--On January 17, 2006 7:21:45 PM -0500 Phillip Susi <[email protected]>
wrote:
Your understanding of statistics leaves something to be desired. As you
add disks the probability of a single failure is grows linearly, but the
probability of double failure grows much more slowly. For example:
What about I said was inaccurate? I never said that it increases
exponentially or anything like that, just that it does increase, which
you've proven. I was speaking in the case of a RAID-5 set, where the
minimum is 3 drives, so every additional drive increases the chance of a
double fault condition. Now if we're including mirrors and stripes/etc,
then that means we do have to look at the 2 spindle case, but the third
spindle and beyond keeps increasing. If you've a 1% failure rate, and you
have 100+ drives, chances are pretty good you're going to see a failure.
Yes it's a LOT more complicated than that.
If 1 disk has a 1/1000 chance of failure, then
2 disks have a (1/1000)^2 chance of double failure, and
3 disks have a (1/1000)^2 * 3 chance of double failure
4 disks have a (1/1000)^2 * 7 chance of double failure
Thus the probability of double failure on this 4 drive array is ~142
times less than the odds of a single drive failing. As the probably of a
single drive failing becomes more remote, then the ratio of that
probability to the probability of double fault in the array grows
exponentially.
( I think I did that right in my head... will check on a real calculator
later )
This is why raid-5 was created: because the array has a much lower
probabiliy of double failure, and thus, data loss, than a single drive.
Then of course, if you are really paranoid, you can go with raid-6 ;)
Michael Loftis wrote:
Absolutely not. The more spindles the more chances of a double failure.
Simple statistics will mean that unless you have mirrors the more drives
you add the more chance of two of them (really) failing at once and
choking the whole system.
That said, there very well could be (are?) cases where md needs to do a
better job of handling the world unravelling.
-
--
"Genius might be described as a supreme capacity for getting its possessors
into trouble of all kinds."
-- Samuel Butler
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
