Device Sharing

Dave Peterson dap at
Wed Feb 26 06:29:41 PST 1997

* From the SCSI Reflector (scsi at, posted by:
* dap at (Dave Peterson)
Hello All,
The purpose of this message is to generate discussion about the sharing
of devices (specifically tape transport's) between multiple host's and to
convey the need for consistency between the various standard bodies
regarding the usage of reserve and release.
The sharing of transport's is becoming more desirable as companies migrate
"open system" machines to the traditional glass house for data and storage.
In the open system arena there is no consistent or well-defined method
to share a tape device. Tape devices have reserve/release commands to
facilitate sharing, but few (if any) device drivers
issue a reserve at open and a release at close. Basically it is left to
the application.
The SCSI-2 spec (at least the one I have) states that the reserve and release
commands are mandatory for sequential devices. After looking at SSC rev 9 I
noticed the reserve and release commands are now optional for
sequential-access devices. Currently PLDA_18 requires reserve and release
for sequential-access devices.
I believe reserve and release should be mandatory for sequential-access
devices and device driver's should issue a reserve at open and release at close.
The ability to detect the device is already reserved at open and not release 
the device at close time (int the device driver) would also be beneficial 
for applications that open and close the device multiple times during it's
With the advent of Fibre Channel and SCSI-3 I believe now is a good time to
address these issues.

Dave Peterson           phone : 612-391-1008
Senior Engineer 
Network Systems Corp.	email: dap at

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