SSDs

Overview

Solid-state drives (SSDs) are fast becoming the must-have PC components of 2010. Serving as primary storage space for high-end computers, SSD's super-fast transfer rates and near-instant accesses offer genuine advantages over traditional spindle-based magnetic drives.

Now manufactured and marketed by at least 20 companies and arriving in different capacities, specifications and price-points, same-sized SSDs' performance can vary wildly, and this TekSpek will explain why.

SSDs are made up of a type of non-volatile flash memory known as NAND. These NAND chips, contained inside the aluminium casing (left), don't require power to retain information written to them, and they define the overall capacity of the drive. Interfacing with a separate controller that acts as a go-between between the NAND modules and operating system, the quality of NAND, controller and associated firmware predicates overall performance.

By understanding the workings of the various controllers and NAND memory, potential purchasers can glean a better understanding of SSD performance.

NAND memory

As of May 2010, the majority of shipping SSDs are presented in a 2.5in form factor and cost, approximately, £140 for a 64GB model; £250 for a 128GB drive, and £500 for a quality 256GB SSD. The majority of that cost lies with the NAND memory that makes up the overall capacity.

NAND memory production is led by Samsung, Toshiba, and Intel/Micron (IMFT). Cutting-edge SSDs use 34nm NAND with MLC (multi-level cell) technology that, as the name suggests, increases capacity through using multiple bits per cell. The downside of using more bits per cell is that speed and reliability can be compromised, so SSD manufacturers need to balance the competing interests of capacity, reliability, speed and price when thinking about solid-state drive production. Enterprise-class drives use the faster, more expensive SLC (single-level cell) NAND.

Looking towards the future, later on in 2010, Intel/Micron will introduce 25nm-based MLC NAND - the smaller manufacturing process enabling greater densities and capacities - and one should expect to either faster or more-spacious drives at a given price point.

Controllers

NAND memory defines SSD capacity and potential performance, but the drive's controller has a huge impact on how efficiently the NAND is used. One can think of the controller as the brains of the outfit and the NAND and the horsepower.

Appreciating the complex job entrusted to the controller - acting as the regulator between NAND and the operating system - choosing the appropriate model is critical with respect to overall performance.

The controller's job is to leverage the performance from the NAND chips by way of multi-channel aggregation and to keep traffic (data) moving in and out of the SSD, at peak speeds, without stalling performance.

There are seven main companies that manufacture controllers for SSDs: Indilinx, Intel, Toshiba, Samsung, Marvell, SandForce, and JMicron. Each controller can be programmed via firmware updates, and optimisation is an ongoing process for the nascent SSD industry. We'll take the more popular premium ones into consideration for this TekSpek.

Indilinx Barefoot

The majority of premium SSDs shipping today use the tried-and-tested Barefoot controller from Indilinx. Examples of such drives include Corsair's Nova; OCZ's Vertex; G.Skill's Falcon; SuperTalent's UltraDrive, and Crucial's M225. The newest iteration of the controller, as found on the Corsair Nova, interfaces with 34nm NAND. It uses an auxillary chip, usually 64MB in size, to buffer operations between the NAND and SATA output.

Manufacturers may use the same controller and NAND and yet specify different performance speeds for drives. The reason for doing so lies with the aforementioned firmware, which can be tweaked to provide greater performance and/or reliability. The firmware also controls whether the drive supports the performance-restoring TRIM function.

Generally speaking, Indilinx Barefoot-powered drives will offer sequential throughput of 250MB/s read and 200MB/s write. As a guide to price, Corsair's Nova 128GB currently etails for £290, or £2.26 per GB.

Intel G1 and G2

Chip giant Intel brought performance very much to the fore with the release of Intel-branded drives featuring the company's G1 controller over 12 months ago. Now superseded by the new-and-improved G2, complete with TRIM support and increased performance, and interfacing with IMFT-produced NAND, drives based on the controller - Intel's X25-M and Kingston SSDNow Gen 2 - tend to ship with sequential speeds of 250MB/s write and 100MB/s read.

Perhaps artificially limiting the read speed for market segmentation purposes, Intel's controller comes into its own in a workstation/server environment, where the SSD's random accesses and IOPS are more important than straight-line speed. Considerably faster than Indilinx drives in this scenario they make a compelling case as hybrid enterprise/enthusiast SSDs. Intel's X25-M G2 160GB currently etails for £335, or £2.09 per GB.

Marvell

Crucial uses a Marvell 88SS9174 controller and 34nm IMFT NAND in its RealSSD C300. The controller provides the greatest sequential throughput (straight-line speed) of any consumer-orientated SSD in Q2 2009. The 256GB RealSSD C300 is able to push over 350MB/s reading and 215MB/s writing and therefore is equipped with a SATA 6Gbps interface to the system.

Backed up by a 128MB buffer chip and supporting TRIM, the Marvell controller does well in both straight-line speed and random access performance, although it is eclipsed by the Intel G2 in purely random-access scenarios. Expect to pay £310 for a 128GB model, or £2.42 per GB.

SandForce

The newest entrants on the SSD block are a couple of controllers from SandForce. These controllers, SF-1200 and SF-1500, don't require an auxillary buffer chip because of the way they operate.

SandForce makes a point of stressing optimisations pertaining to writes for the NAND memory. In particular, the company's DuraWrite technology seeks to extend the life of SSDs by minimising on write amplification - the amount of NAND writing actually required for certain-sized data, where larger-than-needed blocks are copied into main system memory and then back to the NAND - to a figure below one. In short and in plain English, you don't want to write more blocks than is absolutely necessary, and lesser controllers often have write amplification of 5x, or more.

Corsair's Force series use the SandForce SF-1200 and offer potential throughput speeds of 260MB/s for both reading and writing. The special sauce in the controller makes it very, very fast for high IOPS enterprise-class operations such as small-file random reading and writing, yet it's just as home in a server as in a high-end enthusiast PC.

Keeping performance sharp with TRIM and healthy over-provisioning - where significant capacity is left spare for the controller to keep data chugging through at near-new levels - SandForce-powered drives, although in their infancy, are, on balance, the best available.

Corsair's F100 costs £320 for a 100GB model, or £3.20 per GB. The premium attracted by the drive is somewhat mitigated by the class-leading performance.

Summary

Solid-state drives may all look the same and ship with similar specifications, but it pays to investigate the underlying technology, particularly the NAND and controller, to determine which the right drive is for you.

A worsening £-$ exchange rate and constraints in NAND supply, primarily attributed to Apple, means that the cost per GB for a quality drive is over £2. This leads to a 64GB drive - the smallest you would realistically need to house an operating system and commonly-used programs - costing £120, or more, so SSDs remain aspirational components for most folk.

The combination of NAND, controller and firmware predicate performance. Indilinx-powered SSDs tend to be jack of all trades, Intel models are very speedy for server/workstation-level work, Marvell's is super-quick for sheer data throughput, and the nascent SandForce SSDs offer the best mix of them all...albeit at a significant price premium.

There is no perfect SSD in Q2 2009, but a sensible examination of each drive's credentials will help narrow the plethora of choice to the model that best fits your need.

Prices, too, should begin to fall as cheaper NAND is produced on the upcoming 25nm process. SSD's star is on the rise, and a model may well make the perfect Christmas present for 2010.