On 4/26/12 2:17 PM, J.P. King wrote:

Shared storage is evil (in this context). Corrupt the storage, and you
have no DR.

Now I am confused. We're talking about storage which can be used for
failover, aren't we? In which case we are talking about HA not DR.

Depends on how you define DR - we have shared storage HA in each datacenter (NetApp cluster), and replication between them in case we lose a datacenter (all clients on the MAN hit the same cluster unless we do a DR failover). The latter is what I'm calling DR.

That goes for all block-based replication products as well. This is
not acceptable risk. I keep looking for a non-block-based replication
system that allows seamless client failover, and can't find anything
but NetApp SnapMirror.

I don't know SnapMirror, so I may be mistaken, but I don't see how you
can have non-synchronous replication which can allow for seamless client
failover (in the general case). Technically this doesn't have to be
block based, but I've not seen anything which wasn't. Synchronous
replication pretty much precludes DR (again, I can think of theoretical
ways around this, but have never come across anything in practice).

"seamless" is an over-statement, I agree. NetApp has synchronous SnapMirror (which is only mostly synchronous...). Worst case, clients may see a filesystem go backwards in time, but to a point-in-time consistent state.


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