While random read performance in a typical MLC-based SSD can get up to 10,000 to 20,000 IOPS, random write performance is significantly less. Even a so-called “high-performance” SSD today delivers roughly less than 1000 IOPS of write performance (Fig. 4). This is generally caused by a high write amplification factor and by a need to restrict writes to extend the drive’s endurance. Typically, SSD makers address the write performance issue with two methods.
One is by adding DRAM caches, described earlier. However, this isn’t a long-term solution because it only speeds up writes until the cache is full (the first few minutes of use, at best). In any event, adding the gigabyte or more of DRAM cache that’s really needed to impact performance would make the SSD too expensive to sell.
The other method is to over-provision the drive. This gives the SSD controller more room to manipulate the data and reduces the amount of time the drive is doing housekeeping operations, e.g., garbage collection (performed on blocks of data no longer needed by the host).
NAND flash memory, like other memory chips, has naturally occurring defects that render portions of die unusable. Most SSDs provide error protection for up to one sector for each 1015 bits read. Assuming a 250-Mbyte/s read speed and a 40% operating duty cycle, a sector error would result on average every 14.4 days based on the following formula:
1015/(8 × 250 Mbytes/sB × 40%) = 14.4 days
In contrast, high-performance HDDs offer error protection for up to one sector for each 1016 bits read, but they’re transferring data much more slowly than an SSD. Assuming a 120-Mbyte/s read speed at the same operating duty cycle, a sector error would result on average every 9.9 months based on the following formula:
1016/(8 × 120 Mbytes/s × 40%) = 9.9 months
To address this problem, SSD manufacturers either reduce the warranty period for their devices or they leave it to redundant array of independent discs (RAID) logic in the host computer system. Shorter warranties aren’t acceptable to most mainstream users, and using host-based RAID causes a high number of rebuilds and further reduces the SSD’s performance. And, of course, in a notebook environment, RAID isn’t a good option.
One of the stealth issues impacting the use of SSDs is the lack of encryption in a typical drive. For most products on the market today, data can be recovered from the SSD by simply removing the SSD cover and attaching a clip to the flash-memory chips—a process far easier than trying to read the contents of a passwordprotected HDD that’s been removed from its host. Enterprises will demand much better security guarantees, though, before they use SSDs in large quantities, especially in laptops.
REQUIREMENTS FOR BROAD SSD ADOPTION
The fundamental cost issue with today’s SSDs can be largely overcome with the use of MLC flash, but that flash must be made reliable enough and deliver enough performance to be practical for use in enterprise servers and laptops. There are four requirements for doing so:
- Better write endurance: The industry must develop new techniques to reduce the write amplification in MLC-based SSDs (thereby increasing the endurance) to meet the five-year expected life of enterprise-class HDDs, and it must do so without imposing daily write limitations, shorter warranties, or costly DRAM caches.
- Better write performance: MLC-based SSDs should perform like HDDs—there should be no difference between write and read performance. This will also require significantly reducing the write amplification factor imposed by today’s SSD technology.
- Lower error/defect rates: Error rates and error correcting code (ECC) protection for MLC-based SSDs must be better than it is for today’s enterprise HDDs, without over-provisioning or relying on system-level RAID techniques.
- Full security: SSDs will need some form of built-in encryption to prevent data theft before enterprises will trust their use in laptops.
- Reduced complexity, size, and costs: SSD designs must eliminate DRAM and combinations of SLC and MLC to reduce packaging complexity, size, and costs.
The market is more than ready for practical SSD storage devices, just as soon as SSD manufacturers overcome the challenges that limit performance, endurance, and complexity and can offer fast and reliable devices at reasonable prices. Advances taking place today will address these key issues, leading to a very bright future for MLC-based SSDs.
ALEX NAQVI, president and CEO of SandForce Inc., received an MSEE from Oregon State University, Corvallis.