[485] in Info-AFS_Redistribution
Re: Will clients use the "nearest" server?
daemon@ATHENA.MIT.EDU (Lyle_Seaman@transarc.com)
Tue Dec 10 07:23:06 1991
Date: Fri, 6 Dec 1991 10:33:54 -0500 (EST)
From: Lyle_Seaman@transarc.com
To: INFO-AFS <@nsfnet-relay.ac.uk:INFO-AFS@transarc.com>
Cc: BBoard <bb+transarc.afs.psupport@transarc.com>
In-Reply-To: <Added.wdDilKD0Bi81IKF08m@transarc.com>
Excerpts from transarc.system.info-afs: 5-Dec-91 Will clients use the
"neare.. Peter Lister@cranfield.a (1276)
> Question: Will clients use the physically nearest server where there is a
> choice of read-only volumes, hence reducing backbone traffic?
Do clients use the server which reacts fastest? If not how do they
> choose? Is it possible to weight clients in favour of a local server?
> Does the ordering of db servers in CellServDB have any effect? Or is it
> entirely random?
In AFS 3.1, the clients choose a read-only replica randomly. This was
done in an attempt to provide rudimentary load-balancing for the common
case, where all the servers were located on the same network. In AFS
3.2, it is possible to weight clients in favour of particular servers,
via an extension to the fs command suite (which uses a couple of new
PIOCTLs, in case somebody wants to write some kind of an external
load-balancing service). In addition, the initial weights are chosen
based on the relative IP addresses of the client and the various
servers. Ie, a server whose IP address is in the same subnet range as
the client will be chosen in preference to a server on a different
network. The hierarchy is "same subnet, same network (class C,B, or
A), different networks". Ties are broken randomly. Choosing the
server which reacts fastest was considered, but was tentatively rejected
since it can be destabilizing, leading to network and/or server
meltdown. A safe adaptive mechanism would be valuable, and can be added
as an external daemon using the new PIOCTLs.
Once a client has found a server for a volume, does it ignore others
> until the server fails?
No. If the ordering is re-arranged, the client will contact the new
preferred server for all accesses to a volume from that point on.
> When part of a volume is cached, do clients favour the server that the
> first part came from when pulling more of it?
Not "favour", per se. The list of preferences is checked, and the top
server is chosen. Practically speaking, that will probably be the same
as "the server that the first part came from".
> Lastly, can anyone point me to a "further info" document that might help?
In /afs/transarc.com/public/lws you may find CMserverprefs.ps, which
goes into a bit more detail, if you really want it.
