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Solid State Disks (SSDs)

AssuredSAN solutions allow SSDs to be used as a tier of storage where you need the fastest access to specific data. 

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Introduction to SSD’s

Solid State Drives, generally referred to as “SSDs”, have the same physical sizes and interfaces (such as SAS, FC and SATA) as Hard (or “spinning disk”) Drives, but instead of a spinning disk that holds the data, the information is stored on non-volatile memory chips. The advantages of SSDs are lower power consumption, more rugged, and higher performance. The disadvantages are higher cost per gigabyte, lower maximum capacity per drive, and a shorter write cycle life.

Market interest in SSDs has been growing in response to increased support of SSD technology from major operating systems. Microsoft Windows 7 now has native support for SSDs, removing some of the traditional mechanical rotating media assumptions when dealing with volumes (e.g. data defrag, etc). Sun’s ZFS is also catering for SSDs by assigning them a special “cache” status.

No Moving Parts

Since there are no moving parts, the power usage for SSDs is much lower than traditional hard drives. They also do not have a “power up spike” from restarting spinning disks. Like USB memory drives, SSDs do not need power to retain data other than what may be used to support an on-drive read/write cache.

With no moving parts, SSDs are not affected by shock and vibration which impact spinning disk drives. They also do not have air density issues and can operate at high altitudes.
Since access to all data points is immediate, read performance of SSDs can be significantly faster than traditional hard drives; this is considered the primary benefit for SSDs. Most models also perform better at writing as well, although not as dramatically.

Cost and Capacity of SSD’s

The cost per GB of SSDs is many times higher than even high-performance 15,000 RPM SAS drives due to the cost of the memory chips that are used to create SSDs. SSD technology has actually been available for decades, but has been cost prohibitive for all but niche applications. The recent rapid decline in prices is what is making this technology available to a broader customer base.

While the memory capacity per chip is growing rapidly, so is the storage density of disk drives. The majority of the market for SSD technology is in 2.5″ or smaller drives, limiting the number of chips than can be used for a single drive. Using MLC technology (detailed elsewhere), 2.5″ Solid State drives are reaching 300+ GB in 2010, but this is still small by Hard Disk Drive standards.

“Hot spots” and “Write Cycle Management”

Memory chips have a much more limited ability to “flip the bit” (write a 0 or 1) than disk drives, and may have a shorter functional life. The risk is that memory locations in the drive will have more active writes and create “hot spots”.

This concern is being addressed with firmware in the drives to balance write cycles across the drive as well as caching heavy write activity. Modifications to operating systems, for example to automatic disk optimization and other maintenance functions, may also be able to extend the SSD’s life. The quality of write cycle management is one of the areas Dot Hill evaluates when qualifying these new drives.

Applications:

  • Higher number of attachments for streaming video
  • Setting up ‘tier 0′ vdisk as a large data cache for high performance for transaction processing.
  • Government applications or military applications. SSD’s higher tolerance for shock and vibration high altitude, and temperature ranges may eliminate the need for hermetically sealed drives.
  • Demanding transactional eCommerce sites. SSDs can substancially improve web page recall speed and increased number of customer transactions per storage unit.
  • Used with Sun servers where Solaris ZFS has a method for allocating SSD’s as a storage cache pool in an external storage array such as Dot Hill’s 2732 with SSD drives.