The Skinny on Solid State Disks

September 22nd, 2009 3:54 pm
Posted by Douglas Eadline
Tags: , , , , ,

What you need to know about the latest trend in storage hardware

The Solid State Drive (SSD) has had a break-out year. Unlike traditional mechanical hard disk drives that use spinning platters with movable read/write heads, SSDs have no moving parts. The SSDs is made entirely out of a special type of flash memory -- the same kind of NAND flash memory found in thumb-drives and memory sticks. Overall, SSDs are faster, quieter, more energy efficient, but less dense than the traditional spinning platter drive.

The staple of the storage industry has been the mechanical drives, where I/O rates are limited to the mechanical properties of a drive. Unlike other semiconductor trends, the sustained write rate has barely doubled from 50 to 90 MB/second over the past 8-10 years. All this is about to change as the use of flash memory will allow stroage to take advantage of a semiconductor growth curve similar to that of processors and memory. (i.e. mechanical drives are limited by the physical motion of spinning disks).

Perhaps the most important feature offered by SSDs is the read and write performance. The IOPS (I/Os per second) rate for an SSD is usually two to five times that of a traditional mechanical hard drive. When reading, performance is mostly constant because the seek time is virtually instantaneous and does not depend on the physical location of the data on a platter. As a result, file fragmentation has almost no impact on read performance. In addition, because there are no moving parts, SSDs use as little as one-fifth the power of a mechanical drive. Another interesting feature is the SSD failure mode. Most SSD failures tend to happen when writing. In contrast, mechanical drives tend to have most failures when reading. Thus, once data is written, it is more likely it can be read from a failed SSD.

SSDs do suffer from “degradation” over time that results in reduced performance and limited lifetimes (i.e. there are a limited amount of read/write cycles avaliable for NAND memory). Vendors have taken this into account and include wear leveling algorithms in SSDs that spread write access evenly over the entire device.

If you are interested in the exploring SSDs, there are some key points to consider. First, SSDs are not the best solution in every case. Currently, their capacity is much less than that of traditional mechanical drives and as such may not be suitable for some of the large HPC data sets. In addition, the cost is per MB is higher and are more susceptible to data loss from energy and power surges.

Second, the very fast read times offered by SSDs has made them a good candidate for improving OLTP (Online Transaction Processing) systems where frequently read tables and indexes can be accelerated. Check both the read and write IOPS, as there is usually a big difference between these values. Typically, the read speed is 10 times the write speed resulting in asymmetric performance. In terms of clusters, using SSDs for NFS mounts or read-only data may be helpful.

Third, because SSDs are new, questions still remain about how much of that speed they can deliver for the long haul (due to degradation). Typically, an SSD will show an initial decrease in performance and then level off. Even with a performance drop over time, SSD drives are almost always faster than traditional hard drives. The JEDEC standards organization plans to publish two standards by the end of this year for SSD endurance metrics.

Finally, pay particular attention to “write endurance,” this number should pertain to random writes. For instance, an Intel® X25-E Extreme 64 GB SATA Solid-State Drive is rated for 2 petabytes of lifetime random writes.

In terms of software, one difficulty facing the industry are the legacy assumptions built into file systems. These assumptions will need to be challenged in order to take advantage of SSD technology. For instance user applications and file systems will need to account for the asymmetric read/write performance of SSDs. Many computer applications rely on synchronous patterns of read/write operations, wherein a given write or update must be completed and the write confirmed before additional application read requests can be issued. With SSDs this process may need to be reconsidered.

There is no doubt that SSDs are the future of storage. Indeed, SSDs are even changing the way we compute. For example, CAE (Computer Aided Engineering) applications can use the speed advantage of SSDs in their out-of-core algorithms. The power of semiconductor manufacturing technology combined the speed of NAND flash memory are about to make the storage market stand still!

Comments

Pingback from Cluster Connection » The Skinny on Solid State Disks | All About Solid State Drives (SSD)
Time September 22, 2009 at 9:29 pm

[...] more… Tags: disk-drives, made-entirely, mechanical-hard, solid state drive, solid state hard drives, solid-state, spinning-platters, write-heads [...]

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Author Info


Dr. Douglas Eadline has worked with parallel computers since 1988 (anyone remember the Inmos Transputer?). After co-authoring the original Beowulf How-To, he continued to write extensively about Linux HPC Clustering and parallel software issues. Much of Doug's early experience has been in software tools and and application performance. He has been building and using Linux clusters since 1995. Doug holds a Ph.D. in Chemistry from Lehigh University.