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NAND Flash Copyback Command - Hidden Potential for Drive Corruption

Posted by Samuel Nakhimovsky on Tuesday, 03 Jun 2014

Usage of NAND Flash COPYBACK Command Causes Drive Corruption and Data Loss

One of the "Beneficial" features that was added into NAND Flash operation was the COPYBACK Command.

Also known as the INTERNAL DATA MOVE (IDM) operations, Copyback was promoted by the NAND Flash vendors as a performance increasing feature which enabled the data page to be copied from one memory location to another within a NAND device without transferring the data off-chip to a Flash controller.

Copyback Command

As the Copyback operation required only was a fraction of time compared to a full data page move and reprogram sequence, it quickly became a favorite choice to enhance the Wear Leveling or Garbage Collection operation within SSD execution.

However, because the data was copied directly from one memory location to another all associated errors (bad bits, etc.) and Error Correction (ECC) codes we copied verbatim to the new location. In the event where the old memory location had bad bits, the data and the ECC codes would be copied exactly as they were read, without any error correction.

In a regular SSD operation, when data is read by the host, the Flash controller transparently corrects any detected errors through an ECC engine.Once a level of bit errors approaches a theoretical maximum that the ECC engine is capable of correcting, the Flash controller re-writes the corrected data page to a new location, zeroing out the accumulated errors.

For a small subset of the customers, whose drive usage model involved heavy data writes and infrequent reads, the Copyback command feature proved to be a real Trojan Horse. While the performance improvement was noticeable due to frequent data moves the drives started to fail after lengthy field deployment.

Extensive Failure Analysis was required to determine the cause of drive corruption. After looking specifically at each memory block it was evident that the high rate of bit errors was a direct cause of the overused Copyback command.

Fortasa Memory Systems Flash Controllers have DISABLED the usage of the Copyback command. While our products are not fully optimized for top performance, we guarantee that our end products will not fail due to uncorrectable ECC errors consitututed by the Copyback command usage. Fortasa Solid State Storage Solutions are the highest reliability memory devices guaranteeing many years of dependable and reliable operation.

Please contact Fortasa for reliability information of any Fortasa product.