[OpenAFS] shadow volumes?
Sun, 10 Jul 2005 17:47:36 -0400
These are, sort of, new in OpenAFS.
A vos copy, or a vos create followed by a vos shadow, allows you to
replicate the contents of one AFS volume, to another, otherwise
unrelated volume. The copy might be on a different fileserver from the
original. Shadow has an incremental option, so a series of shadow
operations can be done to incrementally update the copy. So essentially
the topic is backup. No new type of AFS volume (eg, "shadow volume")
has been created.
A vos clone operation triggers cloning of a volume, giving you
additional clone versions in a volume group to the traditional ones, if
you want them. Without Jason's patch, these clones don't appear in
VLDB. This is the part of the patch that needs most looking at--but I
didn't, so, I don't say anything about it :)
Jason's patch tweaks these operations somewhat to fit a backup strategy
he explained partially in an earlier post. Something like,
1. Periodically shadow rw volumes to surrogates on another server,
marking them readonly
2. In between shadow updates, use "vos clone" to get a COW snapshot of
the surrogate, so one or a few snapshot versions are available
3. Put the clones in VLDB (but I didn't look at what the entries look like)
There are issues you run into quickly with cloning. One is, limit of 4
(?) non-traditional clones of a volume stored on a namei fileserver.
You can make as many volume copies as you like, at the cost of the disk
to store them. I think there's a bit more to do here, but _I think_
that's the state of things at present. I was going to ask Jason for
more details on his sequence of operations, but I didn't get around to it.
Christopher D. Clausen wrote:
>Someone recently posted a patch (to openafs-devel) about shadow volumes.
>What exactly are shadow volumes? When and why would I want to use them?
>I found no mention of them in the IBM documentation, on the AFSLore wiki
>or anything more than just the name itself mentioned in the openafs-info
>Any additional info would be appreciated! Thanks!
>Christopher D. Clausen
>OpenAFS-info mailing list
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