Believe it or not (or simply check Wikipedia), Solid State Drives (SSDs) have been around in their current form for nearly thirty years, however it's only within the past two years that they've really managed to gain any ground in more mainstream corporate usage. In this article I'll be discussing the various aspects of these drives, and when and where to use them.

Proving that everything is a compromise, these insanely high performing drives do have some disadvantages. The most obvious drawback of SSDs are the cost, but durability and sequential writing are also drawbacks to these devices which we'll investigate over this two part entry.

Buyer Beware

The most highly touted limitation around SSDs are their limited shelf life. As each bit is written to the drive, it's stored in an element called a "cell". These cells can only be written to a certain number of times before their performance starts to degrade, and while some drives can accept as many as 100,000 writes (SLC drives) other drives (MLC) can be written to as little as 1,000 to 10,000 times before a cell becomes unusable.  As a result most drives are under allocated, so a 69GB disk is physically 100GB of capacity, the remaining 31GB is used as spare capacity that is gradually used up as cells degrade and are replaced by a process called "wear levelling". Therefore bear in mind that the drive *will* fail a some point, so you should always have a hardware refresh planned for these disks.

There are actually two main types of SSD on the market at the moment, Multi-Level Cell (MLC) and Single Level Cell (SLC). In SLC drives each "cell" or element in an SSD is set to either one or zero, a straight forward binary setting we're all familiar with. However MLC drives can set up to four levels in a single cell, providing not just a one or zero setting, but a "two" and "three" as well.  This means that MLC drives can store more data per cell, resulting in larger capacities per chip. This would seem like a good thing however it comes at a cost. First, because the cells are not binary they need to be translated to and from the binary format that the computer understands. Also, because the cell has several settings, in terms of tolerance and setting it's like trying to adjust your shower temperature control every time you set a bit. The settings need to be fairly precise to store a definitive value, get it wrong and you're not sure if you have set a "2" or a "3", resulting in corrupt data. As the cell ages this setting becomes harder to achieve. As a result MLC drives are slower performing and have a shorter life, but they are however cheaper as a result. These MLC drives are currently available for things like the desktop and home PC market, where cost is a greater priority than reliability and performance. Meanwhile SLC drives are usually used in enterprise level storage and systems, and while much more reliable and faster, are around five times the cost.

SSD Performance Comparison

To give you a rough idea of what kind of performance to expect from the solid state disk, the following basic statistics are provided below.

• Hard Disk Drive (HDD) between ~120 to 200 I/O per second (IOPS)
• Solid State Drive (SLC SSD) is 28,000 I/O per second

Performance Throughput Sustained:

• Solid State Drive is 220 MB/s Read - 122 MB/s Write
• Hard Disk Drive (15K RPM) is 125MB/s

As you can clearly see by the above figures, when you've got something IOP driven like a database with small, random IO, the SSD is clearly the better performing drive. However if you've got something more sequential, such as media files or a single redo log, SSDs have a very similar performance to normal HDD drives. Therefore you have to be really careful about where you use SSDs, otherwise it's very difficult to justify their additional cost.

You'll also notice that it takes twice as long to write as it does to read, this is because the cell has to be "wiped" before it can be rewritten to. So one has to be careful using SSDs in environments with heavy write activity.

In the next part of "Solid State of the Art?" we'll look at where to consider using Solid State Drives in more detail.