Re: IO transfer limits

[john clyne <clyne@ucar.edu>]

Increasing SCSI_MAX_PHYS_SEGMENTS to 256 has doubled the IO size to 1MB.
Thanks to all!

cheers - jc


Mike Christie wrote:
> 
> john clyne wrote:
>> Can anyone give me some guidance on where in the IO stack I might be
>> running
>> into a 512KB limit on IO transfer sizes to an external FC device? I've
>> checked IO scheduler parameter
>> (/sys/block/<dev>/queue/{max_sectors_kb,max_hw_sectors_kb}. Both are set
>> to
>> 32767. I'm using Qlogic HBAs (qla2312), but I don't see any relevent
>> parameters. I'm running RHEL 4.0 with a 2.6.9-34 kernel. Any pointers
>> would
>> be greatly appreciated.
>> 
> 
> There are also scatterlist limits.
> 
> /sys/class/scsi_host/hostX/sg_tablesize is a limit for the number of
> scatter list entries. For qla2xxx it is 255.
> 
> The scsi layer sets the queue's max_phys_segments to 128 by default. I
> thought there was ia scsi compile time option to increase this, but
> maybe you have to just modify the SCSI_MAX_PHYS_SEGMENTS define by hand.
> 
> So with the default value and with 4 K pages if you end up getting pages
> that cannot be clustered you will end up with 4K * 128.
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