By Luke Tymowski, Systems Administrator, Calgary
Earlier, we said solid-state drives (SSDs) have become much more reliable. Manufacturers have improved their development process, higher quality memory is now cheaper and available for use in consumer-targeted SSDs, memory controllers have improved, and some manufacturers like Samsung are able to design and build every part of the SSD themselves.
Another reason is the TRIM command (TRIM isn't an acronym ' it's just written like that). The memory chips used in SSDs have a limited lifespan ' they can sustain only so many writes before they fail. An SSD's controller shuffles memory blocks around to even out the wear. But there's a catch: the controller cannot tell on its own which blocks are being used, and which are free.
The operating system, on the other hand, say, MacOSX or Linux, knows which blocks are being used, and which are free. When you delete a file, the operating system uses the TRIM command to let the SSD's controller know that it can reuse those blocks, preventing unnecessary writes. MacOSX added TRIM support only near the end of Snow Leopard's release (10.6.8). But there's a catch. MacOSX enables automatic TRIM support only for SSDs purchased with Macs (the SSDs are typically manufactured by Toshiba and Samsung but have Apple firmware). If you purchase a third party SSD, MacOSX won't automatically enable TRIM. But you can use an app called TRIM Enabler to get the TRIM support. (Install it, enable TRIM, reboot your Mac, and you're good to go ' remember to tip the developer though.) Linux has supported the TRIM command since the 2.6.8-25 kernel (late 2008), but again, you have to manually enable TRIM support.
All SSDs have a buffer of unused blocks. For consumer drives, this might be 5-10% of the drive's published capacity. That is, a 240 GB SSD's real capacity might be 256 GB. But you aren't able to use that extra 16 GB ' the SSD uses it as spares to shuffle blocks around.
Enterprise SSD drives boast longer lifespans and better performances than consumer SSDs. Part of that is because they use higher-quality memory. But they also dedicate 25% or more capacity as spare. So a 200 GB enterprise SSD might really be a 264 GB drive. The more spare blocks the drive has available, the faster it can shuffle writes around. (Want more speed and a longer life from a consumer SSD? Don't use your drive's full capacity.)
See this AnandTech article for more details.