At 07:16 AM 4/4/2007, Peter Kovacs wrote:
This may be a silly question but: will not 3 times as many disk drives
mean 3 times higher probability for disk failure?
Yes, all other factors being equal 3x more HDs (24 vs 8) means ~3x
the chance of any specific HD failing.
OTOH, either of these numbers is probably smaller than you think.
Assuming a HD with a 1M hour MTBF (which means that at 1M hours of
operation you have a ~1/2 chance of that specific HD failing), the
instantaneous reliability of any given HD is
x^(1M)= 1/2, (1M)lg(x)= lg(1/2), lg(x)= lg(1/2)/(1M), lg(x)= ~
-1/(1M), x= ~.999999307
To evaluate the instantaneous reliability of a set of "n" HDs, we
raise x to the power of that number of HDs.
Whether we evaluate x^8= .999994456 or x^24= .999983368, the result
is still darn close to 1.
Multiple studies have shown that ITRW modern components considered to
be critical like HDs, CPUs, RAM, etc fail far less often than say
fans and PSUs.
In addition, those same studies show HDs are usually
a= set up to be redundant (RAID) and
b= hot swap-able
c= usually do not catastrophically fail with no warning (unlike fans and PSUs)
Finally, catastrophic failures of HDs are infinitesimally rare
compared to things like fans.
If your system is in appropriate conditions and suffers a system
stopping HW failure, the odds are it will not be a HD that failed.
Buy HDs with 5+ year warranties + keep them in appropriate
environments and the odds are very good that you will never have to
complain about your HD subsystem.
Also rumor has it that SATA drives are more prone to fail than
SCSI drivers. More
failures will result, in turn, in more administration costs.
Hard data trumps rumors. The hard data is that you should only buy
HDs with 5+ year warranties and then make sure to use them only in
appropriate conditions and under appropriate loads.
Respect those constraints and the numbers say the difference in
reliability between SCSI, SATA, and SAS HDs is negligible.
Cheers,
Ron Peacetree
