[OpenAFS] readonly/readwrite

[Derek Atkins]

Corey Kovacs <ckovacs@DEPAUW.EDU> writes:

> i.e. I create readonly volumes for system stuff that I want to have
> consistant across machines (and not let users muck with). for
> example if I want to add the latest and greatest copy of my favorite
> window manager and X, I could make a readonly volume for say
> /usr/X11 and call it usr.X11, or should it be root.cell.usr.X11?
> then create a mount point at say /afs/cellname/usr.X11 and symlink
> it to /usr/X11. Then when I make changes to /afs/cellname/usr/X11
> I'd do a vos release? Is this correct?

Well, no.  Whenever you create a volume using 'vos create' you are
creating a RW volume.  You create a RO clone by using 'vos addsite' to
add a replication site, and then you use 'vos release' every time the
data changes, to push the data from the RW to the RO volumes.

Clients access volumes based upon how they are mounted.  By default,
volumes are mounted 'RO', which means the client will use the RO
volume if one exists, falling back to the RW volume is there are no RO
volumes in the VLDB.  In order to make changes to files, you need to
access the RW volume directly, and then release it.

The problem with your solution is that you have no way to access the
RW volume.  When you create a mountpoint at /afs/<cell>/usr.X11, it
will default to the RO volumes.  You still need a way to get to the
RW volume.

MIT does this the way Transarc (used to?) recommend it: When you
create your root.afs, you make a mount for <cell> as the normal
mountpoint, and you make a mount for .<cell> as the read-write
mountpoint:

	fs mkmount -dir my.cell.name -vol root.cell -cell my.cell.name
	fs mkmount -dir .my.cell.name -vol root.cell -cell my.cell.name -rw

Due to the way parenting works, you can then access all the RW volumes
in your cell under the /afs/.<cell>/hierachy, whereas you access the
RO volumes (where they exist) via the /afs/<cell>/hierarchy.

> Then for users, I create a r/w volume called home or users or whatever,
> then mount
> it at say /afs/cellname/home. Now do I create volumes for each user, then
> mount them
> at places like
> 
> /afs/cellname/home/user1
> /afs/cellname/home/user2
> ...
> then creating a symlink from /afs/cellname/home to /home on each machine?
> Or can I
> simply go into that volume and do a mkdir user1, user2 ...

This is up to you.  You probably just want to have /etc/passwd use
/afs/<cell>/home/user as the user's homedir.  If you symlink /home ->
/afs/<cell>/home, then you lose the ability to have local accounts on
machines.

> so to recap (kind of) readonly for stuff like system binaries, and
> read/write for
> stuff like home directories. And the only things that nead to be
> "released" are
> read only volumes after I make a change to them. Is this right?

This is correct.

-derek

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord@MIT.EDU                        PGP key available