On Wed, Feb 18, 2009 at 10:36 AM, Ulrich Windl
<ulrich.wi...@rz.uni-regensburg.de> wrote:
>
> On 18 Feb 2009 at 8:44, Erez Zilber wrote:
>
>>
>> On Tue, Feb 17, 2009 at 11:35 PM, Mike Christie <micha...@cs.wisc.edu> wrote:
>> >
>> > Erez Zilber wrote:
>> >> Hi,
>> >>
>> >> I'm running a setup of open-iscsi connected to a target. When I run
>> >> I/O from the initiator (e.g using dd) with transaction size of 128kB,
>> >> I sometimes see that 2 128kB requests are aggregated to a single 256kB
>> >> request. This is rare, but it happens from time to time. Can I disable
>> >> this feature? Who is responsible for that? Is it scsi-ml?
>> >>
>> >
>> > block layer.
>> >
>> > /sys/block/sdX/queue/max_sectors_kb
>>
>> Thanks, but this will limit the I/O size for all I/Os. What I forgot
>> to mention is that sometimes I also send larger I/Os (e.g. 512kB).
>> With the proposed solution, these large I/Os will be sent as multiple
>> 128kB I/Os (and affect the performance). Isn't there a way to simply
>> avoid this aggregation?
>
> Hi!
>
> May I curiously ask why you would want that at all? In my experience, the 
> larger
> the requests, and the more requests are packed into one packet, the better the
> performance.
>

That is correct, but sometimes you allocate resources according to
some assumptions (e.g. commmand size not bigger than X). If the
assumptions breaks, bad things happen...

Erez

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