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?
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