On Wed, Feb 18, 2009 at 10:09 PM, Boaz Harrosh <bharr...@panasas.com> wrote:
> Mike Christie wrote:
>> 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?
>> Not that I know of when going through the block layer. I think you will
>> have to ask lkml.
>> I think the only way to control it is the  bsg/sg/passthrough route
>> since that does not do merging. The other alternative is to just hack
>> the code to do what you want :)
> You can select the no-op I/O elevator and you can also use direct IO
> like with sg_dd from the sg_utils package

I'm using noop already, but that didn't help. I'll try to ask in lkml.


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