[stgt] Improving iSCSI passthrough tape performance

[Phillip Frost]

I'm using iSCSI to get access to a fibre channel tape library (a Tandberg T40) for a Xen guest by way of a passthrough device. The initiator in the guest is open-iscsi. It works, but it's slow. Using a benchmark tool provided by my backup software, I can get around 50 MB/s to and from the tape on the Xen host which has the physical FC HBA. From the guest OS, via iSCSI, I get about 11 MB/s.

The trouble is I don't have any idea how to go about finding the cause of the slowness. I can say I'm using version 1.0.17 backported to debian squeeze, top shows tgtd using about 30% CPU, iperf puts the network connection between target and initiator around 800 Mbit/s, and my targets.conf looks like this:

<target iqn.2012-04.com.example:t40>
    <backing-store /dev/tape/t40/drive0>
        device-type pt
        bs-type sg
    </backing-store>

    <backing-store /dev/tape/t40/changer>
        device-type pt
        bs-type sg
    </backing-store>
</target>

Any suggestions on where I should look next?
--
To unsubscribe from this list: send the line "unsubscribe stgt" in
the body of a message to majordomo at vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html