James Bottomley wrote:
On Tue, 2008-06-03 at 09:59 +0200, Arne Wiebalck wrote:
Hi all,
I got a problem using the SG_IO ioctl with our tape drives.
In order to examine errors on our drives in more detail, I
would like to make the sense bytes available to the
application using the REQUEST_SENSE command.
However, there's a discrepancy between the kernel output in
/var/log/messages and what I see using SG_IO within my
application: while the kernel sees
kernel: st0: Error with sense data: scsi1 : destination target 0, lun 0
kernel: command = Space 01 00 0d d5 00
kernel: Info fld=0x1, Current st0: sense key Medium Error
kernel: Additional sense: Read retries exhausted
kernel: Raw sense data:0xf0 0x00 0x03 0x00 0x00 0x00 0x01 0x12 0x00 0x00
0x00 0x00 0x11 0x01 0x00 0x00 0x00 0x00 0x37 0xf7 0x10 0x01 0x00 0x00
0xf7 0x37
(which is realistic) the ioctl reports something like
70 00 00 00 00 00 00 12 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 10 02
(since the first bit is not set, the sense bytes are not even valid,
as far as I understand).
That is a poor usage of the word "VALID" in the standard.
See: http://www.t10.org/ftp/t10/drafts/spc4/spc4r14.pdf
section 4.5.3 on "Fixed Format Sense Data".
The VALID bit (byte 0, bit 7) indicates whether there is
valid data in the information field (bytes 3 to 6 inclusive).
What the SG_IO ioctl is reporting is well-formed sense data
saying there is no error, and no additional sense qualifiers.
It has been cleaned out as James explained.
Doug Gilbert
So, could it be that the sense bytes are already cleared when I request
them? They cleared/set by the next SCSI cmd, I assume? But shouldn't
they be valid even then?
I also tried the sg3-utils to query the drive's sense bytes, and they
report (almost) the same sense bytes as SG_IO inside my application
does. Sending an INQUIRY cmd using sg3_utils works fine, btw.
Maybe that's all a plain usage error, so please find the code I use
below.
Anything obvious I am doing wrong here? All comments appreciated.
Yes: SCSI automatically requests sense in response to a check condition.
So, the sense should be attached to the SG_IO command that receives the
error. You can't do an additional request sense for it because the
sense has already been cleared by the automatic request sense the
mid-layer did.
Does the sense data really not get returned by the SG_IO command that
actually encounters the check condition return?
James
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