Load leveling can be performed in hardware or software. Several
microcontrollers are attached to “raw” flash. These typically
incorporate load leveling in the device drivers. Products
like Datalight’s FlashFX Pro family handle a range of NAND and
NOR flash devices in addition to providing the same interface for
NAND flash controllers.
Hardware-based interfaces offer a number of important advantages,
including a consistent microprocessor interface. This can
cause more of an impact than most designers appreciate because
of the changes in “raw” flash. Moving to newer flash-memory
chips doesn’t typically require major accommodations. Still, it’s
one more issue that requires at least device driver changes.
SandForce’s SF-1500 SSD controller highlights this approach
(Fig. 7). Specifically targeted at MLC flash, it delivers a minimum
five-year lifetime and throughput on the order of 30k IOPS (I/
Os/s) for random read/write and 250-Mbyte/s throughput for
sequential read/write operations. This translates to 5k IOPS/W
versus 20 IOPS/W for a hard disk.
The DuraClass technology employed by SandForce also
implements Redundant Array of Independent Silicon Elements
(RAISE), essentially RAID with chips. This, combined with
advanced dynamic wear leveling and advanced error correction
coding (ECC) support, allows a SandForce-supported solid-state
drive (SSD) to reach the lifetimes and performance requirements
for enterprise storage.
Alternatives will be hard-pressed to match this unless similar
approaches are taken to mask the limitations of MLC flash. For
example, many alternatives force daily write restrictions to attain
a guaranteed five-year lifetime. SandForce can support a singlechip
controller solution in a 512-Gbyte, 1.8-in. SSD.
MASS STORAGE
SSDs are part of the mass-storage solution set. They have
killed the 1-in. hard-drive market and are increasing their share
in the 1.8-in., 2.5-in., and even the 3.5-in. market. They’re also
making a big difference in form-factor solutions that don’t follow
the normal hard-drive configurations. That’s because SSDs
can easily be placed on a circuit board, an option difficult to
attain with a hard drive.
Nonetheless, hard disks still beat SSDs when it comes to the
upper limit on capacity. They also win from a price/gigabyte
standpoint. The boundary where an SSD will be used instead of a
hard disk continues to move, but this simply means more options
for designers and users alike.
The 1.8-in. drive is the favorite for mobile devices. This is
where the choice between flash and hard drives is more difficult
for consumers. It’s easier for designers, though, since SSDs and
hard drives are both readily available in this form factor. (Price
and capacity tradeoffs still exist.)
Most of the action is in the 2.5-in. space. It includes external
drives like Fujitsu’s 500-Gbyte Handy Drive (Fig. 8). This capacity
was the top end for 3.5-in. drives not too many months ago.
The form factor has also significantly influenced the design
of servers due to the fact that a large number of drives can easily
fit into a 1U rack. Even more importantly, the number greatly
exceeds the minimum for RAID configurations, leading to growth
in this controller market. An eight-drive RAID system is no longer
a novelty. It’s become a standard option instead, with even large
drive counts showing up in high-end storage systems.
The drive capacity for a 2.5-in. drive still pales compared to
its 3.5-in. sibling. Size isn’t everything when it comes to RAID
systems, though, where system rebuild times are lower for smaller
drive configurations.
Don’t count out the 3.5-in. market. Drives like Seagate’s Barracuda
LP are coming in with 2 Tbytes of storage looking to fill
the capacity cravings for video storage in digital video recorders
(Fig. 9). If the movie studios ever recognize the opportunity
they have with this growing amount of storage, the 3.5-in. drive
market will go through the roof. As is, it might be tough to keep
up with demand.
RAID continues to play a part with 3.5-in. drives, especially
for consumer applications. However, keeping this hidden from
users is crucial. It’s easy to explain adding more storage to a consumer
and even reduced capacity to improve reliability via RAID.
Understanding the difference between RAID 1 and RAID 5 is a
whole other matter.
STAYING INTERCONNECTED
No coverage of storage would be complete without mentioning
the increasing importance of interconnects. For consumeroriented
products and a wide range of embedded applications,
this means USB and SATA. USB is an indirect interface for hard
drives and potentially a direct interface for flash drives.
External SATA or eSATA is cropping up in a number of products,
including external drives, but it will complement rather than
display USB. USB 3.0 will arrive in time to address the higherthroughput
drives. For now, though, High Speed USB 2.0 will
suffice with its 480-Mbit/s transfer rate.
SAS and Fibre Channel will be found at the enterprise level.
Fibre Channel systems will often comprise SATA or SAS hard
drives and potentially, or rather eventually, SSDs.
There are more options than ever when it comes to storage, but
those choices won’t be easy. There are alternatives.