make sense when they do not spin at all. Volume managers, software that presents collections of disks as a block device, led to similar inefficiencies in disk-based storage, obscuring information from the file system.

Modern file systems such as Write Anywhere File Layout (WAFL) 5 ZFS and B-tree file system (Btrfs) 1 integrated the responsibilities previously assigned to volume managers and reorganized the layers of abstraction. The resulting systems were more efficient and easier to manage. Poorly optimized software mattered when operations were measured in milliseconds; it matters much more on flash devices whose operations are measured in microseconds. To take full advantage of flash, users need software expressly designed for the native operations and capabilities of NAND flash.

overwrite the contents of one page. The FTL could copy the block into memory, modify the page, erase the block, and rewrite it in its entirety, but this would be very slow—slower even than a hard drive! In addition, each write or erase operation wears out NAND flash. Chips are rated for a certain number of such operations—anywhere from 500– 50,000 cycles today depending on the type and quality, and those numbers are shrinking as the chips themselves shrink. A native approach to block management would quickly wear down the media; and to compound the problem, a frequently overwritten region would wear out before other regions. For these reasons, FTLs use an indirection layer that allows data to be written at arbitrary locations and implements wear leveling, the process of distributing writes uniformly across the media. 2

the State of SSDS

IllustratIon by joel ormsby

For many years SSDs were almost exclu-
sively built to seamlessly replace hard
drives; they not only supported the
same block-device interface, but also
had the same form factor (for example,
a 2.5- or 3.5-inch hard drive) and com-
municated using the same protocols
(for example, SATA, SAS, or FC). This
is a bit like connecting an iPod to a
car stereo using a tape adapter; now it
seems that 30-pin iPod connectors are
more common in new cars than tape
decks are. Recently SSDs have started
to break away from the old constraints
on compatibility: some laptops now
use a custom form-factor SSD for com-
pactness, and many vendors produce
PCI-attached SSDs for lower latency.

Bridging the Gap

The algorithms that make up an FTL are highly complex but no more than those of a modern file system. Indeed, the FTL and file system have much in common. Both track allocated versus free regions, both implement a logical to physical mapping, and both trans-