Solid-State Drives: How to Choose the Right One?

If you really want to make most of your investment in SSD, there are a few technical terms you must understand. You should also know the architecture of an SSD and the different components that make up an SSD along with what all is used in SSDs these days.

What makes up SSDs?

Solid State Drives are made of solid-state memory constituting either of flash based memory cells or DRAM. An SSD that is used for storage is mostly flash based as it is non volatile and does not need constant power supply to retain data. DRAM on the other hand is volatile and is mostly used as cache memory. Since SSDs are based on flash memory, they deliver higher data transfer rates over standard HDDs. Unlike HDDs, the SSDs do not have any movable part, which ensures that they generate no noise and are not susceptible to mechanical failures that could result from vibration, shock, heat, etc. Lastly, SSDs have exceptionally fast response times, almost instantaneous, while even the best hard disk drive appears sluggish when compared to an SSD. Every SSD you pick would constitute of a controller which is basically a processor responsible for executing firmware level codes. Besides doing jobs like error correction and garbage collection the most important task in context of SSD that a controller performs is wear levelling. Controller maximizes the life of SSD by distributing the 'writes' across the flash memory which is also known as wear levelling. The second important component is the memory itself which as mentioned earlier is flash based. Finally the interface forms the final critical component of a typical SSD; most of the SSDs that we use today come with SATA 3, the latest and faster version of SATA interface. The earlier version SATA 2 performs slowly when compared with latest the version. If your motherboard and SSD supports SATA 3, your performance would double when compared to SATA 2 interface on either the SSD or the motherboard. In one of the tests we found average transfer rates of 198 MB/s on SATA 2 and 375 MB/s on SATA 3 with the same SSD.

Mostly NAND-based logic

Most SSD storage is made from NAND-based flash. For the uninitiated, NAND is a kind of logic gate, key to the architecture of solid-state devices. The other gate structure is NOR-based but this is rarely used these days.

SLC or MLC

Single level cell or multi level cell is yet another type of NAND flash technology used in SSDs these days. SLC NAND stores one bit per cell and has high endurance. MLC NAND on the other hand, as the name suggests uses two bits per cell. This delivers higher capacity, but at the same time wears out the storage faster than SLC NAND. Newer 3-bit per cell and 4-bit per cell NAND flash are targeted for applications with limited number of writes.

TRIM

SSDs store data in pages which are grouped together into a block. NAND flash memory cells can only be written to when they are empty. However, if they are not empty they need to be erased first before a write operation can be performed reliably. One more important aspect to understand here is that the write operation in SSDs can be done at page level but erase operation affects a complete block. So, if a write operation has to be performed on previously written pages, the contents of the entire block have to be stored in cache before it is effectively erased on the flash medium. The overwritten page is modified in the cache so the cached block is updated, and only then is the entire block (with updated page) written to the flash medium. This phenomenon, known as write amplification, leads to performance degradation. The TRIM command is designed to enable the operating system to notify the SSD of the data pages that are invalid due to erasure by the user or operating system itself. In this way, an SSD knows that it should not relocate data from those logical block addresses during garbage collection. This will result in fewer writes to the flash, reducing write amplification and increasing drive life.

SSD as cache

To overcome the pain of reinstalling OS (along with all the application) on a new SSD, there is an option called 'SSD cache'. Such a cache has low storage capacity and comes with a bundled software package that determines which applications are frequently used, and then caches them on the SSD. Simply attach an SSD cache to the available SATA port and install caching software, and you are saved from reinstalling everything from scratch.

Kingston HyperX 120 GB 2.5 SATA Disk

It is a well-designed solid state drive available in the standard 2.5-inch form factor for compatibility with notebooks and desktops. A 2.5-inch to 3.5-inch mounting bracket is included with the SSD for compatibility with standard hard drive mounting. The SSD drive has a rugged feeling to it and takes an average time of 10 - 15 seconds to reach the login screen from power on, and is straightaway ready to use after you have logged on.

The SSD includes the latest SandForce controller technology with premium NAND Flash, reducing load times while increasing performance and endurance. It provides high-speed SATA Rev 3.0 (6 GB/s) transfer speeds for larger bandwidth, which power users require for advanced gaming, multitasking and multimedia computing power.

The drive has TRIM command support, which allows the OS to inform the SSD about blocks of data that are no longer in use, so the SSD can wipe that data internally, freeing up space. This allows them to store only the data they need and perform memory optimization to ensure the fastest possible write speeds. We tested the drive's speed on Intel Core 2 Duo and 2GB RAM and found the drive registering a speed of 271MB/sec.

The SSD further includes DVD for SSD disk cloning software, along with a screw driver to assemble the SSD. The USB enclosure, SATA cable and USB cable are also available with the box.

Intel SSD 520 Series

This family of SSDs has members with capacity ranging from 60 GB to 480 GB. The one we received for review had a capacity of 240 GB. It is built with 25 nanometer NAND Flash Memory and has standard 2.5 inch form factor. Support for SATA 3.0 (6 Gbps) means this drive would give you higher transfer rates if your board supports SATA 3.0. A 2.5-inch to 3.5-inch mounting bracket is included with the SSD for compatibility with standard hard drive mounting. This SSD also comes with data protection features comprising of Intel Advanced Encryption Standard New Instructions (Intel AES-NI) 256-bit encryption capabilities and includes on board data compression for better performance.

Intel projects this SSD as one that is high on performance and it seems they are not wrong given the high average transfer rates we got in our tests. We tested the transfer rate of this drive using HD Tune 2.55 benchmark on a Core i7 3.30 GHz machine with 1 GB RAM and Windows 7 OS and found average transfer rates of 318 MB/s. This is substantially higher when compared with Kingston Hyper 120 GB SSD which gave 271 MB/s.

Agility 4 Drive

Agility 4 is an affordable and performance equipped SSD which is built around the company's flagship Vertex 4 SSD series. The SSD boasts of a SATA III interface and is an asynchronous NAND equipped drive like the Agility 3. However, Agility 4 comes with OCZ Technology's proprietary 'Indinlix Infused' Everest II Solid State Flash Processor instead of the LSI SandForce 2281 SSD Flash Processor used in Agility 3. The advantage of using the Indinlix is that the read/write speed of compressible as well as incompressible data is almost same.

The SSD drive is nicely designed, lightweight and heat, noise & shock resistant. We ran a couple of tests over the SSD to check its performance. In HD Tune benchmark test, it registered an average transfer rate of 143.1MB/sec with excellent access time of 0.2 ms. Also, it showed a burst rate of 43.0 MB/sec with the CPU usage of 2.9%. In the AS SSD Compression benchmark test, the average read & write rate was about 260 MB/sec & 250 MB/sec respectively. It got an excellent cummulative score of 602 for overall read/write benchmark test.

If we compare this drive with Kingston 128 GB SATA III, we find that Kingston SSD has almost similar read rate (270 MB/sec) but slightly lower write rate (222 MB/sec). Agility 4 gets a marginal edge over Kingston in the AS SSD benchmark test which had a score of 548.

Corsair Force GT

Corsair has three lines of solid state drives: Neutron, Force, and Accelerate. While accelerate series consists of SSDs similar to SanDisk ReadyCache, the other two series are regular SSDs. The one that we received is marketed as high performing SSD for those who want very high performance out of their local storage. Besides performance other differentiating feature about this drive is that it supports Windows 7 TRIM command, which enables better write performance and longer SSD life. Other features of this drive include support for SATA 3 interface and bundled 3.5 inch case for compatibility with standard PC chassis.

We tested the transfer rate of this drive using HD Tune 2.55 benchmark on a Core i7 3.30 GHz machine with 3 GB RAM and Windows 7 32-bit OS and found average transfer rates of 354 MB/s on SATA 3 interface. This is lower than Intel 335 Series which gave a score of 375 MB/s. If this performance is compared with Intel SSD 520 which gave average scores of 318 MB/s then Corsair Force GT is definitely ahead.

Intel SSD 335 Series

The 335 SSD series is sandwiched between 330 and 520 series, the prime differentiator between 335 series and other two is the underlying MLC NAND flash memory used. While 330 and 520 series are based on 25 nm NAND the 335 series is based on 20 nm NAND process. To put all this in 'English', 335 series is built using technology that allowed Intel to put more transistors on the die which in turn enables more capacity and at the same time price is kept under control. Besides these benefits an SSD built using 20 nm processes would consume lesser power when compared to those using 25 nm. Intel 335 series has only one member with 240 GB capacity in 2.5 inch form factor and a SATA 6 GB/s interface (for quick data transfer). If you have a machine that supports only 3.5 inch drives then one can use the bundled 3.5-inch mounting bracket.

We tested the transfer rate of this drive using HD Tune 2.55 benchmark on a Core i7 3.30 GHz machine with 3 GB RAM and Windows 7 32-bit OS and found average transfer rates of 198 MB/s on SATA 2 and 375.5 MB/s on SATA 3. This is higher than Intel SSD 520 series 240 GB SSD which gave 318 MB/s. This high performance is also accompanied with lower price, while Intel's price for the 520 series is $318 and for 335 series is $184.

SanDisk ReadyCache

This is not a traditional SSD like the others. It's basically a SSD to be used as cache to speed up performance of existing SATA 3 drives. Therefore, we've not ranked this with the others. To overcome the pain of reinstalling OS (along with all the applications) on a new SSD, SandDisk has come up with an innovative solution called ReadyCache SSD. You simply have to attach this SSD to an available SATA port (SATA 3 is supported by this SSD) and download ReadyCache software by the name of 'ExpressCacheSetup.exe' from www.sandisk.com/readycache. Install this software, which is 132 MB and activate it using the activation key that ships with the drive. Once activated, you need to restart your machine three times so that installed software knows which all packages are readily used by you and cache them on the SSD. The software also allows you to delete the cache. One critical point to note here is that ReadyCache works only with Win 7 OS, and does not support other OSs, which limits its usability.

To test this SSD, we used HD Tune 2.55 benchmark where this drive clocked average transfer speeds of 326.5 MB/s. This speed is lower than Intel SSD 335 Series which clocked 375 MB/s, but comparable to Intel SSD 520 Series which clocked 318 MB/s. All these tests were performed using SATA 3 interface.