[ntp:questions] Selective peerstats collection

David Woolley david at ex.djwhome.demon.invalid
Sat Dec 8 11:35:07 UTC 2012

Hal Murray wrote:
> In article <k9q1fg$vd0$1 at dont-email.me>,
>  David Taylor <david-taylor at blueyonder.co.uk.invalid> writes:
>> Thanks for your suggestions, Hal.  At the moment, I don't have a handle 
>> on what to expect the lifetime of an SD card used as a Linux system 
>> disk, so therefore how much effort to put into finding a solution.
> What's the cost of a SD card?  Pick a medium size one and run the
> experiment with full logging.  You might get lucky or unlucky, but
> it will give you one data point.

I presume this is being done as a hobbyist project, where the value put 
on human time is very subjective.

Even commercially, you can have situations where SD card is part of 
hardware sold as part of some managed service, and where one is expected 
to send a technician out to replace it, making the cost more like £50-100.

The real problem, though is there seems to be no reliable information on 
life times.  It is fairly easy to find the raw lifetime of a cell 
(3000-5000 cycles) but the actual way that external writes are mapped 
into row writes tends to be a carefully guarded secret and any clues 
seem to be missing from packaging and marketing material.

What I found out is a suggestion that more expensive, branded, 
controllers are likely to do "wear levelling".  However, there is was 
also a suggestion that all cards, do not update in place, except for 
some sort of index memory, which is more resilient than the main memory. 
  They copy a row to a new location on each update.

How wear levelling can improve on the basic copy operation was not clear 
and nor where things like the row size and when the rewrite actually 
took place (using a write back cache of one or more rows would reduce 
the actual number of physical updates, but what is the policy for 
forcing a write).

All this sort of information seems to fall under the class of trade 
secrets that a buyer needs to know, to make intelligent buying and 
design decisions, but the release of which is vetoed by the vendor's 
marketing people, nervous of their IPR.

In the case of the Pi, they are designed for an extremely price 
sensitive market, and good quality SD cards cost about 25% of the cost 
of the computer board.  Cheap cards may well expire very quickly when 
used as a computer filesystem, rather than a still camera's "film".

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