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Putting up a SAN in a phased implementation

We are looking at putting up a SAN in a phased implementation. Currently, we have four "divisions", each with an independent set of servers supporting approximately 2000 users. We first want to consolidate servers and storage within each division using SCSI-based attached storage then re-use the SCSI storage cabinets and servers in a SAN and server cluster, using a FC-to-SCSI router or bridge. Any expansion of the SAN beyond the SCSI storage cabinets would be with FC storage. Is this a viable solution?

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Sure is, but it's going take a heck of a lot of bridges! Most bridges today come with one or two FC ports and around four FWD high-density SCSI ports. You can put around 15 disks per SCSI bus (watch performance though), and there will be no RAID capability if you attach the disks directly.

What I would do is put a bunch of RAID SCSI adapters in the PCI slots of two very fast servers, use "Y" adapters on the ends of the busses with external termination and SHARE the busses with another node using an MSCS cluster to share out the SCSI storage as file shares. This would give you RAID, and high availability for the SCSI storage, and would create your own highly available NAS appliance. Another method would be to purchase host-based virtualization hardware or software, and let the virtualization engine share out the connected SCSI disks. (The bridges can be used in this approach also).

Also, Compaq has some cool software called "Virtual Replicator" that will let you serve out your host-based SCSI disks to client servers over IP. The disks look and feel as if they are physically connected to the client servers and applications can be installed on them just as if they were local to the client. You can create a storage POOL on single server of up to 2 terabytes in each pool using this approach.

You may find it beneficial to put out an RFP (request for proposal) to the major storage vendors for your environment. You may find they will be willing to "buy back" some or all of your SCSI disks to help you reduce the cost of an initial SAN investment. The age of the disks will determine the total of the buy-back.

Chris

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This was first published in April 2002

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