Re: IO transfer limits

[john clyne <clyne@ucar.edu>]

So what's the best course of action for increasing the 512kb IO transfer size
limit, changing SCSI_MAX_PHYS_SEGMENTS (currently 128 and apparently can be
set no greater than 256), or increasing the page size? Or is there another
option.

thanks - jc


James Bottomley wrote:
> 
> On Wed, 2007-01-10 at 15:07 -0800, john clyne wrote:
>> 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?
> 
> Well ... that's not the maximum possible ... just the maximum number of
> SG elements.  If the buffer is completely fragmented in physical memory,
> then it will be 1MB ... however, linux does a reasonable job of getting
> physically contiguous pages into a large memory allocation, so you could
> get transfers that are larger than the 1MB limit by quite a way.  And,
> of course, any system that has a larger page size or uses an iommu will
> go over this 1MB as well.
> 
> Jmaes
> 
> 
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