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Tiered storage: Heterogeneous vs. homogeneous

Ezine

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Variable protection
One recent approach to bringing prices down is to vary the RAID levels within the array. Arrays have long had the ability to protect some disks with RAID-1 and others with RAID-5. RAID-1 has 50% overhead, meaning that each disk has another used just for protection. RAID-5 has much less overhead, usually 20% or 25%, but sometimes as low as 11%. The overhead depends on the proportion of data volumes to parity volumes in a RAID set. A typical RAID-5 set containing four data disks and one parity disk would have 20% overhead and would be expressed as a 4+1 set.

HDS has long offered both RAID-1 and RAID-5 (3+1 and 7+1) in its Lightning arrays, but IBM Corp. only recently introduced RAID-10 in addition to the usual RAID-5 on its Shark. EMC Corp. used to be reluctant to offer levels of protection below mirroring. Its old RAID-S was roughly equivalent to 4+1 RAID-5, but was rarely used. Recently, EMC added parity RAID to the DMX series which is analogous to 3+1 or 7+1 RAID-5. They now claim that the majority of Symmetrix DMX systems use at least some parity RAID.

These arrays also have a variety of lower-performance bulk disk available. EMC and HDS claim to support dynamic conversion of LUNs between RAID-1 and RAID-5 protection. This allows an administrator to modify the protection without the downtime associated with migrating storage between different arrays.

Given a base

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cost of $50,000 per terabyte for raw storage, one terabyte of RAID-1 will cost $100,000, while a RAID-5 (4+1) set would cost just $62,500. A larger RAID-5 (8+1) set would lower this to $56,250, but would offer much less protection. This sounds great, but it's not the whole story. Fifty percent of the cost of an enterprise storage array is nonhardware, including software, maintenance and implementation services. Plus, there's a steep base price for the storage array itself before taking the cost of disks into account. For this reason, the savings of varying RAID levels can prove illusory. If physical disks make up just 25% of the cost of storage, saving 50% only shaves 12% off of the total cost.

One newer vendor rates a mention here. 3PAR, Fremont, CA, has a completely virtualized array--the InServe--that not only supports multiple RAID levels but can dynamically reconfigure volumes to use different levels. It allows an administrator to vary the protection, cost and performance of an individual volume. What makes this different from the leading arrays is 3PAR's low entry point and scalability. The InServe can start with just two controllers and grow to include eight. 3PAR promises a base price similar to the traditional midrange arrays and capacity well beyond the big boys of the enterprise space. They also have a feature called thin provisioning that can vastly reduce the storage required, but that's beyond the scope of this column.

Even if varying RAID protection levels are often less than rewarding, the benefits of keeping all of an enterprise's storage on a single type of array are significant. Put a network-attached storage (NAS) head on an enterprise array and all three tiers of storage can be hosted on a single storage platform. This makes management simpler, reducing the need for cross-training and heterogeneous management software. And the ability to seamlessly move data between tiers by varying the protection applied to it is tremendously important to maintaining a utility model. Who would want to change storage tiers if the change required an outage? "Homogeneous tiered storage" on this page shows what this environment would look like.

Your choice depends on your desired outcome. If you are concerned primarily with reducing capital costs, then the traditional tiered strategy is for you. Midrange arrays--especially with the introduction of ATA disks--will always beat enterprise arrays on price. But if your environment is dynamic or you require the simplest management possible, then the enterprise array approach is definitely worth a look.

This was first published in December 2003

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