Re: IO transfer limits

[john clyne <clyne@ucar.edu>]

So given that the HBA max_sectors_kb looks sufficiently large, my
interpretation of your article is the next place I need to look is at
ajusting SCSI_MAX_PHYS_SEGMENTS. Sounds like i can only increase this to
256. Does that than give me a max transfer size of 256 * page_size (4k) =
1MB? Is 1MB the limit or am I missing something?

thanks - jc


Douglas Gilbert 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.
> 
> John,
> I discuss the subject in this page:
>    http://www.torque.net/sg/sg_io.html
> in the section titled:
> Maximum transfer size per command
> 
> Mike C. has given you the answer for the block device
> interface (e.g. via /dev/sda); you should be able
> to do about 8 times better via the scsi generic
> interface (e.g. /dev/sg0).
> 
> Doug Gilbert
> 
> 
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