Precious boot speed: Solid State Drives explained

Precious boot speed: Solid State Drives explained

With the solid state drive rising in popularity for PC builds, Jon Gillooly explains what you need to know

For many years the major bottleneck in PCs has been the hard drive. While everything else has migrated to an electronic implementation, the hard drive has remained one of the last bastions of mechanical technology within the PC (fans and optical drives being the other two).

This has meant that while semiconductor manufacturers push further and further with performance and complexity, hard drive spinning hasn't gotten any faster than the 15,000rpm speed attained by Seagate with its Cheetah enterprise drives.

For the past seven years the fastest desktop hard drives have been from Western Digital's 10,000rpm Raptor range. These drives have occupied a specific desktop niche thanks to their low capacities relative to 7200rpm drives, one in which they are used for booting and frequently used applications, while large-capacity 7200rpm drives provided the bulk storage within a PC.

However in recent years this ‘boot drive' niche has been under threat by Solid State Drives. These are collections of flash memory that interface with a PC via a special controller chip. To the PC they appear as a hard drive, but the flash memory is potentially much faster than a mechanical hard drive can get.

Speed warning
We say potentially, because there are some special caveats to Solid State Drive performance, related to the way in which Flash memory is made. While a well-designed SSD on the right operating system is a massive speed boost, under some circumstances an SSD can perform at the level of a hard drive or even slower. These issues are tied to the fundamental design of Solid State Drives, so a basic understanding of what is going on under the covers helps a lot.

By far the most common type of Solid State Drive consists of flash memory, controller chip and SATA interface packed into a metal shell. The vast majority of these drives are built to the same dimensions as a 2.5in hard drive, so they can slot into bays in laptops and desktop PCs designed for these drives. There are also some larger-capacity 3.5in models, which are functionally identical to the 2.5in ones. The third category, which has until now been the remit of enterprise, is the PCI-Express based Solid State Drive, which plugs into a PCI-E slot in order to get around the speed limitations of Serial ATA (but more of that later).

Open up a 2.5in SSD and you find a rather mundane-looking printed circuit board. This board is dominated by Flash memory chips, which are driven by a controller chip. Both of these components are incredibly important for performance and pricing of drives.

There are two major types of flash memory that can be used on SSDs. Single Level Cell (SLC) flash can contain one bit of information per cell, which makes it highly reliable, but also quite expensive due to the amount of silicon needed to manufacturer large capacities. SLC flash is largely used in costly enterprise products; consumer drives use a cheaper type of Flash.

Like Plasma televisions, SSDs will one day die. But the technology is now sophisticated enough that this lifespan is likely to be much longer than the product will be in use for.
Like Plasma televisions, SSDs will one day die. But the technology is now sophisticated enough that this lifespan is likely to be long after you've finished with it.

This cheaper flash is called Multi-Layer Cell (MLC). It can store several bits of information per cell, which makes it slightly less reliable but much cheaper to make thanks to a higher density of data leading to less silicon needed per megabyte. It has been the widespread adoption of MLC flash that has made the current crop of affordable consumer SSDs possible.

Taking control
But the storage medium is only a part of the Solid State Drive story. The real magic lies in the controller chip and how it manages storage. Controllers are made by several different manufacturers, and have in recent times been used to stratify pricing and performance across ranges of consumer drives. The fastest controller chips at the moment come from a company called Sandforce, whose S1200 powers the best-performing SSDs on the market.

A controller chip is the primary interface between the memory and your PC's operating system. It is responsible for functions like wear levelling and garbage collection, which are an intrinsic part of keeping Solid State Drives working to optimum capacity. Basically, when data gets erased from a Solid State Drive all that happens is that references to where that data is are removed from the partition table. The flash cell is still physically holding that data.

Problems arise when that supposedly empty piece of flash gets new data written to it. When this happens the drive has to delete the existing, unreferenced data and then replace it with the new stuff. This ends up taking a lot longer than a simple write and means that a full Solid State Drive will perform poorly if this old data doesn't get purged.

To this end SSDs have been designed to hook into an operating system function called TRIM. TRIM is a command that tells the drive to clean out this empty data during idle periods, enabling a consistently fast level of performance for the drive. TRIM support is only available on some operating systems - Windows 7 and Ubuntu 10.10 (Maverick Meerkat) being the most prominent, although Ubuntu does involve some messing about to get it working.

For older operating systems you will need to check and see if your drive manufacturer has an application available that will clean up your drive (this process is known as Garbage Collection). As a rule of thumb though, if you want to upgrade to an SSD you will want to run Windows 7

The secondary issue with Flash is that it degrades ever so slowly with each write operation. This means that spreading out data storage across all the flash in an SSD is important to ensure that some cells don't wear out quicker than others. This process of wear levelling is one of the most important functions of an SSD's controller chip.

Some of the misconceptions about SSD lifespan revolve around wear levelling and early SSDs. These were found to have poor operational lifecycles because the firmware didn't have great support for wear levelling. Now that controllers have improved, these limited lifespans are no longer an issue, with the quoted Mean Time Before Failure (MTBF) of SSDs now comparable to or even better than that of desktop hard drives.

In other words, like Plasma televisions, SSDs will one day die. But the technology is now sophisticated enough that this lifespan is likely to be much longer than the product will be in use for.

Solid State Drives are here to stay, with the price dropping and more and more manufacturers coming into the market. Because of the way that they are designed, performance is largely down to the controller chip on consumer level MLC Flash based drives. This is something that we will always mention in reviews of SSDs, but for now drives based upon the Sandforce 1200 controller are the ones to beat.

This feature appeared in the January, 2011 issue of PC & Tech Authority Magazine
Copyright © PC & Tech Authority. All rights reserved.

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