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SSD Synchronous NAND Print
November 2012

We have previously talked about NAND memory in terms of SLC (single level cell) versus MLC (multi level cell).  SLC refers to one bit per cell and MLC refers to 2 bits per cell as of November 2012.  This note introduces a different dimension of NAND and it is Synchronous versus Asynchronous in terms of timing.  Both SLC and MLC can be Synch or Asynchronous.

Similar to the standard main memory of PC called DRAM being Dynamic Random Access Memory, NAND is graded in terms of read and write interface speeds.  There is indeed an open industry workgroup for standardising NAND speeds.  This organisation is ONFI for Open NAND Flash Interface  It defines component-level interface specifications as well as connector and module form factor specifications for NAND Flash and has published version 1 and 2 and is working on version 3 as in November 2012.  Interface speeds are as shown below. 

v1 for 50MB/s (we call this asynchronous in this context)
v2 for 133MB/s (we call this synchronous in this context)
v3 for 400 (in progress)

We have looked at a popular brand of SSD and have observed both versions of NAND were used and sold on 2 price levels.  Version 2 model consumed 3W of power and version 1 consumed 0.9W only when active.  The biggest difference is in performance.   Version 1 achieved a lower “read and write” performance benchmark than Version 2 on incompressible data and is close on compressible data.  See slide 1. 

(Hover mouse over to enlarge)



These SSD models employed a drive controller that excels in performance by applying compression to the process of writing data from host to NAND.  However, some data types such as multimedia files are already compressed and they cannot be further compressed by the SSD controller.  These data types are called incompressible data in the context here.

We came across another source of benchmarking information online which talked about the difference between Synchronous and Asynchronous.  We found the results of performance benchmarking with Crystal Write Mark and AS SSD Copy to be eye opening. See slide 2 and 3.

Version 2 performed better than Version 1 on both compressible and incompressible data!