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The right way to provision storage

Ezine

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LUN sizes
Administrators have to make decisions at numerous points in the storage provisioning process. These decisions revolve around such things as the specific RAID level, the size of the LUNs, as well as zoning, masking, striping and concatenating LUNs. For example, if the application requires mirroring for protection, you'll need to provision double the amount of storage and set up the necessary paths.

In general, experts recommend provisioning LUNs at least 30% to 50% larger than the expected size. "This is not wasting capacity," says Jerry Namery, chief technology officer at Winchester Systems Inc., Burlington, MA. "Especially when you're provisioning for database performance, this leaves room to move things around without hitting a performance wall."

Depending on the vendor, you may have more or fewer choices. Let's use LUN sizing, a storage admin's primary decision, as an example. Ideally, administrators want to size the LUN on the basis of performance requirements, efficient capacity utilization or protection. To provide greater performance, for example, you need to provision the LUNs to encompass more spindles.

For example, Forsythe Solutions Group's Arland might recommend provisioning 100GB LUNs on a midrange array for database performance. But he would stripe that 100GB across an entire 16 to 24 disk RAID group, which would allow plenty of spindles for performance. "For performance, you want smaller drives and more spindles in your

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RAID group," he says.

On the other hand, "big LUNs--anything over 200GB--I would reserve for low-performance applications and archival storage and use fewer spindles," adds Arland. In this case, the RAID group might consist of just four to eight drives.

The goal of sizing is twofold: getting the right amount of appropriate storage while "avoiding being stuck with small fragments of storage which kills utilization," says Namery. The trick is to use smaller LUNs, which leave you more flexibility. Fragmented storage happens surprisingly often, usually when someone starts provisioning the array without mapping it on paper first.

LUN sizing also differs from vendor to vendor. "It helps to know which array you're using. Some of the enterprise storage systems allow less freedom of choice when it comes to LUN size than midrange arrays," says Datalink's Anderson.

Similarly, vendors have different limitations on internal striping of the drives. "The high-end Hitachi arrays don't do internal striping," continues Anderson. "You can still do RAID but not RAID 0." If his client needs RAID 0 and the array won't do it, Anderson provisions a smaller LUN and stripes at the host.

The largest enterprise arrays, originally intended for use in mainframe environments, will often come preconfigured with many small LUNs, around 8GB to 9GB. "The high-end arrays like EMC's Symmetrix DMX are designed to work best with small 8.4GB LUNs," says GlassHouse's Weinstein. "It's built into the design and the microcode of the array. The best practice then is to configure the entire array at once into many small LUNs."

These small LUNs can be concatenated into larger LUNs, adds Arland. In contrast, midrange array vendors may sell the array already cut into large LUNs, often 200GB in size.

With midrange arrays, the best practice is to configure an entire row or shelf of drives at one time, striping the LUNs across multiple spindles, says Weinstein. But don't feel restricted to small LUNs. In the end, "performance has less to do with the size of the LUN than with how many disks it's spread across," he concludes.

When he was at Nielsen Media Research, Stevenson often relied on the prebuilt LUNs that came with the array. "We knew their I/O capabilities," he says. The prebuilt LUNs worked well with Nielsen's standard RAID 5 configuration. "The only question left was how many spindles to put in the RAID set," adds Stevenson.

This was first published in October 2006

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