File systems usage patterns and NAND lifetime

[Ed McNierney]

Deepak -

Thanks very much for the report and the notes; this is great stuff.   
Of course, my first question is to wonder how long that GPS has  
actually been in use by customers <g>.  Any other real-world NAND data  
would certainly be worth sharing with the team.

	- Ed

On Oct 10, 2008, at 2:17 AM, Deepak Saxena wrote:

>
> I attended and Embedded Linux Conference [1] last week  at which I
> saw a great talk on "Managing NAND Over A Product Lifecycle" [2].
>
> The speaker presented the case of determining whether a choosen
> NAND HW and SW combination will survive the estimated lifecycle
> of a product. As an example, he used a GPS device his firm worked
> on in which they had some very specific usage data such as:
>
> - The average runtime for the device is 4 hours a day, during
>  which we will see 100bytes/second of application logs
>  written, 2300 bytes written for the addressbook,
>  1KiB/second used for temporary storage as mapes are
>  decompressed.
>
> - The user will on average update the map data from his/her
>  PC every such that it requires 3GiB writes/quarter.
>
> - OS and application updates require 32MiB/quarter.
>
> There were many other data points, please refer to the slides
> for full details.
>
> With this data, they were able to  generate an I/O model of the
> application that was used to drive nandsim, an in-kernel NAND device
> simulator. By doing this, they were to replicate the product's  
> expected
> lifetime before user replacement (3 years) in a matter of a few days.
> nandsim + the UBI reporting mechanisms were used to generate detailed
> reports of the wear leveling behaviour of the system, how the  
> filesystem
> reacted to bitflips, bad pages, etc. Using this they were able to
> determine how to layout their filesystem and to meet the lifecylce
> requirement. After this was done, they used the same I/O model was  
> used
> to rapidly drive a real device toward failure modes to see how it
> would react. If it didn't survive for the expected lifecycle,
> they could analyze the data and figure out what settings to tweak.
>
> In this talk I also learned about the MLC NAND property of "read
> disturbance", where a read to one page can cause a bit-flip on an
> adjacent page.
>
> I found the talk fascinating and it has made me wonder if we
> have any idea what our typical deployed usage patterns might
> look like?  How often does the journal write to disk and how
> big is each write write?  How often do systems reboot and
> require a full filesystem read vs simply suspending/resuming?
>
> Related to this topicm I am also wondering  what is the expected
> usable life of the XO? We're used to product replacement every few
> years, sometimes faster depending on the product segment, but I
> doubt countries that are investing $millions expect to only get
> 2-3 years of use out of the XO.
>
> ~Deepak
>
> [1] http://mvista.com/vision/
> [2] http://www.mvista.com/download/fetchdoc.php?docid=329
>
> -- 
> Deepak Saxena - Kernel Developer - dsaxena at laptop.org
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