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DAS, NAS and SAN replication solution

What software should I be looking for that can replicate among DAS, NAS and SAN storage? I need both mirroring and snapshots. I'll need a network-based solution because I have many servers with at least three different platform types and storage vendors haven't been determined yet.

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I doubt there is any one product or vendor that can work for you. The main problem is that NAS, snapshot and replication products tend to be proprietary in nature and replication is often platform-specific.

The following may help you think through this. I think of mirroring, snapshots and replication as three different functions:

1. Mirroring is duplication of data by creating two I/Os from a single I/O. Disk mirroring is created on host systems through OS or volume management software. Disk mirroring is a local option that depends on the platform and the local connectivity characteristics. Certainly works with DAS and SAN and most NAS products support it.

Store and forward style mirroring disk subsystems (for example, EMC SRDF, IBM PPRC, Hitachi TrueCopy) are mostly for SAN products. I don't know of any NAS products that support it.

2. Snapshots comes in three basic flavors: File system based (NetApp), subsystem based and volume manager/virtualization based. All three are considerably different. It's too much to go into in this forum. I think snapshots are an extremely important function for business continuity, but there are a lot of details to work through. You need a strategy for snapshots as well as a decent understanding of how you will establish operations to work with them. They will change your daily operations and they require constant, ongoing administration. Platform specific operations for flushing cache (file system buffers) matter a whole lot.

3. Replication (to me) is the transport of data objects (files -- tables) over a TCP/IP network. The transfer is made from system to system not between storage devices or subsystems. These tend to be platform specific also so a replication product for Windows 2K might not work as well as you hope (if at all) for Unix platforms or Linux.

I tend to think of replication as a less-capable, less expensive form of snapshot. It's not quite as good for business continuity but it's a lot better than nothing. And, if the data is not changing THAT much, it can be a good solution.

The cost of bandwidth will determine a great deal about what you'll eventually be able to do.

Regards,
Marc

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This was first published in April 2003

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