The State of Solid-State Disks
Prices are dropping, and solid-state disks are no longer confined to niche markets.
By John Jory
Solid-state disks are high-speed, random-access storage peripherals based on DRAM technology. Originally developed to get memory-speed response from storage devices and to increase the reliability of storage devices, today these disks are used to ease I/O bottlenecks in computer systems running relational database applications. As such, they are sometimes referred to as database accelerators.
While there are fewer reliability problems with magnetic disk drives due to RAID implementations, response times remain a significant issue. In fact, even with today`s fast-access disks, more than 90% of the time it takes to transfer a 4KB block is due to the mechanical delays of moving the read/write head and rotating the disk--not the CPU speed. This means that a system`s performance is limited by that of the peripheral storage device (i.e., disks).
This is especially true of systems running relational databases such as Informix, Oracle, and Sybase. A number of separate I/Os are often needed to complete a single transaction. To locate data, for example, users must first access the index file. In a multi-user system, the disk containing the indexes can become the bottleneck. If the disk holding the indexes performs, say, 75 I/Os per second, but the disk needs to be accessed more frequently than that, a queue forms. The CPU must then manage the queue, which adds overhead to the system and further slows down the system.
By replacing the conventional disk with a solid-state disk, the response time can be reduced, allowing thousands of I/Os per second instead of less than 100. In financial applications, such as stock trading systems, this decrease often translates into increased revenue for many companies. In a batch-processing application, the runtime can be reduced so it does not overflow into the next shift.
A solid-state disk drive emulates a standard SCSI-interface hard-disk drive. Most installations mount the solid-state disks on a separate SCSI bus (either 20MBps Fast/Wide or 40MBps Ultra SCSI), which allows them to use the entire bandwidth of the bus for maximum performance. However, since solid-state disks use either Fast/Wide SCSI or Ultra SCSI, their transfer rate is only 5 to 10 times faster than conventional disks.
As newer, faster buses become available (e.g., 100 MBps Fibre Channel or 80MBps Serial Storage Architecture), the performance advantage of solid-state disks over conventional disks will increase.
However, solid-state disks do not have to replace all conventional disks on a system to have a significant effect on performance. Simply mounting your "hot files" (about 3% to 5% of your files) on these disks will improve your system`s performance considerably.
Most operating systems provide utilities that determine which disks and therefore which files are used most frequently. Although the trend is to install 1GB to 4GB solid-state disks, the majority of applications can benefit by installing units of 0.5GB or less.
How much better will your system perform by adding a solid-state disk? If an application is I/O intensive, as are many database applications, you will probably notice a significant improvement.
But to determine how much, if any, improvement you will see and to determine which files are hot, you need to use the utilities that come with most operating systems and databases (e.g., PERFMON in Windows NT and SAR, IOSTAT, VMSTAT and GLANCE in Unix). Of course, performance ultimately depends on a number of factors, including application design, database version, number of processors and I/O channels, memory, and your operating system.
Another significant benefit of solid-state disks is added reliability. By mounting the "hot files" on a solid-state disk rather than on a conventional disk, thrashing-- a common cause of premature disk failure--can be avoided.
Also, cost, historically the main drawback to solid-state disks, has dropped significantly. Solid-state disks now sell for under $40 per MB, a 10:1 decrease over the last seven years.
Who`s Offering Solid-State Disks?
Not all systems need solid-state disks, so many system manufacturers don`t include them in their standard product offerings. Therefore, the solid-state disks are usually sold either after market to correct performance problems or to systems integrators and VARs who understand the high I/O requirements of applications and include solid-state disks in their solutions.
After-market sales are made by a number of channels. Computer manufacturers occasionally suggest solid-state disks as solutions to specific problems. Consultants, often experts in relational databases, sometimes recommend them. And system administrators occasionally deal directly with solid-state disk manufacturers to improve performance. More often, however, VARs or system integrators provide the bundled solution.
Solid-state disks are available in a variety of physical packages, including industry-standard 3.5-inch and 5.25-inch chassis for internal mounting, tabletop, and tower enclosures and 19-inch rack-mount chassis.
They also typically include operator control panels that permit manual backup of data to the internal disk, RS232 ports that allow automatic battery checking and remote diagnostic capability, and automatic data shadowing to a separate storage device without host intervention--all of which are not available on traditional rotating disks.
Since users` "hot files" are absolutely critical, error detection and correction (EDAC) data integrity circuitry is built into the memory. All solid-state disk suppliers offer some form of EDAC. Single-bit error correction with double-bit error detection on 8-byte words is the minimum level of data protection offered.
Some solid-state disks have single-bit error correction and multi-bit error detection on 4-byte words. The best products offer EDAC circuitry that also corrects multi-bit errors.
Solid-State Disks Smooth Out I/O Bottlenecks
Cyrk Inc., a provider of promotional products and marketing services, focuses primarily on large corporate customers. Cyrk (Polish for "circus") designs and manufactures sports apparel and accessories for com- panies, including Pepsi, Philip Morris, Caterpillar, and Mars. With sales topping $250 million, Cyrk has more than 900 employees worldwide. At its Gloucester, MA, headquarters, employees access the enterprise network daily. Applications include transaction processing, accounting, accounts payable/ receivable, general ledger, purchase order, and costing.
The demands on Cyrk`s IS department were overwhelming. Eight 2.9GB hard drives--including four used to mirror the system--labored under the company`s I/O demands. The supporting software applications running on the three Sybase-based SunSparc 1000E platforms (each packing four 85Mhz processors) demanded 200MB of temporary database usage.
Relentless transaction processing across the network caused I/O bottlenecks within the database, resulting in delays for employees tracking inventory, sourcing products, invoicing customers, and filling orders.
After an analysis of the situation, it was determined that most applications, such as inventory and purchasing, involved scans of voluminous data to generate the necessary reports. "Users had to wait for data to be retrieved from storage devices before reports could be generated," said Sudhir Bajaj, Cyrk`s CIO.
Bajaj knew he needed a solution. Outside of modifying the software (a formidable effort without the source code), the only other logical consideration was to relieve the I/O bottleneck. Consultants suggested solid-state disk technology.
Imperial Technology`s MegaRam-3000 connects directly to Cyrk`s Sun platforms via standard SCSI-2 Fast/Wide interfaces. Because no special drivers or software has to be loaded, the solid-state disks appear to the systems as conventional disk drives. They have a transfer rate of 20MBps per port, an access time of only 0.035 milliseconds, and a capacity range of 67MB to 4GB. Other features include up to three extra SCSI interface ports and advanced error detection and correction circuitry. The multi- porting feature allows the disks to provide redundant data paths to a single host or access to the disks from multiple host computers.
Redundant power supplies and internal backup disk drives preserve data in the case of a temporary power loss. If power fails, data is transferred to the internal disk via an independent bus, regardless of the duration of the power failure. Data is automatically restored to the solid-state memory when power is restored.
"With the addition of the solid-state disks, the I/O wait time was significantly reduced and performance was improved remarkably," says Bajaj. He says Cyrk`s sales, marketing and warehousing staffs experienced a 300% to 400% performance increase after transferring a Platinum database to the SSDs. "This dramatic improvement in database I/O access far outweighed the cost factors and fully justified the price," he adds.
John Jory is vice president of sales and marketing at Imperial Technology in El Segundo, CA.