On 3/25/2017 8:12 AM, Martin Gregorie wrote:
> 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.

Ok, so it is a PDA.  So what is different about using a PDA as a satnav
from using it in any other mode from the perspective of the SD card?

--

Rick C

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