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Date:      Sat, 21 Jan 2017 12:12:17 -0600
From:      Karl Denninger <karl@denninger.net>
To:        freebsd-arm@freebsd.org
Subject:   Re: how to measure microsd wear
Message-ID:  <1d757b3b-67d2-b29b-ba01-89b462b0019f@denninger.net>
In-Reply-To: <1485021485.34897.185.camel@freebsd.org>
References:  <16821b7c-e300-97fc-36e5-a508b22c21b8@zyxst.net> <1485021485.34897.185.camel@freebsd.org>
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[-- Attachment #1 --]
On 1/21/2017 11:58, Ian Lepore wrote:
> On Sat, 2017-01-21 at 15:46 +0000, tech-lists wrote:
>> Hello list,
>>
>> How would one measure microsd wear? Is there a utility like
>> smartmontools (I think this only works for regular hard drives) but
>> for
>> microsd?
>>
>> many thanks,
> There is basically no way to see what's going on in the flash array of
> an sdcard.  The microcontrollers in modern sd cards have complex wear-
> leveling algorithms which are completely transparent to the outside
> world.
This is true.
> On the plus side, most of what you see in the way of warnings and scare
> stories about wearing out sd cards is pure BS.  I've got systems here
> that have been running for literally years on the same sdcard, and that
> card is being used for swap, and routine data storage like syslog (on
> an embedded system that logs status and progress pretty much
> continuously 24x7 for years).  I've seen a few sd cards die over the
> years, but I've never been able to say it was because of how much was
> written to them (indeed, the dead ones I've got weren't in service long
> before they died).
>
This, however, is total nonsense.

I've had multiple SD card failures in build/test/high-volume write
environments on the PI2 series over the last year and change.  There are
two general ways in which you will see failures:

1. The card write-locks itself. This is a defensive move by the
controller when it determines that it cannot reallocate a failed block
during a write (e.g. it's out of spares) OR it takes an unrecoverable
read error.

2. The card loses its allocation map (in which case you're completely
screwed; it will show up as zero size if you manage to get it mounted
somewhere.)

If you get a type 1 failure you can copy everything on the card off;
provided you do not attempt to write it, you will not get errors.  Prior
to a fairly recent MFC if you had soft-updates on and took a Type 1
failure you'd get an instant panic; this has been (I believe entirely)
fixed.

In the event you get a Type 2 failure there's nothing you can do.  In
both cases the card is junk if it happens.

-- 
Karl Denninger
karl@denninger.net <mailto:karl@denninger.net>
/The Market Ticker/
/[S/MIME encrypted email preferred]/

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