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From Robert Coli <rc...@digg.com>
Subject Re: Ec2Snitch & Other snitches...
Date Tue, 22 Mar 2011 16:05:05 GMT
On Tue, Mar 22, 2011 at 7:19 AM, Sasha Dolgy <sdolgy@gmail.com> wrote:
> More, I suppose the question I'm after is, can the snitch method be
> adjusted adhoc (with node restart) or once it's changed from
> SimpleSnitch to Ec2Snitch that's it?

You can change Snitches on a cluster with data on it, as long as you
are very careful about what you are doing and you are in a particular
case which you are probably not in if you want to change your Snitch.

The snitch meaningfully determines replica placement strategy, and in
general when changing snitches you need the replica placement strategy
to stay exactly the same. Unfortunately the point of changing a snitch
is usually.. changing your replica placement strategy. Simplest case
is if the replica placement strategy actually stays the same, like for
example when Digg replaced its custom version of the
PropertyFileSnitch with SimpleSnitch in prep for going single-DC,
because we weren't actually using the functionality of PFS. In that
case, I simply generated a set of input which hashed correctly such
that I had one piece of input per node. I then verified the topology
based on this input before and after changing my snitch, and got the
same results both times, confirming that my change of the Snitch was a
no-op.

A less simple, but still tractable case is if the topology changes
such that one or more replicas is different but at least one is still
the same. In this case, repair would be likely to repair.. most.. of
your data. But honestly if you have to change strategy that much (and
are not running IP-partitioned counts, which make this operation much
more difficult) you probably just want to dump and reload your data
into a new cluster which has the topology and snitch you want.

=Rob

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