http://nbd.sourceforge.net/
"Network Block Device (TCP version)
What is it: With this thing compiled into your kernel, Linux can use a remote server as one of its block devices. Every time the client computer wants to read /dev/nd0, it will send a request to the server via TCP, which will reply with the data requested. This can be used for stations with low disk space (or even diskless - if you boot from floppy) to borrow disk space from other computers. Unlike NFS, it is possible to put any file system on it. But (also unlike NFS), if someone has mounted NBD read/write, you must assure that no one else will have it mounted.
Limitations:It is impossible to use NBD as root file system, as an user-land program is required to start (but you could get away with initrd; I never tried that). (Patches to change this are welcome.) It also allows you to run read-only block-device in user-land (making server and client physically the same computer, communicating using loopback). Please notice that read-write nbd with client and server on the same machine is bad idea: expect deadlock within seconds (this may vary between kernel versions, maybe on one sunny day it will be even safe?). More generally, it is bad idea to create loop in 'rw mounts graph'. I.e., if machineA is using device from machineB readwrite, it is bad idea to use device on machineB from machineA.
Read-write nbd with client and server on some machine has rather fundamental problem: when system is short of memory, it tries to write back dirty page. So nbd client asks nbd server to write back data, but as nbd-server is userland process, it may require memory to fullfill the request. That way lies the deadlock.
Current state: It currently works. Network block device seems to be pretty stable. I originaly thought that it is impossible to swap over TCP. It turned out not to be true - swapping over TCP now works and seems to be deadlock-free.
If you want swapping to work, first make nbd working. (You'll have to mkswap on server; mkswap tries to fsync which will fail.) Now, you have version which mostly works. Ask me for kreclaimd if you see deadlocks.
Network block device has been included into standard (Linus') kernel tree in 2.1.101.
I've successfully ran raid5 and md over nbd. (Pretty recent version is required to do so, however.)
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