Attila Nagy wrote:
>
> Hello,
>
> I'm just curious: is it possible to set up an NFS server and a client
> where the client has very big (28 GB maximum for FreeBSD?) swap area on
> multiple disks and caches the NFS exported data on it?
> This could save a lot of bandwidth on the NFS server and also redues load
> on that.
There is a configuration called "dataless", in which you
have local swap for an NFS booted system; this has been
supported by SunOS since 1991/1992. It can also be used
with FreeBSD, with some rc file tweaking (FreeBSD has
seemingly resisted rc file changes to make this kind of
division easier, as well as diskless). You are also
allowed locacl data storage, with the idea that the OS
and utilities, etc. all come off the remote server.
The major value would be to mark the VFS type as "precache
executables to swap"... the main failure mode for diskless
and dataless SunOS machines has historically been the fact
that, when the machine when to swap in a page in an exectable,
the server was down, and therefore, all the engineers sit and
twiddle their thumbs while their machine is locked up sitting
in the page-in path.
To combat this, you could have an attribute flag on the FS
that indicated it was remote, and thus triggered local swap
caching of the executable file image in its entirety, so that
demand paging over the network was held to a minimum. This
would permit people to continue to do work during a server
outage, since the pages will be there.
This is an idea I've suggested before.
If, on the other hand, you are asking for caching of data
file contents for writable files (unlike executables, which
are read-only except for the server, since when you run a
program, you do not tend to write to its image), then the
answer is "no, not unless you implement NFSv4".
The problem is that, prior to NFSv4, there was not a working
distributed cache coherency protocol, so locally cached data
can become stale; writebacks to the server by one client can
therefore overwrite adjacent but unrelated data written back
by another client, if such writes are not restricted to page
boundaries (or worse).
So that answer is that any system that does this, risks the
corruption of its data in the common case (even the simplest
case, a mail server whose mailbox files are accessed by a
single client machine at a time, will get corruption).
-- Terry
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