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.
