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Choosing the right replication method

We are looking at implementing two Dell PowerVault NAS devices and each PV would have 800GB capacity. The plan would be to have one live server in our main office and a mirrored copy of this server replicated into a remote site. I'm not sure if the NAS devices would replicate instantly or once a day though.

I never see the issue of bandwidth requirements being addressed in relation to mirroring NAS. All I ever see quoted is "substantial bandwidth." Any ideas of what kind of bandwidth I would require for this purpose?
Remote replication is one area where there are many differences between the vendor offerings. Some systems use internal replication functions that work with only like systems over an IP network to mirror data. Others use software from other vendors such as NSI DoubleTake loaded on the NAS box to perform the replication. The replication can be done synchronous or asynchronous.

With synchronous, the data is sent immediately and the application will wait until data is on the remote site. With asynchronous, data is sent and the application can be released so that the copy may fall behind in getting the data on the remote side. Some implementations do periodic copies where the differences only are sent on intervals to minimize the amount of data transferred and the amount of bandwidth required.

The bandwidth is primarily a cost issue -- how much do you want to pay to keep the data at the most current level. It is more a cost issue than a system limitation. Because there are many different ways to do this and many implementations, you need to understand what your requirements are from a business risk standpoint and how much data change will have to be replicated.

If the business risk issues are low, then a periodic copy with an adjustable interval transmitted over an IP connection may be adequate and the least cost. If currency is the main issue, then dedicated, high bandwidth lines may be needed.

There's no simple answer but get your requirements down and then look at the analysis of the individual products to understand whether they will meet them or not. Again, this is an area of high variation amongst vendors. I don't believe the Dell systems provide a remote replication capability as a basic function but offer NSI DoubleTake as an extra charge option so you would either have to add that, use a host program from another vendor, or go to a different solution.

Randy Kerns

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