Re: Expand VM

Date: Thu Jan 23 2003 - 11:55:38 EST

On Thu, 23 Jan 2003 12:56:27 -0300, User & <> said:

> I have one idea , and this is about expand virtual memory on linux boxes
> connected in LAN.
> Example: Linux A is processing come information , and need more memory , so
> with this source , Linux A could access virtual memory on Linux B in LAN.

We've seen *this* done before (remember diskless Sun3-50's?) - the /dev/swap
file would be a large file on an NFS mount from a server. At the time, this
actually made performance sense, because the old 'Shoebox' drives the -50
came with were incredibly slow, and you could actually do an NFS operation
to a larger server (a -280 with Fujitsu SuperEagle disks, for instance) faster
than talking to the local disk.

These days, it's probably easier and cheaper to just buy more RAM and/or disk
for Linux A.

> But i donīt know how translate the virtual address between Linux A and B , to
> have success in acess VM, or how to send all the process for Linux B to be
> processed.

Sending the whole process to Linux B to be processed is called "process
migration", and is a difficult problem. Moving the memory image of the
process is usually pretty easy. What is difficult is moving things like
references to open files, file locks, and so on (what if the process is
actively writing to block 739 of /usr/foo/some.file, and the LinuxB machine
doesn't have a /usr/foo, or the permissions on some.file don't match, or
another process has it locked, or... ) There be nasty dragons in this.

You're probably better off buying more RAM and disk for your A machine.

				Valdis Kletnieks
				Computer Systems Senior Engineer
				Virginia Tech

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