mbuf_jumbo_9k & iSCSI failing

[Ryan Stone]

Having looking at the original email more closely, I see that you showed an
mlxen interface with a 9020 MTU.  Seeing allocation failures of 9k mbuf
clusters increase while you are far below the zone's limit means that
you're definitely running into the bug I'm describing, and this bug could
plausibly cause the iSCSI errors that you describe.

The issue is that the newer version of the driver tries to allocate a
single buffer to accommodate an MTU-sized packet.  Over time, however,
memory will become fragmented and eventually it can become impossible to
allocate a 9k physically contiguous buffer.  When this happens the driver
is unable to allocate buffers to receive packets and is forced to drop
them.  Presumably, if iSCSI suffers too many packet drops it will terminate
the connection.  The older version of the driver limited itself to
page-sized buffers, so it was immune to issues with memory fragmentation.