Flash drives like SSDs havebecome more and more popular. But since the early days of wear leveling there is one problem all NAND Flash devices have in common.
Corrupted data and data loss during sudden power failure!
Over the years there were several solutions, mostly solved in software to make the disks more stable.
But with the introduction of SATA III and the customer demands of higher data rates, the SSD manufactures had to think about how to improve the throughput and the IOPS of the drives.
To increase throughput, DRAM buffers were extended and some controller manufacturers started to compress the data inside the disk to achieve better Flash performance.
In a stable environment like a home computer/notebook this works very well but what happens if these drives are used in demanding industrial applications?
Imagine it like pulling the cord of your PC frequently while writing data to the disk.
Worst case not only data which is buffered in the DRAM gets lost, but also the rest of the data on the drive can be corrupted.
Why does data loss occur in a power fail?
While writing to a DRAM buffered SSD, your data will be temporarily stored in the RAM before the controller transfer it to the Flash. In some cases the data gets compressed first for higher transfer rates (bad if your application has compressed the data before), in some cases an intelligent writing algorithm prepares the data to optimize the Flash Cell usage. Once the power fails, the controller has no chance to transfer the DRAM buffered data into the Flash which results in data loss.
In some cases the controller will get a hold signal, to use the last milliseconds of power to process the last operation safely and prevent the entire drive from getting corrupted.
Why can the drive be totally corrupted?
While writing to the drive, the controller continuously updates the Flash allocation table. This is easily confused with FAT (file allocation table) derived from the file system. The Flash allocation table is an internal map of the drive where the different bits and bytes are stored. Due to the static and dynamic wear leveling, which is necessary to manage the balanced use of the NAND Flash cells and improve the reliability, the controller needs to remember where all the data fragments are distributed over the entire memory.
Essentially the Flash allocation table is also temporarily stored in DRAM but this doesn't effect the drive integrity very much. The data corruption can occur when the updated flash allocation table will be written to the flash while the power fails. The data will be still on the drive, but the map got damaged and the controller cannot find it any more. Over the past years we have also observed that in some customer test scenarios also the firmware of the SSDs can be damaged which ends in a product return to the factory to reset the drive.
How can Innodisks iCELL technology help?
Software can solve a lot of the listed issues if the power fails, but it cannot maintain the power to safely finish data storage.
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| Figure 1: Sudden power outage WITHOUT iCell |
iCELL is a clever combination of software, voltage detectors, capacitors and Innodisks flash controller ID102 to detect an abnormal power situation and maintain enough energy to store all data into the flash, save the NCQ status and end all operations safely before data will be lost, damaged or the drive gets corrupted.
It acts like small UPS (uninterruptible power supply) which sends a signal to the controller that something is wrong and immediately starts action before it gets worse.
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| Figure 2: Sudden power outage WITH iCell |
The following Innodisk products are equipped with iCELL Technology:
- 2.5" SATA25000 (industrial SLC based high performance SSD)
- InnoRobust II SATA (military SLC based SSD)
- Evergreen Plus (optimized MLC based SSD with L2 wear levelling)
We hope you found this post interesting.
If you would like to know more about iCell and Innodisk, please send an email to sales@score-electronics.com or look in the Innodisk section on our webpage.
Innodisk has published a white paper about iCell technology and is available on request.
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