Solid state drives or SSDs are the latest in high performance storage for computer systems. They offer much higher data transfer rates than traditional hard drives while consuming much less energy and also having greater levels of reliability thanks to no moving parts. These attributes make them extremely attractive to those using mobile computers but they are also starting to make their way into high performance desktops as well.
Features and performance can vary greatly in the solid state market. Because of this, it is very important to consider things carefully if you are looking to purchase a solid state drive for your computer. This article will take a look at some of the key features and how they can impact the performance and cost of drives to help buyers make a more informed purchasing decision.
Capacity is a fairly easy concept to understand. A drive is rated by its overall data storage capacity. The overall capacity of solid state drives is still significantly less than what can be achieved with traditional hard drives. The price per gigabyte has been steadily dropping such that they are more affordable. Still, this can cause issues for those that want to store a lot of data on their solid state drive. Typical ranges for solid stat drives are between 32 and 300GB. There are larger drives available but their price is still too high for many to consider them viable alternatives.
The problem is that capacity in solid state drives can also play an important role in the performance of the drive as well. Two drives in the same product line with different capacities will likely have different performance. This has to do with the number of NAND chips on the drive. Typically, capacity is linked to the number of NAND chips. So, a 240GB SSD may have twice the number of NAND chips as a 120GB. This allows the drive to spread out the read and writes of the data between the chips which effectively increases performance similar to how RAID can work with multiple hard drives. Now, the performance will not be twice as fast because of the overhead of managing the read and writes but it can be significant.
Controller / Firmware
The performance of a solid state drive can be greatly impacted by the controller and the firmware that are installed on the drive. Some of the companies that make SSD controllers include Intel, Sandforce, Indilinx (recently bought by OCZ), Marvel, Toshiba and Samsung. Each of these companies also has multiple controllers available for use with solid state drives. So, why does this matter? Well, the controller is responsible for handling the data management between the various NAND chips. The controllers an also determine the overall capacity for the drive based on the number of channels for NAND chips.
Comparing controllers is not something that is easy to do. Unless you are extremely technical, all it will really do is let you know if a drive is a current or past generation solid state drive. For example, the Sandforce SF-2000 is a newer controller generation than the SF-1000. This should mean that the newer one can support larger capacities and have higher performance.
The problem is that two drives from different companies can have the same controller but still have vastly different performance. This is due to the firmware that is included with the SSDs. One firmware may emphasis data management differently than another that can boost its performance for specific types of data compared to another. Because of this, it is important to examine the rated speeds in addition to the controller itself.
Write / Read Speeds
Since solid state drives offer significant performance speeds over hard drives, the read and write speeds are particularly important to look at when buying a drive. There are two different types of read and write operations but most manufacturers will only list the sequential read and write speeds. This is done because sequential speeds are faster thanks to the larger data blocks. The other type is random data access. This typically consists of multiple small data reads and writes that are slower because they require more operations.
The manufacturer speed ratings are a good basic measure for comparing solid state drives. Be warned though that the ratings are at their best under the manufacturer testing. Real world performance will likely be below the ratings given. This has to do partly with the various aspects discussed later but also because data can also be influenced by other sources. For instance, copying data from a hard drive to a solid state drive will be limited to the maximum write speeds for the SSD to how fast the data can be read from the hard drive.
The interface on the solid state drive is going to be Serial ATA. Why will this interface be important than? Well, in order to get the highest performance out of the latest generation of solid state drives means that you will need to have a 6Gbps rated SATA interface. Older SATA interfaces will still offer strong performance especially compared to hard drives but they may not be able to achieve their highest levels of performance. Because of this, people with older SATA controllers in their computer may want to by an older generation solid state drive that has rated maximum read and write speeds closer to their maximum interface speed in order to save some on costs.
Another thing to remember is that interfaces are rated in gigabits per second while read and write times on drives are listed in megabytes per second. In order to determine the limitations on interfaces, I have listed the converted ratings below for the various SATA implementations for readers to better match drives to their PCs SATA versions:
- SATA III (6Gbps): 750MB/s
- SATA II (3Gbps): 375MB/s
- SATA I (1.5Gbps): 187.5MB/s
Remember that these are the theoretical maximum throughputs for the various SATA interface standards. Once again, real world performance will typically be a bit lower than these ratings.
One issue that buyers of solid state drives might not be aware of is that fact that the NAND chips inside of them have a limited number of erase cycles they can support. This means over time, the cells within the chip will eventually fail. Typically, the manufacturer of the NAND chips will have a rated number of cycles that they are guaranteed for. To mitigate the failure of the NAND chips being worn out from constant erasing of specific cells, the controller and firmware will not immediately erase old deleted data.
The average consumer will probably not see a solid state drive's NAND chips fail within the typical lifetime (upwards of five years) of their system. This is because they don't typically have high read and write tasks they are doing. Someone doing heavy database or editing work might see higher write levels though. Because of this, they may want to take into account the rated number of write cycles that a drive is rated for. Most drives will have ratings somewhere in the 3000 to 5000 erase cycles. The larger than cycles, the longer the drive should last.
TRIM / Cleanup
A process of garbage collection can be used within the firmware to try and cleanup the drive for improve performance. The problem is that if the garbage collection within the drive is too aggressive, it can cause write amplification and shorten the lifespan of the NAND cells. Conversely, a conservative garbage collection may extend the life of the drive but significantly reduce the overall performance of the drive.
TRIM is a command function can let the operating system better manage the data cleanup within the solid state NAND. It essentially keeps track of what data is in use and what is free to be erased. This has the benefit of keeping the performance of the drive up while not adding to the write amplification that leads to early NAND cycle degradation. Because of this, it is important to get a TRIM compatible drive if your operating system supports the function. Windows 7 currently supports TRIM while Mac OS X will likely add support for it in OS X 10.7 or Lion revision.