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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