Re: [PATCH 4/5] convert st to use scsi_execte_async

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

Kai Makisara wrote:
> 
> st sends the command using a buffer of one segment. The command is passed 
> to the HBA driver and it sees 8 segments. Clustering seems to work 
> properly (the maximum segment size is set to 65536 bytes by default). 
> 
> Here is what is seen when the block size is increased to 513 kB:
> dd if=tdata of=/dev/nst0 bs=513k count=1
> 
> The dd process hangs in device wait. It turns out that 
> scsi_execute_async() fails. This is an async write and the process later 
> waits for the failed write to finish. The patch at the end of this message 
> fixes this st bug (don't worry about the line shifts, I have some 
> debugging printks in this driver).
> 
> The real problem is that scsi_execute_async() fails. The 513 kB request is 
> 129 pages. Could the reason be related to these defaults in include/linux?
> #define MAX_PHYS_SEGMENTS 128
> #define MAX_HW_SEGMENTS 128
> 

Yeah I think this is due to the MAX_PHYS_SEGMENTS limit. The hw segments 
is set by scsi_lib in scsi_alloc_queue and is the sg_tablesize value on 
the host. Right now all I do is return a error when someone violates one 
of the limits, but I think the right thing to do is have the ULDs take 
some of these values into account when they build their lists. However 
if I do that we will not be able to make large requests since the 
MAX_PHYS_SEGMENTS/SCSI_MAX_PHYS_SEGMENTS will limit them. Umm let me 
rethink.