2.11BSD on two RL02 drives? Probably not, but...

Phil Blundell pb at pbcl.net
Fri Aug 4 08:42:03 CDT 2017


On Fri, 2017-08-04 at 08:53 -0400, Noel Chiappa via cctalk wrote:
> 
> But are SD cards really that unreliable? 

It depends on exactly how you measure "reliable".  There are a few
different things going on, and it differs from one SD card to another.

Firstly, there are multiple types of flash memory that can be used for
the underlying storage.  The flash cells can be either SLC, MLC or TLC
in decreasing order of cost per bit but also in decreasing order of
robustness.  An SLC cell might tolerate 100,000 erase/write cycles,
whereas an MLC cell might fail after only 10,000 and a TLC cell might
be worn out after 3,000.  The act of reading from a flash cell also
disturbs the charge in nearby cells: this effect is not particularly
significant for SLC, which you can generally read without restriction,
but on MLC and TLC the controller needs to keep track of how many times
it has read from a particular block and periodically re-write the data
to refresh it otherwise it will eventually become corrupt.  And
finally, the charge in flash cells does eventually leak away: for MLC
and TLC cards your data may disappear over a timescale of months to a
few years.

Consumer-grade cards will almost always use MLC flash, or possibly TLC
at higher capacity levels.  SLC cards are available but typically they
are the "industrial grade" ones and are only available in smaller
capacities.

Secondly, there are many different controller ICs and the behaviour of
the controller has a significant effect on how well the card works
overall.  Some controllers are better than others at managing bad
blocks and bit errors in the flash.  Some deal better than others with
unexpected power failures.  Some deal better than others with the "read
disturb" effect.  Some deal better than others with random access,
particularly random writes: it's fairly common for the cheaper consumer
cards to only have enough buffer space for one output file to be open
at a time, and if you start trying to write multiple files in parallel
to different areas of the card then the write buffer will start
thrashing and performance will be dismal.

It's also worth remembering that most consumer-grade cards are used in
a way that is not a very close match to a disk emulator.  Cameras
(including cameras in phones) are generally writing a relatively small
number of fairly large files.  They seldom read, and they virtually
never modify a file in place.  Also, because all SD cards come
preformatted as either FAT or exFAT, the pattern of accesses that the
host will make to the filesystem is somewhat predictable and some
controller ICs are specifically optimised for this.

All the above said, although it probably is true that the average
consumer-grade SD card is significantly less robust than the average
SATA SSD that you can buy today, and probably also less robust than the
average spinning hard disk, I suspect they are probably not all that
much less reliable than the average 1980s or 1990s-era hard disk. 
Personally I would be quite happy to use an SD card to emulate mass
storage in a classic computer, and in fact I was thinking just this
morning about buying one of the scsi2sd boards for that very purpose. 
If you are going to be using it in an application that sees frequent
writes then I would try to get an SLC card, or failing that a low-
capacity MLC one.  But otherwise, I would just make sure I had a backup
and live with the possibility that the card might need replacing after
a couple of years.

Phil



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