Solid-State Drive Terminology
© 2012 by KV5R. Rev. August 5, 2012.
Most articles start right out hitting you with new terms and acronyms and, as any real writer knows, there are rules for defining terms when used, which the usual crop of web geeks tend to ignore, so let’s get our definitions straight before diving into the topic.
Terms Related to SSD Technology
- SSD: A Solid-State
Drive DiskDevice that uses digital flash memory instead of mechanical rotating magnetic platters for storing data. Since it isn’t mechanical, it’s much faster, both in access time and transfer rate. SSDs are intended as replacements for mechanical hard disk drives (HDD), providing both much faster speeds, much lower power consumption, and much greater mechanical shock (you can drop them!) resistance—the latter two being particularly useful in laptops. For detailed info, see Wikipedia SSD.
- NAND memory: the logic circuitry of flash memory used in SSDs. It’s a technical term that end-users need not know, but lots of articles and advertisers throw it around needlessly. Simply, NAND memory must be written and read in blocks (good for file storage), while NOR memory may be read and written at the bit-level (good for embedded program execution). If technically curious, see the NAND_Flash entry at Wikipedia.
- SLC and MLC: Single- and Multi-Level Cell: Types of flash memory cells. SLC is slightly faster and much more durable, while MLC is much cheaper. Most consumer-grade SSDs use MLC, while higher-priced SLC memory is used for industrial applications requiring ten-times-greater reliability. For the technically curious, here’s a good white paper from Super Talent.
- Transfer Rate: Refers to the maximum sustained sequential transfer rate, in Megabytes per Second (MB/s). Depends on both the technology (and price) of the SSD, and the computer’s drive interface (see SATA, below). This performance benchmark is useful when transferring large files. Typical SSDs now (July 2012) run 500-560 MB/s on a SATA-III interface, and about 250-280 on a SATA-II interface, and that's about 5-10 times faster than a good 7200 RPM hard drive.
- IOPS: Refers to the maximum number of Input-output Operations Per Second. Depends on same as above. This performance benchmark is useful when transferring many small files. The better SSDs now (July 2012) are running 80,000 IOPS or more. IOPS speed is more a function of the speed of the drive’s controller than the speed of the memory.
- Controller (flash memory controller): a dedicated, embedded microprocessor in the SSD. Various manufacturers such as SandForce, Marvell, JMicron, Intel, Samsung, and Toshiba are in a race to produce the fastest SSD controllers. From a performance perspective, SandForce is currently (July 2012) doing very well, and some SSD manufacturers, including Intel, have recently switched to SandForce controllers. From the buyer’s perspective, an SSD with the latest SandForce controller chip is the likely one to buy, though all of them perform very well, if you manage to avoid getting one that been on a shelf for a while. Various web articles compare various SSD controllers, but look for a very recent one because this technology rapidly changes.
- Wear-leveling: The memory cells in the memory chips “wear”—that is, they have a limited number of erase and write cycles before they fail from silicon degredation. The SSD’s controller, when erasing and writing, chooses cells that have the least amount of use, thus extending the useful life of the SSD wearing cells evenly. When cells do begin to fail, the controller de-allocates them and replaces them from a reserve pool. When the reserve pool is used up, the drive is toast. SSD’s using MLC range from about 3,000 to 10,000 cycles, so look for the highest numbers. Note that higher-density SSDs have shorter lives, since each cell has less silicon to wear. But note also that SSDs that are only ever half-filled should (theoretically) last twice as long. Some studies suggest that an SSD should last 10 years in typical use scenarios, but most experts still recommend reducing unnecessary writes as much as is practical. We will cover ways to do that below. For technical details, see wear-leveling at Wikipedia.
- Garbage Collection: The controller consolidates data and erases freed blocks before they are needed for writing, increasing performance. For technical details, see garbage collection.
- TRIM: Not an acronym, but a command that the operating system can send to a TRIM-capable SSD to tell it that a file is no longer needed, and can be erased from the SSD. Unlike hard drives, data cannot simply be written over, it must be erased first, then written, in a two-step operation. TRIM gets blocks erased ahead of when they are needed, and extends both the performance of the SSD slightly. To use TRIM, both the operating system and the SSD must support it. Windows 7 has TRIM, while XP and Vista do not. For technical details, see TRIM.
- Alignment: The partition boundaries are, in many cases, aligned differently on a hard drive than they are on an SSD. Cloning (see below) a hard drive to an SSD may cause a mis-alignment that causes the SSD to write all blocks across a boundary, such that each write causes the controller to split it across two blocks. A “mis-aligned” SSD will thus run at about half its rated speed, and also wear out faster. We’ll see more about that below. For the technical details, see this article from Paragon Software.
Terms Related to the Computer Interface
- SATA: Serial AT Attachment is the ATA drive host interface used by modern computers, replacing the older and slower Parallel AT Attachment (PATA). The SATA specification has three major revisions and three corresponding speeds: SATA-I (aka SATA 1.5) is 150 MB/s (similar to PATA/IDE), SATA-II is 300 MB/s, and SATA-III is 600 MB/s. A related terms is eSATA, which simply means “external SATA.” The newest SSDs support SATA-III but are backward-compatible with SATA-II, and will run at a correspondingly lower speed, typically 250-280 MB/s on a SATA-II interface. They might also run on SATA 1.5, if the BIOS will recognize it. Which one you have depends on your motherboard chipset, specifically the ATA host interface, as well as the system BIOS. SATA-II became common in consumer computers in 2010, and SATA-III in 2011, so if you system is older, you will not benefit from the speed of an SSD.
- PATA: Parallel AT Attachment (aka IDE): If your computer has 40- or 80-wire flat ribbon cables in it, it has PATA, not SATA. You need a newer computer if you want to use an SSD in it. Yes, there are SATA host adapter cards for older computers, but that’s just more expense and trouble.
- AHCI: Advanced Host Controller Interface; it provides a larger control command set than IDE. When you install the SSD, you will need to go into the BIOS setup and make sure the drive controller is set to AHCI mode, not ATA/IDE, before installing the operating system to the SSD.
Terms Related to SSD Migration
- Migration: Refers to the whole process of replacing your hard drive with an SSD and moving the operating system, applications, and data files to it.
- Cloning: Various software programs are available for migrating by “cloning” the hard drive to the SSD. This is a common operation for drives of the same type and size, but the typical operation with a new SSD is to clone a large hard drive to a smaller SSD. Obviously, the data on the hard drive must be smaller than the SSD’s capacity. The cloning software must be SSD-aware, so that is can both down-size the partition to fit on the smaller SSD (“smart cloning”), and ensure proper partition alignment (“SSD-aware” cloning). The recent (2011+) versions of popular cloning software (such as Acronis, Paragon, Macrium, and AOMEI) have the capability to properly clone a hard drive to an SSD. Cloning has the advantage of being fast and non-technical—typically the user can have the computer up and running as it was in 30 minutes to two hours. But even when using SSD-aware cloning software, there is one big disadvantage: you now have the same cluttered system that you did beforehand, because cloning is a byte-for-byte copy.
- New or Fresh Install: This is where you back up all your data (usually to an external drive), re-install your operating system from scratch into the SSD, then re-install all your programs, then copy back all your data files. This is a long and tedious process that may take at least several hours to get your computer all set up like it was, but it has the advantage of giving you a “clean” system. You should consider this if your computer has been in heavy use for a couple years or more.
- OEM: Original Equipment Manufacturer, id est, your computer’s brand name (usually). OEM is loosely used to refer to any thing or parameter of the computer in a “factory-new,” unmodified state, or anything added by the manufacturer, as in “OEM license key.”
- OS: Operating System—Windows, OSX, Linux, or whatever.
- BIOS: Basic Input Output System. This is the software, stored in a chip in the computer, that gets the various hardware talking and brings it to life. Various things are configurable, and you get into BIOS settings by hitting Del, Esc, or F-something (depending on the BIOS brand) as soon as the computer starts booting. You’ll need to do so to (a) set your system to boot from CD-ROM drive if installing an operating system from CD/DVD, and/or (b) to verify or set your drive host controller to AHCI mode.
- Cache (pronounced like cash, not caysh, or ca-shay): files that are temporarily stored for rapid retrieval, typically web browser cache, keeps you from having to re-download every web page and image you repeatedly visit. The web browser looks in the cache first, and if not there, it proceeds to download the page’s files. A browser cache is a great idea on hard drives, and a bad idea on SSDs, for reasons we’ll see below.
- ISO: International Standards Organization. This term is commonly used to refer to a file that is an exact image of a CD or DVD. An ISO file may be downloaded them burned to optical media using an ISO-burning program, such ImgBurn. You do not simply copy the file to the disc, you must burn it in image mode. For example, I recently purchased Microsoft Access 2010 from Amazon, who gave me a valid license key and a link to download the ISO (for a mere $120). I downloaded the big ISO file, burned it to a CD with ImgBurn, and then installed it. The CD I burned is exactly like one I might have purchased in a store, but for the lack of pretty labeling and little box, and it’s perfectly legal. You can also buy a Windows license on-line and download a Windows install ISO, and make your own Windows disc (it takes a DVD-R). Further, you can freely download and burn a Windows ISO and use it with a valid key you already have. More on this below.