[OpenAFS] Basic OpenAFS questions

Esther Filderman mizmoose@gmail.com
Thu, 26 Jun 2008 14:04:11 -0400

On Wed, Jun 25, 2008 at 1:26 PM, outsider <aonsen@gmail.com> wrote:
> I'm considering using OpenAFS on my home system/LAN, but have a few questions
> first (things thate not quite clear) before I take the plunge.
> 1. Is AFS a tool that is used on top of existing file systems? Do I still
> format my partition with ext3 and then add it to OpenAFS ? Or does the
> partition get formated using the OpenAFS FS?

The former is correct; AFS does not have it's own special disk
formatting.  However, it's important to remember that AFS is
client/server technology.

On the client, OpenAFS requires a special directory (which it will
automagically create) called a "cache".   The cache is used by the
afsd process (which communicates between the client and the servers)
to do things like store pieces of files that have been retrieved from
the servers and keep track of where the files are kept on the servers,
so you don't have to.  (Unlike other network filesystems, users don't
have to remember what server their files are kept on.)

On the server, OpenAFS stores volumes (containers of files &
directories) on sections of disk it calls partitions.  The volumes are
in a special format and cannot be read directly from the server
itself; they must be mounted in an OpenAFS file system [ie.
/afs/yourcellname/whatever] for the data within to be seen.

> 2. On a setup with multiple drives/shares/partitions how does OpenAFS manage
> where data gets written? Does it fill up one volume before moving on the
> next? Does it spread the data evenly between all the volumes?

Davor explained this pretty well.  To a user, an OpenAFS volume looks
like any other directory.  That volume container has a limited quota
(that you, the administrator, set).  Administrators also control what
volumes go in which fileserver partition.   Since a volume is a
container, if the volume fills up, it does not affect the quota of any
other volume.  The only "gotcha" is that if the partition fills up,
all volumes are affected.

(think of a bunch of bags of beans, say, 6 of them.  Each bag can
store 100 beans.  They're in a box that can store 500 beans.   If bag
one has 100 beans, bags 2-5 are still empty, and the box has 100

Now say bags 1-4 have 100 beans each and 5-6 have 50 beans each.  Even
though 5 & 6 aren't full, the box is full (it has 500 beans) so you
can't add more to 5 & 6.

Side note:  Don't fill the box (partition) completely full; moving
volumes from a completely full partition is a nightmare, at best.  )

> 3. If I remove a drive from the OpenAFS system, will that drive still be
> readable on it's own? (I guess this will be answered by question 1)

Davor's short answer of "no" is correct.

If you remove a drive from an OpenAFS server, you can, in theory, put
it on another OpenAFS server and make the volumes readable.  However
this is not a beginners thing!

If you remove a drive from an OpenAFS client, well, it would probably
be just like screwing up any other computer :-).