Solid-State Drives for Point-of-Sale Systems



By C.C. Wu, Innodisk

MLC flash-based SSDs that incorporate wear-leveling and power-protection schemes present superior performance storage for POS applications at a competitive price point.

Overview
Traditional hard drives have been the storage media of choice in the point-of-sale (POS) industry for years, but recently higher performance solid-state drives (SSDs) have come down in price enough to compete. What are the benefits of SSDs for POS systems, and how do you choose the right SSD for your POS application?

POS describes a variety of computers used mainly in the retail and hospitality industry to conduct retail transactions. Whereas desktops, laptops and tablets describe specific form factors, point-of-sale systems range widely in form factor from desktop-like systems used in bars and restaurants and large integrated self-checkout machines in grocery stores, to handheld devices which are changing the face of retail and hospitality.

Faster, more reliable and more rugged than traditional hard drives, SSDs are a natural fit for the diverse needs of retail and hospitality environments. While consumers see all SSDs as the same technology, in fact SSD implementation and performance vary significantly.

SSDs use non-volatile NAND flash memory which can be implemented with top-grade SLC or lower-grade MLC memory cells. To stay competitive, POS vendors will tend to prefer MLC–based drives, but to mitigate the negative effects of lower-grade flash, strong controller technology is needed.

Choosing the appropriate SSD for a POS system requires insight into the performance characteristics of SSDs. Understanding the unique requirements of the POS environment can help designers create more reliable systems while lowering upfront costs as well as total cost of ownership.

Performance
Traditional hard drives are cheaper than ever. With advances in read heads, platter density, and rotational speed, modern hard drives have quite good IO performance—as long as data is being accessed sequentially. The limitations of spinning media which needs to physically seek data means random I/O performance will always be relatively slow.

Solid state drives, meanwhile have inherently superior I/O performance and in recent years have come down in price enough to compete with traditional hard drives. Compared to hard drives, solid state drives are more expensive per megabyte but have much better read performance, superior write performance and negligible seek or random access times. This means extremely fast boot-up times for fixed POS systems and wakeup times for mobile POS devices. File access times, application loading and I/O-intensive OS processes are also greatly accelerated with SSDs.

The greatest obstacle to SSD adoption has always been price, but technological advances have made reliable SSDS available at a price competitive for consumer and business applications.

Figure 1. Compared to hard drives, solid state drives are more expensive per megabyte but have many feature advantages.

Figure 1: Compared to hard drives, solid state drives are more expensive per megabyte but have many feature advantages.

Requirements for POS Applications
Point-of-sales systems are popular in retail and hospitality industries, which present challenges not found in consumer or enterprise environments. Most POS systems in retail and hospitality are located at a fixed place such as a cash register or a self-service kiosk. These fixed POS systems use similar storage form factors and interfaces to desktop PCs, but because of their usage environment, have unique demands. Unreliable power, shock and vibration, temperature changes and even liquid spills are a fact of life for many POS systems in the field. Files need to be accessed quickly and applications loaded fast so customers aren’t left waiting. Reliability, ruggedness and performance are therefore essential in the POS environment.

In recent years, mobile POS solutions have come to market presenting a new set of storage requirements. Handheld POS and inventory tracking devices used by sales representatives, clerks and even waiters not only need to perform just as well as fixed solutions, but also require small form factor storage that is even more resistant to shock and vibration.

While POS applications have higher mobility, ruggedness and reliability requirements than desktop computers, they are actually less demanding in other aspects. Point-of-sales transactions are generally not I/O-intensive, which means disk endurance is less of an issue. Sales transactions don’t take much disk space to store, so storage space is also less important.

Reliability and Endurance
POS applications require extreme reliability and uptime. In this respect SSDs clearly outperform magnetic media. While hard drives have spinning platters and moving heads which can fail or crash, SSDs have no such moving parts to wear out. Mean time between failure (MTBF) for an SSD is generally higher than for a comparable hard drive.

The Achilles heel of NAND flash-based SSDs has traditionally been service life in write-intensive applications, but recent developments have made this an issue of the past. Flash memory cells in an SSD have a limited number of writes before they become unreliable. For enterprise-grade SLC flash, this number is around 100,000 cycles, making it a non-issue. However SLC flash is priced out of reach for most POS applications. Consumer-grade MLC flash is competitively priced, but write endurance used to be quite low—in the thousands of cycles. I/O-intensive servers running old MLC-based SSDs have been known to wear out their disks in months.

MLC service life can be greatly enhanced, however, through innovative drive controller programming using wear-leveling algorithms. Most file systems are optimized for magnetic media and will tend to repeatedly write at the same location. For flash-based media, this will wear out memory cells extremely fast. With wear-leveling, an SSD maintains separate logical and physical addresses to make sure memory cells across an SSD are evenly written to, maximizing the life of the SSD.

Wear-leveling can be implemented as static or dynamic. Dynamic wear-leveling is easier to implement and wears evenly across blocks with “dynamic” or changing data, while cells that contain static data, such as operating system files, are left untouched. Static wear-leveling on the other hand spreads program erase cycles across the entire drive regardless of whether the blocks are holding static or dynamic data. Static wear-leveling is more difficult to implement, but essential for optimal SSD endurance. With wear-leveling technology, a modern MLC-based SSD deployed in a POS environment could expect service life equal to or greater than a hard drive with less chance of drive failure.

Figure 2. InnoDisk’s static wear-leveling process spreads the program/erase count evenly on all blocks.

Figure 2: InnoDisk’s static wear-leveling process spreads the program/erase count evenly on all blocks.

Ruggedness
Unlike desktop computers and servers, POS systems often have to operate in unpredictable environments. Even fixed POS systems placed at a retail cash register or a bar counter can be subject to bumps, vibration or spills. Power delivery to POS systems can also be unreliable.

In terms of shock and vibration resistance, solid state drives clearly win out over hard drives. Without moving parts, there’s nothing to physically break when a SSD is bumped or knocked. The weak point then becomes the physical interface between the drive and motherboard. Innovative features in some SSDs such as anti-vibration mechanical design allow these drives to lock to the connector and maintain their connection when bumped or even when dropped.

Figure 3. InnoDisk’s iCell technology provides a temporary power back-up to safely write all user data from DRAM buffer to the flash memory.

Figure 3: InnoDisk’s iCell technology provides a temporary power back-up to safely write all user data from DRAM buffer to the flash memory.

Since POS systems store important business transactions but operate in environments with unpredictable power availability, data integrity even through power loss is critical. Data corruption from sudden power loss affects both traditional hard disks and SSDs, but with flash storage media it is a critical issue. The SSD controller uses volatile memory to cache write operations as well as maintain physical addressing information. If data is suddenly lost, it can cause extensive data corruption in an unprotected SSD. A robust power protection system needs to both detect an impending power loss (e.g., a sudden drop in voltage) and then have enough backup power to write the crucial data stored in volatile DRAM into non-volatile flash cells. A power loss data protection scheme is an absolute requirement when selecting an SSD for a POS application.

Figure 4. InnoDisk SATADOM module without power cables is ideal for space-constrained designs.

Figure 4: InnoDisk SATADOM module without power cables is ideal for space-constrained designs.

Mobility
For handheld POS devices which need to be compact and light, storage form factor matters. Standard hard drives come in a very limited number of form factors and sizes, while SSDs have many more choices in their form factor. Flash memory can be integrated directly on the board for a handheld POS, but the SSD market has come up with an even more flexible solution. Disk-on-module, where the drive is shrunk and integrated with the connector, allows for SSD storage to be easily integrated into the smallest handheld POS devices. Because of the low-power requirements of SSDs, the power source can actually be integrated on SATA interface DOM drives by using Pin 7, meaning less cabling and even less space requirements. Compared to dedicated on-board memory solutions, SSD solutions like SATADOM allow POS vendors to offer rugged handheld devices with extremely reliable solid state storage.

Figure 5. The InnoDisk pin assignment shows pin7 repurposed as power (Vcc).

Figure 5: The InnoDisk pin assignment shows pin7 repurposed as power (Vcc).

Conclusion
The industry’s tides have changed. Migration from HDDs to SSDs is what makes sense for an effective system. When it comes to selecting storage for a POS system, flash-based SSDs present superior performance characteristics for all but the most budget-conscious applications. MLC flash-based SSDs that incorporate wear-leveling and power-protection schemes present superior performance storage for POS applications at a competitive price point compared to traditional hard drives. Faster I/O performance, better shock and vibration resistance and flexibility of form factor make SSDs the storage of choice for POS.


wu_cc
C.C. Wu is vice president of Innodisk and director of the Embedded Flash Division. He is a frequent presenter at the annual Flash Memory Summit held in Santa Clara, California and speaks on the topics of NAND Flash technology and embedded systems storage.

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