Shareable, networked, swap device?

[Andy Glew]

Does anyone have a shareable, networked, swap device?

Anyone = commercial or academic / public domain, any flavour of UNIX.

Shareable, networked, swap device:
    NOT mkfile/swapon !
    What I seek is a swap device that multiple workstations share
dynamically, not a disk that can support several statically sized swap
partitions (well, not quite statiic, but more swap space isn't
allocated automagically).
    In any typical network of n machines, each machine is allocated,
say, 32M of swap space, for a total of 32*n megabytes.  But, in our
network, typically half of the machines are not being used at any time,
so they typically have around 30M of swap-space free.  At the same
time, a much smaller fraction of our machines are running very large
jobs, and really need swapspace around 64-128M - but these aren't
always the same machines (not designated compute servers).  
    It really would be nice if the unused swap memory of some machines
could be used to temporarily, transparently, expand the swap memory of
others, on demand.  Ie. it would be nice if swap space was a
centralized resource pool, rather than fragmented.
    As I've said above, management via mkfile/swapon is possible, but
klugey.  You want the swap space to be transparently added, without
human intervention, and without causing processes to die with the
message "too big".  Things aren't helped by SUN and friends not
supporting the swapon -l and swapon -d (list and delete) commands
found on System V.
    
How hard would this be to do?
    Modest.  The typical /dev/drum interface, where the kernel assumes
that it has exclusive control over a large space, could be modified.
All that is really needed is an information call to indicate when a
swap page has been freed, so that it can be physically removed from
under one machine's /dev/drum and given to another. And a trap so that
an attempt to access a /dev/drum page that has been removed can be
handled by requesting over the network.  Safety properties, of course,
are a bit harder, and it really would be nicer to have a "give me NNN
pages of swap space call" made to the shareable, networked, swap
device.

Has anyone seen something like this?


This is posted to comp.unix.internals, because any such device is
probably a driver, a server, an interface to /dev/drum, or all three;
to comp.unix.large, because large systems with lots of workstations
are likely to be playing the swap allocation game; and to
comp.unix.admin because such a device would make system administration
on large systems easier.  Followups to comp.unix.inyernals.
--
Andy Glew, a-glew at uiuc.edu [get ph nameserver from uxc.cso.uiuc.edu:net/qi]