The embedded computing industry is being inundated with changes to storage products driven by technological advancements, market forces and environmental concerns. The precipitous drop in solid-state flash drive (SSD) prices has enabled designers, previously turned off by the cost, to now consider solid state as an option over rotating hard disk drives (HDD). With that decision comes a serious list of performance benefits, including improvements in durability and security, which SSDs have over HDDs.
Driven by higher clock rates and parallel interface performance limitations, the storage industry is moving rapidly towards serial interconnects. This affects both SSDs and HDDs and forces system designers to consider their options when faced with this choice.
The European Union's Reduction of Hazardous Substances (RoHS) directive bans lead in electronic components including storage devices. This wellintentioned requirement certainly affects consumer products, but it's tentacles reach far into the industrial and military space as commercial off the shelf (COTS) products are used in these applications. The shift to RoHS components and the serial to parallel interface shift required product end of life (EOL) management including monitoring product roadmaps, making last time buys and identifying secondary sources.
Solid State Flash
Until recently the default solution for most industrial or military storage needs involved storage using rotating drives and one of two widely used parallel interfaces — IDE/ATA or SCSI. Units are ruggedized external to the drive by way of costly shock mounting or thermal management schemes at the board or box level.
Solid-state flash drives in Compact Flash, 1.8", 2.5" and 3.5" form factors have long been available as form, fit and function replacements for rotating drives and they offer higher resistance to nasty environments compared to rotating drives. The barrier to making the shift to solid state was always sticker shock. Now, however, with the drop in solid-state prices at almost 80% in the past 18 months, it begins to make economic sense to consider flash storage. There are two cost points to weigh when considering which storage solution best fits an application — the up front material costs and the longer term costs. Flash storage still costs 15 to 20 times more than rotating storage on a per gigabyte basis. However, when considering the not so obvious long term costs — especially in rugged applications — the picture changes rapidly. Table 1 gives a brief performance comparison between flash and rotating storage.
In terms of rugged operation, solidstate drives are far and away the more capable choice. Nothing kills rotating drives like excessive shock, vibration and temperature conditions. The cost of a ruggedized rotating system goes well beyond the raw cost per gigabyte; in fact, the long term cost to mitigate the effects of harsh conditions is estimated to be anywhere from 5X to 50X the original cost of the drives, depending on the specific requirements. Harsh industrial and military environments share similarities, both in terms of rugged requirements and cost sensitivity, so it's important to look at the big picture. Ruggedization and maintenance costs over and above the initial lower cost of rotating drives can be placed into two categories, and when combined, can easily surpass any savings recognized at the onset of a project or program. It may well be that solid-state flash drives are now less expensive over the long run than rotating drives (Table 2).
In addition to durability, solid-state flash drives bring to the table a host of security features including secure erase (sanitization) and write protect. Data erasure in rotating hard drives is software driven and can take roughly one hour to erase 20GB of data, with no guarantees that residual data images don't still remain for the taking. By contrast, it takes approximately 20 seconds to erase a 16GB solid-state drive, leaving little or no residual data. In both cases, the question of whether or not this sanitization meets mil requirements is a topic of debate and a separate discussion. The PMC SecureStor from ACT/Technico uses an FPGA implementation for hardware enabled secure erase and write protect on a Compact Flash drive. Hardware activation allows the erasure function to continue as long as power is applied, and unlike software driven erasure, is not dependent on the software running in the processor. Read and write access times are comparable between solid state and rotating drives, and flash capacities in 2.5" drives are set to break the 128GB threshold soon. For most industrial applications, current capacities and bandwidth capabilities in both drive types is sufficient.
Hitachi, Fujitsu and Seagate, the three pre-eminent rotating hard drive manufacturers, have either EOL'd (end of life) their 2.5" 7200 RPM enhanced duty IDE/ATA parallel ATA (PATA) drives, or will soon in favor of the same breed of drives in Serial ATA (SATA) format. This follows the industry trend away from parallel interfaces driven primarily by the PC market. Only Seagate plans to manufacture 5400 RPM PATA drives past December with two rugged models targeted primarily for the embedded market. All three companies continue to manufacture 4200 RPM PATA drives.
This trend can also be seen in solidstate flash storage, where most cutting edge capacities are introduced in SATA format. The transition is driven by rising clock rates that now outpace the bandwidths of existing parallel interfaces. SATA drives are faster, support circuitry is far easier to lay out with fewer traces, signal skew problems are virtually eliminated at higher frequencies and cabling issues related to air flow and routing are reduced (Table 3). In addition, SATA drives support hot swap, a major advantage allowing for reduced mean time to repair (MTTR), easy data transport, and equipment declassification in military applications. Several manufacturers have introduced removable storage products based on SAT interfaces. The PMC ShuttleStor offers front panel hot swap capability for 2.5" rotating or solid state drives in PMC form factor.
PATA drives and controllers will continue to be supported for the embedded market, however, the choices may be limited. As with any technology platform, users of PATA drives should know their suppliers' drive roadmaps and, if necessary, make plans to buy ahead of any technology transition. For some applications performance tradeoffs may allow users to opt for a slower PATA drive — for example a downgrade from 7200 to 5400 RPMs — without impact to system performance. Forced to make the transition at an inopportune time, designers should take comfort in knowing there are numerous manufacturers offering SATA to PATA converters that allow use of a new SATA drive in legacy hardware. In this case, the new SATA drive looks to the system like the old PATA drive and requires no device driver changes. SATA drivers on the other hand, are available for the most commonly used operating systems.
The RoHS Revolution
The European Union's Reduction of Hazardous Substances (RoHS) directive banning lead, mercury, hexavalent chromium polybrominated biphenals and polybrominated diphenyl ether set off a chain of events which could ultimately impact most embedded computing hardware. The key element here of course is the lead used in the solder paste on circuit boards including those in hard drives. Most suppliers have made the transition already. As form, fit and function replacements for non- RoHS drives, the only issue is one of requalification, which is less an issue in industrial versus military applications. COTS sourcing implies RoHS components so designers will have no choice but to deal with the transition sooner or later. Think product roadmap, discuss the time line with your suppliers, and prepare to invest in last-time buys to keep moving.
The dynamic storage market offers many opportunities as well as many challenges for embedded system designers. The key to taking advantage of these changes involves an awareness of upcoming technology shifts, maintaining a close relationship with parts suppliers, and working with embedded equipment developers who leverage the latest storage technology advances in their products.