[LEDE-DEV] [PATCH] base-files: seed /dev/urandom

[Arjen de Korte]

Citeren David Lang <david at lang.hm>:

> On Mon, 13 Jun 2016, Daniel Curran-Dickinson wrote:
>
>> On Mon, 2016-06-13 at 22:10 +0200, Etienne Champetier wrote:
>>
>>> Before we try to minimize writes, how much writes are we talking about?
>>> my openwrt routers have multiple months of uptime, and even if we get
>>> down to 1 week, that gets us to 53 writes a year.
>>> How much writes can a flash handle these days?
>>
>> I'm told that even cheap wear-levelling and flash chips from 4-5 years
>> ago with even 4 MB flash that it's really non-issue (and done the math
>> for what would be the case if wear levelling worked as claimed, on all
>> blocks, not just currently unwritten; the real question is whether
>> wear-levelling works as claimed, or more to the point whether there is
>> any *hard data* that says wear levelling does *not* work as claimed, or
>> whether persistent anecdotal stories of worn out flash are based on
>> urban legends, or classic mis-attribution).
>
> Well, we don't have a layer that will implement wear leveling in  
> OpenWRT/LEDE do we? We use the raw flash and the filesystem, so  
> unless the filesystem is moving directories they will be hot spots.

Indeed.

> But even in that case, aren't we talking hundreds of thousands to  
> millions of writes before there is a problem?

Not really. Assuming NOR flash (which is most common in the 4-16 MB  
range), write endurance is usually typically something in the order of  
100.000 times or more. It's the drop in retention time that worries  
me. See

https://www.micron.com/~/media/documents/products/technical-note/nor-flash/tn1230_nor_flash_cycling_endurance_data_retention.pdf

At a temperature of 55 degrees C or above, retention times can be in  
the order of 1 year (or less) for flash that is written a lot. You  
should always keep writes to a minumum (< 1000). On the other hand, if  
writing to flash once every boot is will be an issue, you have bigger  
problems to worry about.

Arjen