Coda File System

Re: Coda servers and WAN links?

From: Bruce Janson <>
Date: Fri, 07 Aug 1998 20:38:14 +1000
    I have been thinking about this too and I was kind of interested in
    knowing about the usage on a metered line. It seems like my coda
    client keeps my ISDN line up all the time. Is there any reason for
    this? Would the same thing happen to if I put a coda server here? 
    It would be fine if it sat around not keeping the line up and then the
    moment that it gets a request for a file, it goes back to the main
    server for that volume and checks that the file is the same.

Here, we use the Linux<->Coda kernel interface (but use
neither Venus nor Vice).  One of our development client PCs is separated
from its server by a (busy) 2Mbits/s WaveLAN wireless link [Hi, Robert].
The client runs a Venus-like cache manager.  The operating system
interface does not allow one to determine directly which blocks of a
writable cached file have actually been modified.  Initially our cache
manager would write back entire files on close() after a write-mode open().
For small modifications to large files this was slow.  The latest version
of the cache manager sacrifices local CPU cycles and cache storage space
in an attempt to reduce this communication cost.  At first open() of a
writable file it fetches the file from the server and then makes a second
copy in the local cache.  After each close() the cache manager compares
the potentially modified cache file with the saved copy and sends to
the server only those blocks which have changed.  It then updates the
saved copy, ready for another block-level `diff' at the next close().

Can/Does Venus act in this mode too?

Of course, if the cache manager could just ask the kernel which blocks
had been changed...

Bruce Janson, Basser Department of              Email:
Computer Science, Madsen Building (F09),        Phone:  +61-2-9351-3423/4
University of Sydney, N.S.W., 2006, AUSTRALIA   Fax:    +61-2-9351-3838
Received on 1998-08-07 07:41:23