File systems usage patterns and NAND lifetime

[Philippe Clérié]

Valerie Henson blogged about SSD's a while back 
(http://valhenson.livejournal.com/25228.html). Since then I've made 
sure I back up anything I have on flash.

Philippe

------
The trouble with common sense is that it is so uncommon.
<Anonymous>
On Friday 10 October 2008 01:17:43 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