Network performances using Tgtd on a USB3 device

Hello,

We just tested the performances of a USB3 device imported using a
iSCSI link (tgtd + iscsiadm between 2 rhel 6 boxes).

The disk is mounted with the "sync" (server side) and written to using
the "O_DIRECT" option (client side). We write to the disk using the
following command:

    dd if=src_from_fast_device of=dest_on_test_device bs=128k oflag=direct

When we test the disk locally, we reach about 50MB/s which is nice.
However, when we use a iSCSI link, we cannot reach more than 6MB/s on
a Gigabyte network.

We tried a disk emulation and a passthrough iSCSI exports without much
performance gain.

Is there some parameters we could ajust on the server or client side
to gain back our writing performance in such a case ?

Is there a way to workaround this situation, while still keeping the
"sync" mount and "O_DIRECT" write so we avoid dirty pages bloating
problems on the client side ?

Thanks for your help,