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-