How Controllers Maximize SSD Life – External Data Buffering

Tempus FugitSince NAND flash is weakened by erase/write cycles then it would make sense to try to reduce those cycles to prolong the life of an SSD right?  That’s what external data buffers are designed to do.

There are many ways to use RAM (either a RAM internal to the SSD controller chip or a discrete DRAM chip on the SSD’s printed circuit card) to stage data in a way that will reduce erase/write cycles.

One is to perform a function called “Write Coalescing.”  This involves gathering several short writes to adjacent SSD sectors to turn them into a single long write from the buffer into the NAND flash.  One large write is less taxing to the chip than are several small writes.  It’s also a lot faster.

Here’s an example: In an SSD without a write buffer a number of short writes to the same general area in the drive may not all occur at the same time.  An early write is likely to already be committed to the flash before a later one is received by the SSD.  When the later write is received the flash block that contains both sectors may need  to be reassigned to another block in order to perform the operation, leading to an additional erase/write cycle.

Write coalescing is not the only way a RAM buffer can be used to reduce write traffic to the flash in an SSD.  Another approach involves buffering several successive writes to the same sector, without actually writing to the flash until several such writes have been performed.  Although the system may believe that it is repeatedly overwriting the same NAND sector, in truth the overwritten sector is in RAM until that RAM is needed for some other task.  Several hundred writes to a single sector may turn into a single write to NAND.  This means that in some cases the endurance of the NAND may be increased by a couple of orders of magnitude, which is a pretty good return for a couple of dollars’ worth of RAM.

RAM is also used to assure that writes occur in full page lengths, since this is the away that NAND most efficiently performs a write.  By matching the length of a NAND write to the chip’s page length the buffer helps to reduce write traffic.  Different NAND chips have different page lengths (generally 1-4kB) so the controller has to be informed of the page length of the exact NAND chips used in the SSD.

Any of these approaches creates a risk that data in RAM will be lost during a power failure.  The work-around for this is to provide internal energy storage that can keep the SSD alive long enough to write the RAM’s contents into the NAND for an orderly shut-down.  The power for this process is commonly provided one of three ways – it can be from a:

  1. Battery
  2. Super Capacitor
  3. Bank of Tantalum Capacitors

We won’t join the debate about which of these is best – it’s not the subject of this post.

Still, it’s clear that adding a RAM as temporary storage can do a lot to lengthen flash lifetime well beyond the limit of its endurance specifications.

This post is part of a series published by The SSD Guy in September-November 2012 to describe the leading methods SSD architects use to get the longest life out of an SSD despite the limited number of erase/write cycles that NAND flash specifications guarantee.  The following list provides the names of all of these articles, and hot links to them:

Click on any of the above links to learn about how each of these techniques works.

Alternatively, you can visit the Storage Networking Industry Association (SNIA) website to download the entire series as a 20-page booklet in pdf format.

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Jim Handy
Objective Analysis
SSD Market Research
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