Re: sg regression in 2.6.16-rc5

[Mike Christie <michaelc@cs.wisc.edu>]

James Bottomley wrote:
> On Wed, 2006-03-01 at 14:42 -0600, Mike Christie wrote:
> 
>>The current sg driver should use alloc_pages() with an order that should
>>get 32 KB. If the order being passed to alloc_pages() in sg.c is only
>>getting one page by default that is bug.
> 
> 
>>The generic routines now being used can turn that 32KB segment into
>>multiple 4KB ones if the LLD does not support clustering.
> 
> 
> To be honest, the original behaviour was a bug.  A device that doesn't
> enable clustering is telling us it can't take anything other than
> PAGE_SIZE chunks ... trying to give it more is likely to end in tears.

Yeah, we hit this with iscsi_tcp. iscsi_tcp does not suport clustering, 
not due to a HW limit, but becuase that is just how it was implemented. 
When we get clustered segments we end up with data corruption or an oops 
depending on the operation. I think the workaround was to set the 
default segment for sg and st to a page or just use the block layer sg_io.

> 
> However ... I'm not sure we actually have any devices that anyone can
> identify which truly can't enable clustering (a lot which have it
> disabled, I suspect, are that way historically because their writers
> didn't trust the clustering algorithm).
> 

ok, I can implement clustering for iscsi_tcp. For now it does not much 
matter since we never supported large sg or st commands.