[stgt] Support for persistent reservations

[Lars Kellogg-Stedman]

Hello all,

The fence_scsi method provided with RedHat's cman package uses SCSI
persistent reservations as an i/o fencing mechanism.  I've tried using
this against an iSCSI target served by tgtd without much luck.

The tool is able to successfully create a reservation by running:

  sg_persist -n -d <device> -o -R -K <key> -T 5

fence_scsi uses --preempt-abort to fence another node (prevent it from
accessing a device) by running:

  sg_persist -n -d <device> -o -A -K <local key> -S <other key> -T 5

This results in the following error from sg_persist:

  PR out: bad field in cdb including unsupported service action

I'm using the version of scsi-target-utils distributed with Fedora 13
(scsi-target-utils-1.0.1-4.fc13.x86_64).  Has anyone else run into
these problems?  Is this expected to work?

The tgt configuration looks like this:

  default-driver iscsi

  <target iqn.2001-04.com.obliquity-tgt-1>
	backing-store /var/lib/libvirt/images/shared.img
  </target>

I see the same error message with tgtd 1.0.9.  Additionally, both
versions appear to freeze up completely if I try to use --preempt
instead of --preempt-abort.

Thanks for your help,

-- Lars
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