[colug-432] Solid State Drives (was: SMART HDD Status): Wear Out

[Rob Stampfli]

On Thu, Feb 23, 2012 at 06:20:40PM -0500, jep200404 at columbus.rr.com wrote:
> On Thu, 23 Feb 2012 18:01:56 -0500, Rob Stampfli <res at colnet.cmhnet.org> wrote:
> Writes wear out flash memory. 
> 
> /usr is for stuff that one expects to not change. 
> /var is for stuff that one expects to be changed. 
> 
> Likely, /usr (and similarly /bin and /sbin) were written to 
> when you installed your OS, then left alone, so the sectors 
> that content was on did not wear out and stayed the same. 
> /var (and perhaps especially /var/cache) was repeatedly 
> overwritten until it worn out. 
> 
> Mount /var on something less likely to wear out, like RAM, 
> or Moon bounce. 

Jim brings up a good point, and perhaps I should have mentioned this
in my original post.  (But, that post was getting pretty long as it
was, and it's always problematic as to what to include and what to
leave out.)

There is only one partition here, '/'.  There is no separate /var.
If there were, I perhaps could understand this better.  But, I can't
see how a static file under /var should be at any more risk of becoming
corrupted than a static file in /bin, when /var and /bin are on the
same file system.

FWIW, I have read a bit about the techniques used in these NAND
devices.  I understand the concept of active wear leveling, where
static blocks get moved around in NAND to even out the erase cycles.
But, I don't think that explains what went on here.

Maybe /var/cache/apt just had the bad fortune of being mapped to a
weak area on the NAND memory.

In any event, I am more curious about how solid state drives and their
spinning counterparts differ.  Should we presume that active file systems
on such a drive are likely to fail at a faster rate than quiescent file
systems?  (I don't think we make such assessments with spinning drives.)
And, are SS drives more bullet-proof than the generic SD card, which was
more likely designed to be slid into a camera rather than a Linux box.
In any event, I hope the answer is yes.

Rob