12 Dec
2014
12 Dec
'14
3:30 p.m.
On 12/12/2014 11:56 AM, Craig Ruff wrote:
You should be able to detect file mark indications in
either the
variable length or fixed block read modes. When the tape (or drive
controller buffer position for those that perform buffering) is
positioned at the file mark, you will get a check condition response
to the issuance of the read command, then you (or the SCSI generic
driver) issue the read sense command (immediately after the read, no
other intervening commands!) and the file mark bit will be set in the
sense buffer. For the generic SCSI devices, you can typically tell
the driver to perform an auto sense for check condition responses.
Just make sure you have properly told the driver where to put the
sense data. I suppose a drive could have a manufacturer specific mode
that could hide file marks, but I've never seen one. We did make use
of manufacturer specific procedures to place the read head beyond the
detected EOT mark so we could recover tapes that had been
accidentally overwritten in the middle of recorded data.
I'm using a simple ASPI interface, so I'm talking to the SCSI controller
directly, issuing CDBs and getting REQUEST SENSE data back, so there's
not really any issue of a driver getting in the way.
As previously mentioned, the SCSI sense data does not reflect a filemark
in variable-length Read mode, but rather 0 bytes transferred with no
check condition indicated (i.e. both sense key and additional sense
code and sense qualifier are 0, indicating a normal completion of read;
ILI is not set in this case either.
About all that I can assume is that this is a quirk of this particular
tape-to-SCSI interface. It's probably not an issue and easily taken
care of in the code, since 0-length normal tape records aren't possible
AFAIK.
--Chuck