The present invention relates generally to methods, and apparatus for storage devices, and specifically to improving performance of non-volatile memory devices. More particularly, embodiments of the present invention provide a hard-read threshold optimization technique which can increase the hard-read ECC performance of non-volatile memory devices.
Non-volatile memory devices such as Solid State Drives (SSDs) are finding new applications in consumer electronics. For example, they are replacing Hard Disk Drives (HDDs), which typically comprise rapidly rotating disks (platters). Non-volatile memories, sometimes referred to as ‘flash memories’ (for example, NAND and NOR flash devices) are used in media storage, cameras, mobile phones, mobile computers, laptop computers, USB flash drives, etc. Non-volatile memory provides a relatively reliable, compact, cost-effective, and easily accessible method of storing data when the power is off.
In NAND flash memories, information is stored by cell levels. A read is performed by comparing a read threshold to the cell voltage. For example, if the cell voltage is higher than the read threshold, we get a 0, otherwise a 1, as a read result. To get soft information for better error correction performance, multiple rounds of reads, or a soft-read, are needed. To make sure most of data can be retrieved from a solid state disk within a certain amount of time, the soft-read can only be triggered with certain small probability. For example, in enterprise applications, soft-read can only be triggered with a probability lower than 0.01% or 0.001%. If the triggering rate of soft-read is higher, customers often consider the SSD as having deteriorated and having reached its end-of-line (EOL) and will replace the drive. Therefore, the hard-read ECC performance plays an important role in determining the life span of an SSD.