(x86/1.6T).
I've started getting "unable to allocate scsipi_xfer" messages from one
of my machines.  Once it starts getting these, it quickly enters a state
where any new process (including, say, reboot, or sync) which would have to
be loaded from disk terminates instantly with "Killed", and shortly
thereafter, it hangs - worst of all, hanging in such a way that you have to
hit the reset button, because it doesn't panic.

This happens fairly consistently after, say, a day or so of uptime.  Only
happening over the last few days, and I'm running -current.

Any suggestions?  This always shows up on the root drive, but I don't know
that this means anything - it's the most heavily used.

-s