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.
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