Solid-state disks offer 'fast erase' features


As the pilot ejects inside enemy territory, the fighter jet triggers an automatic data-destruction sequence. Within 15 seconds, the highly classified mission data on the solid-state disk has been wiped out.

The storage device in this scenario didn't just burn up like the voice recorder in Mission: Impossible. Instead, the system's manufacturers simply took advantage of a key property of the flash memory chips that make up solid-state disks: Data can be erased much more quickly and thoroughly than it can with a magnetic, spinning hard disk. Solid-state disks, or SSDs, don't require six or seven passes to erase all traces of the bits on every track and sector. Once the bits have been reset in every flash memory cell, that data is gone forever, although meeting the most stringent government disk-sanitization requirements may still involve two or more passes.

The process is quick and efficient. "You're talking about seconds," says Gary Drossel, vice president of marketing at SiliconSystems Inc., a manufacturer of SSDs used in government systems. With a typical hard disk, just the process of getting every block on a drive of that size to spin under the read/write head would take almost an hour and a half, and the entire process could take three to four hours on a fast eSATA drive, according to experts at Texas Memory Systems Inc. and Kroll Ontrack Inc.

How Clean is Clean Enough?

Hard disks can leave behind a magnetic residue that, theoretically -- because of something called the hysteresis effect -- could be used to reconstruct the hard disk data. Whether in fact that's practical with today's disk drives is a matter of debate. But does this same potential weakness apply to solid-state drives? No, says Jamon Bowen, enterprise architect at enterprise SSD maker Texas Memory Systems Inc. If normal disposal processes are followed, your SSD data will be irretrievably erased, he says.

"With NAND [flash], you're storing a full amount of electrons on a floating gate, so there's no real way of telling what the value of that transistor used to be. Once you fully erase the drive, there is no ability to recreate the data," says Drossel.

"To my knowledge, a reprogrammed cell does not contain any previous data," says Sean Barry, senior data recovery engineer at Kroll Ontrack Inc. But not everyone agrees. BitMicro Networks Inc. claims that a hysteresis effect can also affect SSDs and has created a hardware-level secure-erasure function to deal with that. For all practical purposes, however, if data-recovery firms can't get the data back after a single secure-erase pass, most nongovernment users needn't worry about it.

Assuming, that is, that the erasure pass got everything. Unfortunately, some cells can be missed during the process. SSDs have extra memory cells beyond what is allocated to the file system. These are used by a "wear-leveling" feature that distributes data across that larger area to extend the life of the SSD. Those cells may be swapped in and out of the area used by the file system.



Instant Erasure




In contrast, SiliconSystems' fast-erase feature requires power, but disconnecting the drive won't kill the process: Erasure continues the instant that power is restored. "There's no way to stop it," says Handy. The technology can be applied to the whole drive or a preconfigured secure "zone" on the SSD that's also protected by a password.

SiliconSystems also offers an SSD self-destruct feature that applies an "overvoltage" to each of the flash chips, physically destroying them. The destruction can be triggered via software or a physical switch, says Drossel. SSDs can also be designed to self-destruct or erase if they are stolen and inserted into any unauthorized machine.

In the private sector, rapid-erasure techniques could be used in point-of-sale systems or kiosks that might contain sensitive customer or sales data. "The data may be gone, but at least it's not in the wrong hands," Drossel says.

More Costly Recovery

The flip side of the level of security SSDs offer is the fact that recovering data from them can be more difficult and expensive than for other media.

Each SSD vendor has its own proprietary method for mapping data from the file system to individual memory cells. "If you don't have the mapping table that records where everything is kept, you have random data distributed throughout the chips," says Bowen. "Everyone follows their own data-placement schemes. Without knowing the details of that, it would be next to impossible to piece all of that together."

That may be true for a hacker, but not for data recovery specialists, who can pull data even when an SSD has sustained physical damage. "Kroll Ontrack has developed methods to recover data without the controller chip available," says Barry. "We've been successful in discovering a number of data layouts for different manufacturers."

Another drawback is that data on SSDs can be far more costly to recover in the event of a physical failure, such as a broken circuit. "When an SSD becomes damaged, it's more difficult to get the data off the raw chips. We've had jobs go as long as three or four months," Barry says. Costs go up if the data is needed quickly and additional staffers are assigned to the project. "That jumps up the service level," he says, "and they pay accordingly."

WinTel and SSD

In the future, changes in how the Windows file system interacts with SSDs may improve both security and performance for end users. Today, when a user deletes a file on a Windows computer, the file system removes the pointers to the locations on disk where the data that makes up the file resides, but the data itself remains until the space is allocated to another file. Only then is it overwritten. That's why erased files can be recovered on hard disk drives, and SSDs operating under Windows are no different. But that may be about to change.


Encryption: SSD's Missing Link




One reason why more manufacturers don't offer SSD is that, while encryption algorithms themselves are standardized, there are no standard implementation methodologies for full disk encryption, so each vendor must roll its own proprietary solution. That adds to the cost. Drossel estimates that encryption would add 10% to the cost of its SSD products today.




Wilkison says in the enterprise-class market, full disk encryption isn't always a requirement, even when sensitive data is involved.

"Hardware-level encryption may be unnecessary because data is being encrypted upstream," he says.

On the end-user side, most people simply don't care about full disk encryption -- on hard disk drives or SSDs. "On the consumer side, it's about reliability and ruggedness. On the military side, they want to encrypt data or destroy it immediately. There's no middle ground," says Bowen.

Eventually, full disk encryption may be offered with all SSDs. But it won't sell at a big premium, Wilkison predicts. Rather, as competition intensifies, he predicts that manufacturers will add it as just another differentiating feature.

But don't look for full disk encryption any time soon. While some vendors say we'll see some implementations in 2009, others say they don't expect to see systems with SSDs that offer full disk encryption until 2010 at the earliest.

Shredding the Evidence

While a fully overwritten drive is unrecoverable, the best way to ensure complete data destruction when disposing of SSDs is to physically destroy them, says Barry. "If you ran that through a grinder and completely chopped that up in quarter-inch chunks ... that is by far the best way to make sure the device is unrecoverable."

But every flash chip must be destroyed, and existing shredders may not be up to the job. "Shredders for disk drives might not be adequate for SSDs because the chips are so much smaller [than disk drive platters]," says Bowen. SSDs have arrays of tiny flash chips -- anywhere from eight to 30 per device. Any that are missed by the shredder would still be readable by data-recovery specialists such as Barry.

Next: QuickStudy: Extensible Access Method (XAM)

Copyright © 2009 IDG Communications, Inc.

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