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.
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