Solid State Drives (SSDs) Are Not Perfect

Apple’s latest MacBook Air models represent the first MacBooks exclusively equipped with an SSD, or solid state drive. Instead of spinning platters, these drives use non-volatile flash memory (like that inside the drive hanging off your keychain). The SSD in the MacBook Air delivers silent, cool, reliable, and fast operation; while they are more expensive per gigabyte than a not silent, not as cool, not as reliable, and not as fast conventional hard disk drive.

While SSDs carry many benefits over HDDs, I asked what one might need to take into account when operating a computer with an SSD that they might not have otherwise have had to when using a conventional HDD. For one, while a hard drive is often the component that survives a liquid spill better than any other in a modern notebook, an SSD is much more susceptible to electrostatic discharge, particularly one without any type of enclosure like that featured in the new MacBook Air. But what differences arise if you manage to keep your machine away from a beverage-wielding five year old, or an impromptu dorm room party?

SSDs handle delete operations differently than traditional hard drives. When you empty your trash, your Mac simply flags those blocks as “not in use” in the file system. On an SSD, the flash memory cells can only be written to when truly empty. Over time, the less empty your SSD becomes, the slower the drive becomes, particularly when writing. When writing new data to an SSD, the more inactive blocks there are, the more an SSD must read, then erase, modify, and then write; this is different than a traditional hard drive in which writing occurs the same whether a block is actually empty or not.

Some operating systems and SSD controllers have begun to support “TRIM,” a command which allows an operating system to inform the drive as to which blocks can be wiped internally upon file deletion. This keeps them empty and maintains hard drive integrity. Mac OS X does not have this sort of garbage collection implemented as of this writing.

A recent review at Anandtech of the new MacBook Air found marginal degradation in speed after intentionally filling up the drive with garbage and random writes, and then writing sequentially. Anand Lal Shimpi & Vivek Gowri state, “First, through normal use the drive should be able to recover its performance over time (assuming you give it enough spare area). And second, if there’s any idle garbage collection in Apple’s custom firmware for the Toshiba controller it should be able to keep the drive running at peak performance even without TRIM supported in the OS.”

So is SSD write speed degradation a non-issue? Ultimately, even an SSD with deflated read/write times is going to be faster than most any conventional hard drive. For those who are concerned about their custom SSDs, there is a $40 utility called DriveTester and a guide to try it for free While it’s hard to say for sure how the MacBook Air’s SSD will wear, I’m sure we will be hearing about improvements to hardware and software (Lion) to support the ever-increasing adoption of SSD storage.

I’ll be conducting my own tests in the coming weeks on my SSD-equipped MacBook Air and will post a follow up article.