With Apple almost certain to introduce a new MacBook Air, questions have turned to the specifics of the hardware to be used. A leaked pre-production photo features an odd memory configuration (not to mention four batteries), a device I immediately recognized as an SSD-on-a-stick. With this high-profile introduction of a new SSD stick form, I thought it was time to cover these unconventional new storage formats.
Most conventional SSDs use conventional disk interfaces, normally serial ATA (SATA). These plug right into just about any modern computer using standard internal SATA power and data connectors. But small form factor PCs like the MacBook Air need a more-compact SSD, leading to some clever (and confusing) engineering.
Most modern computers use the PCI Express and USB busses for internal peripheral connections other than storage. Most folks are familiar with the full-size USB ports and PCI Express slots found in desktop machines, but portable computers (and a few others, including my iMac) need something more compact.
Introducing PCI Express Mini Card
This led to the development of the PCI Express Mini Card standard, commonly called Mini PCI-E. Like the similar ExpressCard format, Mini PCI Express includes both USB and a single PCI lane, along with a host of other pinouts for flexible use in portable machines:
- A single PCI Express lane
- USB 2.0
- System Management Bus (SMBus) – a 2-wire lightweight communication bus derived from I2C for power-on/off commands
- Pins reserved for GPS use
- Pins for diagnostic LEDs showing Wi-Fi activity
- Pins to communicate with a GSM SIM card slot located elsewhere
- Reserved pins for an additional PCI Express lane
- 1.5 and 3.3 volt power
This is one full-featured slot, as you can see, packing many useful features into a compact connector.
Nearly every modern laptop has at least one Mini PCI-E slot, and they are frequently used for Wi-Fi and 3G radio devices. Apple often uses them for their “AirPort” Wi-Fi adapters, so machines as diverse as the desktop iMac include Mini PCI-E slots.
Mini PCI-E SSDs
As SSD performance increases, conventional disk interfaces like SATA and SAS are becoming increasingly-saturated. And block storage protocols like ATA and SCSI make little sense when communicating with NAND flash devices like SSDs. So PCI Express SSDs have become increasingly popular lately. Companies like Fusion IO and SandForce are aggressively pushing these devices, and the performance numbers are impressive.
Unsurprisingly, the availability of a compact PCI Express Mini Card slot in portable machines has led to the development of tiny PCI Express SSDs for these applications. A wide variety of options are available but, as with ExpressCards, caution is in order since not all are equal. Manufacturers have developed a variety of Mini PCI-E SSDs using the various pins in this tiny slot:
- Native PCI Express SSDs can use the single included PCI Express lane and are compatible with most computers
- Cheaper USB SSDs (essentially glorified thumb drives) can use the included USB pins and are also compatible with most computers, though much slower
- A de-facto standard for SATA over Mini PCI-E called mSATA is seeing wider adoption but is incompatible with standard Mini PCI-E slots since it uses (reserved) pins 5 and 7 for SATA communication
- Various alternative methods of sending SATA signals have been developed, including the format used in the popular Asus Eee PC, which are also widely incompatible
This proliferation of Mini PCI-E SSDs has caused confusion and frustration among users. Many have purchased Mini PCI-E to SATA adapters designed for the Eee PC, for example, hoping to connect SATA devices to their machines. Finding compatible devices is a trial-and-error process and is played out repeatedly in forums dedicated to machines like the JooJoo and Mac Mini.
Apple chose another format entirely for the new MacBook Air, designing their own connector rather than selecting a standard like PCI Express or even mSATA. The variety of incompatible Mini PCI-E cards is frustrating to enthusiasts like myself, and I had hoped that the entry of a major company like Apple would bring sanity to this disorder.
As with full-sized PCI Express SSDs, Mini PCI-E SSDs hold great promise for performance, reliability, and power consumption and their compact size is perfect for a variety of applications. I look forward to examining and reviewing these devices in the future!