The question of whether or not to do synchronous or asynchronous replication between storage arrays does not come up often but I suspect it will as more and more people expand their business continuity infrastructure.  It’s an important question because it can have a serious impact on the production environment.

With EMC’s Mirrorview/S (sync) there is a distance limitation of between 50km and 200km depending on what fibre optics you are using (short/long wave vs. dwdm).  Mirrorview/A (async) is more widely used over an IP WAN connection but can also be used over fibre as well.

Mirrorview/S -

Pros:

• Synchronous – Exact copy of data on production
• Little to no data lost

Cons:

• Distance limited (60km using short wave gbics, long wave gbics or optical extenders, 200km using dense wave division multiplexors)
• WAN link more expensive (fibre vs. copper/ip) unless Fibre Channel over IP converters are used and those are still a little expensive

Mirrorvew/A -

Pros:

• Cheaper WAN link between sites (IP usually)
• Writes to prod don’t have to wait on mirror site to write
• Not distance limited like sync replication

Cons:

• Data can be lost depending on write intervals from prod to DR site

What you need to know -

Array-based mirroring is a great way to protect multiple hosts in an environment instead of buying per-server or per-application replication.  As I’ve discussed before, the biggest drawback is that it provides a restartable copy which isn’t the same as an active-active cluster application transaction-level replication (i.e. Oracle Dataguard, Exchange CCR, MySQL Master/Slave replication).  Be careful of adequate LAN/WAN line quality, poor comm lines can cause insanely painful headaches (troubleshooting, added latency, etc).  Get line tests done to determine available bandwidth, line quality and latency.

Comments welcome.