On Thu, Apr 26, 2012 at 5:45 PM, Carson Gaspar <car...@taltos.org> wrote:
> On 4/26/12 2:17 PM, J.P. King wrote:
>> 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.

Sure, if we assume apps make proper use of O_EXECL, O_APPEND,
link(2)/unlink(2)/rename(2), sync(2), fsync(2), and fdatasync(3C) and
can roll their own state back on their own.  Databases typically know
how to do that (e.g., SQLite3).  Most apps?  Doubtful.

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