by Joe Gleinser
16. February 2010 23:35
Moving large amounts of data offsite is a difficult thing to do. It is difficult to beat tape drives and an automobile for rapid data replication. Obviously this method has serious limitations. With the rise of Shared Storage, most replication today occurs at the Storage layer. Though application layer replication offers major advantages, it is usually pursued in addition to storage replication.
What is storage replication? Most SANs and many NAS devices offers licensed replication as a feature. GCS' two favorite devices, the HP Lefthand and Dell Equallogic, include these licenses at no cost. By placing a SAN in-office and one at a remote data center, then connecting the sites with sufficient bandwidth, the SANs will replicate data between the two at the block level. This is an efficient method. Unfortunately, the SANs cannot differentiate between legitimate data changes (ie, a saved Word doc) and temporary data changes such as SQL log files. Everything is pushed across the connection. This tends to require large amounts of bandwidth.
The HP Lefthand offers both asynchronous and synchronous replication options. With asynch replication changed data is pushed at scheduled intervals. With synchronous replication the devices attempt near real-time replication. The HP Lefthand only offers synchronous when there is less than 20ms of latency between the sites. The HP Lefthand also offers bandwidth management on the SAN to help restrict the amount of bandwidth consumed on your site-to-site connection.
A readily apparent benefit of SAN replication is virtual machine replication. If my VMs are stored on my SAN, they are replicated to my remote sites. This dramatically reduces recovery time in a variety of outages.
GCS also recommends Microsoft DFS-R for replication of file stores. Simple, reliable and efficient, this technology allows Windows NAS devices to replicate between sites and provide failover in the event of an outage.