[OpenAFS] Moving Volumes

[Craig_Everhart@transarc.com]

Excerpts from mail: 28-Feb-02 [OpenAFS] Moving Volumes "Michael
Aldrich"@reserv (1324*)

> If I am moving volumes from server to server, can I expect an outage? My
> AFS server currently contains volumes attached to my web site. Will this
> cause an outage for my site?

Let's say that it takes 2 minutes to move a volume.  There will be an
outage for some smaller interval while the move happens.

By the way, should you be using RO volumes for your web servers?  That
keeps them available even when your RW is unavailable.

The way moving a RW volume works is this.  (a) The RW volume is cloned
to a temporary clone volume.  (b) This temporary clone volume is then
shipped (via volume dump and volume restore) to the destination site. 
(c) An incremental dump of the RW volume is shipped to an incremental
restore on the destination site.  (d) The source volume is deleted as
all location sites are told where the volume now lives.  I'm ignoring
some details here, but they don't matter for your purposes.

Access to the RW volume is suspended selectively during parts of the
above, and that's when your web servers might potentially be suspended
as well; they will of course wait without the filesystem showing
anything other than a delay (their system calls will complete normally).
 So during step (a), access to the RW is suspended, but that's only a
few seconds.  During step (b), access is *permitted*, and this is likely
to be the longest phase of the operation--as all files in the RW volume
are shipped across a network.  During step (c), access to the RW is
again suspended, but only changed files are shipped--ones that changed
during the (b) phase.  So if nothing changes during the (b) phase, step
(c) will execute quickly.  Step (d) takes at most a few seconds.

		Craig