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


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

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