"bitrot" on magnetic media: is there such a thing?

[John Bass]

In article <247 at desint.UUCP>, geoff at desint.UUCP (Geoff Kuenning) writes:
>In article <1978 at sdcsvax.UUCP> jc at sdcsvax.UUCP (John Cornelius) writes:
>
>>The wisdom of leaving your winchester running, even if the system it is
>>connected to is not running, cannot be too heavily stressed.  Winchesters are
>>designed for a continuous operating environment, not a sporadic one.  There is
>>a school of thought that being nice to your disk drive involves turning it off
>>when it is not in use.  I recognize that this thinking has some intuitive
>>basis but it is, alas, quite incorrect.
> 
> I wonder if John could give us some references to support this contention.
> In particular, one of the failure modes I have seen in Winchesters is
> bearing failure.  Bearing wear is directly related to on-time, not to
> the number of startup/shutdown cycles.

Sorry, but bearing wear is also a function of the number of cold starts,
running temp, and thermal cycling. On-time is just one componet in the
life factor. Furthermore the media/head life is also a function of
start/stops, as is the life of the spindle motor control circuit in most
smaller drives (startup current rush).

> 
> Let's remember that a lot of Winchesters are spec'ed with MTBF's of
> 10,000 hours or less.  There are 8760 hours in a year, so if you leave
> your Winchesters on 24 hours a day, you can expect the average one to
> fail after about 14 months.
> -- 

Most vendors don't spec the number of cold start cycles, the number of
host start cycles, or the effects of thermal cycling on life. I don't think
very many drives will run over 1,000 hours of a power/thermal cycling
combination.

I think that a survey of 10 drives under continuous service compared to
10 drives under cycling of 1 hour on/off will result in a VERY skewed
comparison favoring continuous duty when plotted again operating time.

This cycling rate is not out of line, given most desk top micro usage
is for a very short interval.
-- 

John Bass (DBA:DMS Design)
DMS Design (System Design, Performance and Arch Consultants)
{dual,fortune,polyslo,hpda}!dmsd!bass     (805) 541-1575