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Choose the right iSCSI array

Once you've decided that iSCSI storage fits your needs, it's time to choose the heart of your SAN: the disk array. There's an iSCSI array out there for just about everyone.

In our last look at iSCSI, Solving the riddle of data storage with iSCSI, we saw that consolidating your storage into an iSCSI SAN can improve your disk space utilization and simplify your life when you need to provision new servers or your servers run out of space.

Now that you've decided that an iSCSI storage area network would make your life easier, it's time to choose the heart of your SAN: the disk array. Today you can buy iSCSI arrays from vendors including iSCSI specialist startups EqualLogic Inc. and LeftHand Networks Inc. and EMC Corp., the 800-pound gorilla of the storage world. iSCSI array prices range from $5,000 to several hundred thousand dollars.

The first thing you need to do is figure out how you plan to use your new array. Do you need 3 TB for a disk-to-disk backup target or storage for your new Exchange or SQL Server cluster? Do you need storage for a few servers, or are you implementing a SAN so you can support a hundred blade servers for your Web and terminal server needs? The answers to those questions will help you put together a list of features your iSCSI array must have to support your servers.

Disk space is key to success

For a disk-to-disk backup system all you really need is lots of space. You could use a basic iSCSI array like Promise Technology Inc.'s Promise VTrak 15200 that simply presents its RAID sets as iSCSI logical units (LUNs). Since, with few exceptions -- like clustered servers -- an iSCSI LUN can only be accessed by one host server at a time, these systems can only handle a few host server connections that need large blocks of disk space.

Smarter systems let you create one or more RAID sets and then slice and dice LUNs from them. You can then create a 1.5 TB RAID-5 set out of seven 250 GB drives and assign 50 GB to one server and 800 GB to another leaving the rest of the space free for ad hoc or development servers. Remember to check how many servers a prospective array will support. Some, like the Dell/EMC AX100i are limited to as few as eight servers, which would make them the wrong solution for a Web server farm of 50.

Make the most of snapshots

We frequently recommend SANs for our clients running Exchange, SQL Server and other transactional systems to take advantage of a SAN array's snapshot capabilities. In the event of a server crash or a corrupt database, you can restore the server in minutes to it's state at the time of a snapshot rather than the hours it could take to restore a large database even from a disk-to-disk backup. In addition, you can take snapshots every hour while the server is active so your data loss, or the amount of logs you need to roll forward, is substantially smaller.

Consider how your structure will grow over time

Next, look at expandability. While traditional systems, like EMC's Clariion CX series, let you add capacity by hanging additional shelves of drives off the controller module, you still have to buy a controller big enough to handle your eventual growth or face a costly forklift upgrade when you outgrow it. Nashua, N.H.-based EqualLogic, San Jose, Calif.-based Intransa Inc. and LeftHand Networks, in Boulder, Colo., have a better idea. Their systems allow multiple drive arrays and controllers to form a single virtual array. As you add arrays, you also add controllers, with their cache memory and gigabit Ethernet interfaces increasing system performance. That allows you to start small and grow to 100 TB or more without the forklift.

If you're really on the consolidation bandwagon, you can combine NAS for file storage and iSCSI for block storage on a single integrated device. Check out Snap Appliance, a division of Adaptec Inc., Hewlett-Packard Co. and others using FalconStor Software Inc.'s iSCSI storage server for Windows to Network Appliance at the high end. Conventional wisdom says that your iSCSI traffic should be isolated from general network traffic, so make sure that an integrated system has enough gigabit Ethernet interfaces to provide the level of redundancy you need on both networks.

Backing it all up

Finally consider replication. Some iSCSI arrays, including those from EqualLogic and Intransa, support snapshot replication where an array at your primary site takes periodic snapshots of your sensitive data LUNs. Then, it replicates the changed blocks to another array at your disaster recovery site. Others, including EMC's CX500i, support synchronous replication, where all changes to the primary array are sent to the secondary array in real time. Others, like those running LeftHand Networks' SAN iQ, can do either.

There's an iSCSI array out there for just about everyone. Figure out what's important to you, talk to some vendors and I'm confident you'll find one that fits your needs.

Howard Marks has been in the personal computer industry from the very beginning. Starting at Lifeboat Associates in 1979, he founded Networks Are Our Lives in 1981 as a builder of custom single- and multi-user CP/M computers. Today, NAOL focuses on network design and documentation, storage consolidation and management projects, and it generally helps organizations clean up their network messes.

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