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LAN And SAN Unite: Page 4 of 4

PHYSICAL FITNESS
The problem with hard disks isn't with data capacities or bus transfer rates; both have increased exponentially while costs have gone down, a trend that's likely to continue. It's with physically moving the drive's heads across the disk surface, a mechanical process that needs to be repeated every time a new file is read or written. The fastest drives take about 4 milliseconds to do this, meaning that they're limited to about 250 operations per second. Moore's Law doesn't apply to moving parts like actuators and drive heads, so this number isn't likely to increase.

Meanwhile, servers are pounded by an exponentially growing number of requests. Rich Internet applications and always-on remote connections make the problem worse, as clients request data in small, frequent chunks that each require the drive to move. Conventional solutions--massive overprovisioning or caching data in RAM--are expensive and power-hungry. Though dynamic RAM uses less power than constantly spinning hard disks of the same capacity, the catastrophic data loss in the event of an outage means that it can't be used alone. Volatile DRAM can only augment hard drives; it can't replace them.

Flash memory doesn't suffer from the same problems as disk drives: It has no moving parts, so the number of operations per second is measured in the thousands. And compared with both hard drives and dynamic RAM, it requires very little power. Its weakness is that it wears out even faster than hard drives: A flash drive can only be rewritten at most 100,000 times before it dies--not an issue for consumer electronics or laptops that only access the disk occasionally, but a serious problem for servers whose storage is in almost constant use.

"Flash has limitations, very much so," admits Amyl Ahola, CEO of Pliant Technology, a startup aiming to make flash memory competitive with hard disks in storage targets. "At the enterprise level, it's a nontrivial task to put an architecture together to get lifetimes that are at least as long as a hard drive."

As yet, there's no way to make a flash drive with infinite rewritability. STEC, whose 73-GB and 146-GB flash drives are used in the initial EMC array, gets around the limitation by using a technique called wear leveling. This distributes write operations evenly over an entire disk surface, ensuring that all parts of the chip will wear out at the same time. It estimates that this extends the lifetime to about 2 million write cycles.

Pliant has extra error-correction techniques that it says will extend the life of flash drives even further, building an ASIC that will detect and avoid parts of a flash chip that are about to fail. Its products are still in the lab, expected to ship by the end of the year through major storage OEMs. The big downside is price: Pliant estimates that drives will cost around $20 to $30 per gigabyte. For hard disk prices, use the same numbers but change the dollars to cents.

Physical Network Options In A Converged Network
  Pros Cons
Ethernet
>> Well-understood by LAN architects
>> All IT organizations will need it anyway
>> Enables file-based policies for storage management
>> 10-Gbps gear is still expensive, slow compared with competitors
>> Key standards still in flux
Fibre Channel
>> Widespread support from storage vendors
>> Little support outside the world of storage
InfiniBand
>> Lowest overhead and latency
>> Proven in high-performance computing and remote memory apps
>> Still seen as a niche technology

Still, flash memory prices are falling all the time, and the technology could pay for itself. Pliant estimates flash memory eventually will cost around the same as hard drives, once the overprovisioning necessary for high-performance applications is taken into account. The lower numbers of drives needed for flash also translates into savings in space and maintenance, as well as much lower power consumption.

Even if a migration from hard drives to flash memory eventually reduces the size and number of storage targets, there's little sign that they'll be moving back inside servers. The flexibility inherent in virtualization means that storage resources increasingly will be abstracted away from applications, which won't know or care whether their data is stored locally, on a hard disk, or in flash memory. Virtual servers will need virtual storage, with a virtual network in between.

Illustration by Christoph Neiman

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