On 07/10/12 05:26 AM, Brian Wilson wrote:
Yep, thanks, and to answer Ian with more detail on what TruCopy does.
TruCopy mirrors between the two storage arrays, with software running on
the arrays, and keeps a list of dirty/changed 'tracks' while the mirror
is split. I think they call it something other than 'tracks' for HDS,
but, whatever.  When it resyncs the mirrors it sets the target luns
read-only (which is why I export the zpools first), and the source array
reads the changed tracks, and writes them across dedicated mirror ports
and fibre links to the target array's dedicated mirror ports, which then
brings the target luns up to synchronized. So, yes, like Richard says,
there is IO, but it's isolated to the arrays, and it's scheduled as
lower priority on the source array than production traffic. For example
it can take an hour or more to re-synchronize a particularly busy 250 GB
lun. (though you can do more than one at a time without it taking longer
or impacting production any more unless you choke the mirror links,
which we do our best not to do) That lower priority, dedicated ports on
the arrays, etc, all makes the noticaeble impact on the production
storage luns from the production server as un-noticable as I can make it
in my environment.


Thank you for the background on TruCopy. Reading the above, it looks like you can have pretty long time without a true copy! I guess my view on replication is you are always going to have X number of I/O operations and now dense they are depends on how up to date you want you're copy to be.

What I still don't understand is why a service interruption is preferable to a wee bit more I/O?

--
Ian.

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