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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Predictive failures
Date: Wed, 17 Apr 2024 08:42:03 -0700
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On 4/17/2024 5:18 AM, Chris Jones wrote:
> On a vaguely related rant, shamelessly hijacking your thread:
> 
> Why do recent mechanical hard drives have a "Annualised Workload Rate" limit 
> saying that you are only supposed to write say 55TB/year?

Are you sure they aren't giving you a *recommendation*?  I.e., "this
device will give acceptable performance (not durability) in applications
with a workload of X TB/yr"?

I built a box to sanitize and characterize disks for recycling.  It seems
a typical run-of-the-mill disk performs at about 60MB/s.  So, ~350MB/min
or 21GB/hr.  That's ~500GB/day or 180TB/yr.

Assuming 24/7/365 use.

In a 9-to-5 environment, that would be (5/7)*60TB (to account for idle time
on weekends) or ~40TB/yr.

Said another way, I'd expect a 55TB/yr drive to run at about (55/40)*60MB/s
or ~80MB/s.  A drive that runs at 100MB/s (not uncommon) would be ~100TB/yr.

> What is the wearout mechanism, or is it just bullshit to discourage enterprise 
> customers from buying the cheapest drives?
> 
> It seems odd to me that they would all do it, if it really is just made up 
> bullshit. It also seems odd to express it in terms of TB read+written. I can't 

As this seems to be a relatively new "expression", it may be a side-effect of
SSD ratings (in which *wear* is a real issue).  It would allow for a rough
comparison of the durability of the media in a synthetic workload.

> see why that would be more likely to wear it out than some number of hours of 
> spindle rotation, or seek operations, or spindle starts, or head load/unload 
> cycles. I could imagine they might want to use a very high current density in 
> the windings of the write head that might place an electromigration limit on 
> the time spent writing, but they apply the limit to reads as well. Is there 
> something that wears out when the servo loop is keeping the head on a track?

I've encountered drives with 50K PoH that still report no SMART issues.
I assume they truly are running 24/7/365 (based on the number of power cycles
reported) so that's *6* years spinning on its axis!  (I wonder how many
miles it would have traveled if it was a "wheel"?)

Most nearline drives pulled from DASs seem to be discarded (upgraded?)
at about 20K PoH, FWIW.  Plenty of useful life remaining!

[FWIW, I've only lost two drives in my life -- one a laptop drive installed
in an application that spun it up and down almost continuously and another
that magically lost access to it's boot sector.  OTOH, I've heard horror
stories of folks having issues with SSDs (firmware).  So, just put all the
rescued SSDs I come across in a box thinking "someday" I will play with them]