe@suse.de> wrote:
> > On Wed, Jul 31 2002, Mukul Kotwani wrote:
> > > Hello!
> > > Couple of other questions..forgot to ask in the
> > > previous email!
> > > 
> > > 1) When we do 512 byte sequential reads, the mid
> > > level( or someone else) seems to be combining
> the
> > > requests and sending them down as one big 60k+
> > > request. Can this combining of requests be
> > disabled
> > > somehow?
> > 
> > not the mid layer, it's the block layer doing that
> > for you. you can
> > limit the size of each i/o with the max_sectors
> part
> > of the scsi host
> > template.
> > 
> > > 2) If we do 512k IOs, the mid layer seems to be
> > > breaking it up into 64 k chunks. The Scatter
> > gather
> > > size is set to 240, so there is no reason for it
> > to
> > > break it up. Is there something I am missing? Or
> > is
> > > there a tunable I can set to increase the IO
> size
> > sent
> > > down to the low level driver?
> > 
> > again, the mid layer does not do any combining or
> > breaking up of i/o
> > requests. the block layer will assemble requests
> > according to the rules
> > set for it, these rules you control with the host
> > template settings.
> > these are things like max size, max segments, max
> > segment size, etc.
> > 
> > > 3) What exactly does the max_sectors in the host
> > > structure do?
> > 
> > see above :-)
> > 
> > -- 
> > Jens Axboe
> > 
> > -
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