On Fri, 24 Mar 2017 22:01:39 -0400, rickman wrote:

> On 3/24/2017 8:30 PM, Martin Gregorie wrote:
>> On Fri, 24 Mar 2017 21:09:33 +0000, James Harris wrote:
>>
>>> That's the first I have heard of flash failure modes. From a quick
>>> read-up it seems that writes start to fail. Presumably they are only a
>>> problem if they go undetected. So don't flash drivers read back what
>>> they have written?
>>>
>> There's not a lot about easy-to-find stuff about flash failure modes,
>> but there was a good thread about it on this newsgroup about three
>> weeks ago.
>> Here are the links and references I kept from it:
>>
>> 1) There are three kinds of devices:
>>
>>    - those which do no wear levelling at all. A given logical disk
>>      block always maps to the same physical block, i.e. the same
>>      transistors. The blocks holding frequently re-written data wear
>>      out quickly.
>>
>>    - those which do dynamic wear levelling, so each time a given
>>    logical
>>      disk block is written, the hardware chooses, *from those currently
>>      not in use*, a different physical block to map it to. This helps a
>>      lot, but only if the device has a good number of unused blocks to
>>      choose from. Don't run this kind of device close to full.
>>
>>    - those which do static wear levelling. Blocks which hold in-use,
>>      but infrequently modified, data are occasionally rotated into more
>>      heavily used cells, so that the whole device free and in use wears
>>      out at the same rate.
>>
>>    - Thats a summary.
>>      More here: https://en.wikipedia.org/wiki/Wear_leveling
>>
>>    - the summary is more or less verbatim from John Aldridge
>>
>> 2) "Flash memory card design" covers the relationship between pages,
>>    erase blocks and allocation groups and describes their impact on
>>    device life and throughput:
>>
>>    https://wiki.linaro.org/WorkingGroups/KernelArchived/Projects/
>> FlashCardSurvey?action=show&redirect=WorkingGroups%2FKernel%2FProjects%
>> 2FFlashCardSurvey
>>
>> 3) Andrew Gabriel posted a good piece about
>>    Enterprise_vs_Consumer Flash media
>>
>> All were in a thread in this newsgroup called "High traffic in MySQL
>> can corrupt SD?". The first post was on the 1st of March, so your
>> newsreader may still have a copy. Failing that, there's always Google
>> Groups, which should have the whole thread.
>>
>> ======
>>
>> Take a look at this: http://www.bunniestudios.com/blog/?p=918
>>
>> which gives an excellent education of why you should be careful where
>> you buy SD cards and who you buy them from. Well worth the read.
>>
>> Lastly, here are my thoughts on how best to avoid damaging and/or
>> corrupting SD cards. This is mainly discussing their use in PNAs that
>> are being used as navigation aids, but it is also directly applicable
>> to using them in RPis though the cards used to hold these programs and
>> the associated maps, log files etc can be quite a lot smaller that
>> you'd ever plug into an RPi:
>>
>> http://www.gregorie.org/gliding/pna/sd_card.html
>
> What exactly is a PNA?  Your web page uses the term, but doesn't explain
> it.  At first I thought it would be some sort of satnav device, but then
> you refer to, "PNAs that are being used as navigation aids".  If only
> some of these devices are used for navigation, then I would think we
> aren't talking about navigation devices....???
>
> https://en.wikipedia.org/wiki/PNA
>
> http://www.urbandictionary.com/define.php?term=PNA
>
> So what is a PNA?  Is this something like an iPad?

Sorry: thought it was more generally known term. Stands for Personal
Navigation Assistant, i.e. a PDA (Personal Digital Assistant) with a
built-in GPS.

Probably the best-known PDAs were the Compaq iPAQs, though HP sold them
too.

PNAs are hand-held satnav units. The best-known over here were made by
Garmin, Binatone, Medion and Dell, the Streak. They generally have
walking and/or automotive navigation software in firmware and many of
them ran under WinCE or Windows Mobile. These would generally run other
programs from SD cards.

Both PDAs and PNAs have largely disappeared with the death of Win CE/
Mobile and the arrival of cheap smartphones, which is a pity because the
best PNAs had trans-reflective screens that are much more readable in
direct sunlight than any phone I've seen. The best current replacements,
if a phone can't hack it, are e-Ink eReaders, e.g. the smaller Kobos.

Size can be important, because PNAs often need to be pocketable or are
installed on flexmount in front of an instrument panel and so must be
small enough to avoid hiding instruments mounted in the panel. I use
units with a 3.5" screen for that reason, but almost nobody I know uses
anything with more than a 5" screen.



--
martin@   | Martin Gregorie
gregorie. | Essex, UK
org       |

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