Using RAID 0 with unequal capacity drives - K8V SE Deluxe

[bill]

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

I am assembling a machine using the ASUS K8V SE Deluxe and would like to
use the on board SATA RAID 0 capability.

If I use two Seagate SATA drives (160 & 200 Gb) in a RAID 0 (strip)
configuration, what happens to the extra 40 Gb on the larger drive? Can
I partition the 200 Gb drive into a 160 and 40 Gb to enable the use of
the 40 Gb as another drive? Is there a performance hit for using two
unequal capacity SATA drives?

 

[Guest]

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

>I am assembling a machine using the ASUS K8V SE Deluxe and would like to
>use the on board SATA RAID 0 capability.
>
>If I use two Seagate SATA drives (160 & 200 Gb) in a RAID 0 (strip)
>configuration, what happens to the extra 40 Gb on the larger drive? Can
>I partition the 200 Gb drive into a 160 and 40 Gb to enable the use of
>the 40 Gb as another drive? Is there a performance hit for using two
>unequal capacity SATA drives?

You will have one SCSI drive as far as your system knows that is
320GB. You can't use any extra space and the cache on each drive will
be disabled. Large file tranfers will be better but small ones will be
worse. Seek time will not be faster There was a time when not using
identicle drives caused a performance hit but these days not so much.

 

[milleron]

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

On Sun, 25 Apr 2004 10:36:49 -0400, Bill <bilsteinin@hotmail.com>
wrote:

>I am assembling a machine using the ASUS K8V SE Deluxe and would like to
>use the on board SATA RAID 0 capability.
>
>If I use two Seagate SATA drives (160 & 200 Gb) in a RAID 0 (strip)
>configuration, what happens to the extra 40 Gb on the larger drive? Can
>I partition the 200 Gb drive into a 160 and 40 Gb to enable the use of
>the 40 Gb as another drive? Is there a performance hit for using two
>unequal capacity SATA drives?

You lose the 40 GB. The striped array will appear to be a single
320-GB drive which you can partition any way you want AFTER creation
of the array, but you cannot partition BEFORE you create it.
(Literally speaking, of course, you could, but in the process of
creating the array, those partitions would be lost.)

The question about the performance hit is a good one. I don't know
the answer. FAQs on the subject advise using identical drives if
possible, though, so perhaps a size-mismatched RAID 0 does read and
write more slowly. At any rate, I believe that any difference would
be imperceptible and detectable only with benchmarking.

--Stop reading here unless you want to hear my standard RAID-0 rant.--

You do realize that even size-matched striped arrays themselves do not
provide any perceptible performance improvement for desktop use, don't
you? This statement is not intuitive because intuition tells you
that any read or write should take only half as long on a RAID 0 as it
would on a single disk, but in practice, there is amazingly little
difference. Most (all ?) onboard RAID controllers utilize the CPU to
do their thing, and this induces latency approximately equal to the
read-write-time savings. Unless one is doing extensive video editing
or other sophisticated multimedia work, there's not much use for RAID
0 outside of servers and definitely not on machines used primarily for
gaming (RAID 1 is another matter entirely). I have a 3Ware PCI-based
RAID 0 that does not utilized the CPU, and even that provides no
appreciable performance enhancement. Newer chipsets routing data
through the IDE bus rather than the PCI bus may actually have the
potential to give perceptible improvement but even these continue to
use the computer's CPU, and, for now, HDDs don't seem to be able to
stream data faster than the PCI bus can handle it.
The main problem for me, though, is that striped arrays positively
DOUBLE the chance of a disk failure -- i.e., no appreciable
performance improvement with twice the chance of HDD failure. I
cannot see the upside to this scenario.

My next homebuilt will have a small WD Raptor for the OS and one or
more very large 7200RPM drives for everything else. Haven't ruled out
RAID 1 for the OS, but I'll never install RAID 0 in a home machine
again unless the technology changes.

Ron
Ron

 

[Guest]

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

>striped arrays positively
>DOUBLE the chance of a disk failure --

Well..... that's where you lost me. That sentece is false. If he has
one drive and it goes bad he loses all his data. If he stripes with
two and loses one, he loses all his data. Almost a complete wash. Your
statement is almost like saying since your neighbor owns two cars he's
twice as likely to crash. A quick look at his insurance policy will
show that they don't believe that.
No RAID setups( 0, 1 or 5) are a substitute for backing up anyway.


Digital Media, Mike Newcomb writes:

"Point 2: It is my understanding that A RAID 0 system does not
"double" your failure rate or risk of failure. The impact from a
statistical stand point is something less than "double" the risk. In
order to ascertain what the actual risk increase is, a number of
factors, such as MTBF (Mean Time Between Failure) rate, must be
considered.

See, link for forumla/light
reading:http://www.itl.nist.gov/div898/handbook/apr/section1/apr182.htm

In reality, the additional risk is insignificant when virtually all
drives have a MTBF rate of more than 100,000 hours and you will
probably have your system for less than 5 years. (Hint: there is 8,760
hours in a year)".

 

[Guest]

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

Andrew J wrote:

>
>>striped arrays positively
>>DOUBLE the chance of a disk failure --
>
>
> Well..... that's where you lost me. That sentece is false. If he has
> one drive and it goes bad he loses all his data. If he stripes with
> two and loses one, he loses all his data. Almost a complete wash. Your
> statement is almost like saying since your neighbor owns two cars he's
> twice as likely to crash. A quick look at his insurance policy will
> show that they don't believe that.
> No RAID setups( 0, 1 or 5) are a substitute for backing up anyway.

Actually he is quite correct: the probability that a stripe set
will fail is essentially proportional to the sum of the probability
of the individual drive failures.

Suppose that the probability that a drive will fail in a certain
time interval "T" is "P", with P being a very small number.

Then in you have a single drive, the probability that the drive
fails during an interval "T" is obviously P.

If you have two striped drives, then the probability of the losing every
thing on the stripe set is:
P(1-P) (Drive 0 fails, Drive 1 survives)
+(1-P)P (Drive 0 survives, Drive 1 fails)
+ P^2 (Both drives fail)
---------------------------------------------
2P - P^2 TOTAL propability of losing the stripe set

Since P is very small, P^2 is negligible relative to 2P
and is typically ignored.



>
>
> Digital Media, Mike Newcomb writes:
>
> "Point 2: It is my understanding that A RAID 0 system does not
> "double" your failure rate or risk of failure. The impact from a
> statistical stand point is something less than "double" the risk. In
> order to ascertain what the actual risk increase is, a number of
> factors, such as MTBF (Mean Time Between Failure) rate, must be
> considered.
>
> See, link for forumla/light
> reading:http://www.itl.nist.gov/div898/handbook/apr/section1/apr182.htm

That page does *not* support your contention.
S'matter of fact, the formula on that page for Fs(t) is
essentially just another route to what I have done above:
Fs(t) = 1 - (1-P)(1-P) = 2P - P^2


As well, there is the statement along the left hand side
of that web page "Add failure rates and multiply reliabilities
in the series model." That statement is just a first-order
approximation of the formulas on that page for Fs(t) and Rs(t).


>
> In reality, the additional risk is insignificant when virtually all
> drives have a MTBF rate of more than 100,000 hours and you will
> probably have your system for less than 5 years. (Hint: there is 8,760
> hours in a year)".
>

There is (was?) a nice article about MTBF, as applicable to
hard drives and RAIDs at IBM's site. Home of many of the
world's best scientists and engineers, in case you have
forgotten.

They cautioned against RAID 0 because ...
If a single drive has an MTBF(single) of "T", then
(as a first order approximation)
MTBF(RAID 0, n drives) = MTBF(single) / n^2

In other words, if you stripe 2 drives, then the MTBF
of the stripe set is ONE-QUARTER of the MTBF of a single
drive.


As well MTBFs can be extremely misleading.
As you said, there are only a few thousand hours in a year.
Nobody tests a batch of drives for 20 years in order to
get real empirical evidence of how long the drive can be
expected to last. Instead they test a lot of drives for
one or two years and extrapolate from that.

Statistical modelling like that can be extremely accurate
if all of the underlying assumptions are valid - but they
can be wildly inaccurate if just one assumption turns out
to be invalid. IBM found that out the hard way with their
deathstar hard drives, WD found that out with their
DiamondMax9 drives, and Fujitsu found that out ...



And MTBFs can be extremely misleading in other ways.
If half of a batch of drives will fail in 10,000 hours
and the other half will fail in 190,000 hours, then the
MTBF is 100,000 hours but I sure as heck wouldn't want
one of those drives.

In other words, those huge MTBF numbers means squat - I
would much rather know the probability that a drive will
fail during some more useful time interval.

 

[bill]

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

Thanks for the help. I think I will go without the RAID 0 set up. I had
not considered the failure factor. I would have to agree that two
drives are four times more likely to fail. MTBF only addresses
hardware. Software corruption is far more common and very likely.
Without RAID 0 only one drive at a time would be at risk. Since one of
the drives would contain only data files, it could be easily restored
and in my experience soft failures on data drives only affect individual
data files, not the whole disk.

 

[ANON]

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

The cars analogy is incorrect, unless both are being used at the same time
in which case the 'failure' rate is increased.

"Andrew J" <ajpk3@hotmail.comremove> wrote in message
news:ampn8097nkn4v0qspnujiokk6m3itjrcup@4ax.com...
>
>
> >striped arrays positively
> >DOUBLE the chance of a disk failure --
>
> Well..... that's where you lost me. That sentece is false. If he has
> one drive and it goes bad he loses all his data. If he stripes with
> two and loses one, he loses all his data. Almost a complete wash. Your
> statement is almost like saying since your neighbor owns two cars he's
> twice as likely to crash. A quick look at his insurance policy will
> show that they don't believe that.
> No RAID setups( 0, 1 or 5) are a substitute for backing up anyway.
>
>
> Digital Media, Mike Newcomb writes:
>
> "Point 2: It is my understanding that A RAID 0 system does not
> "double" your failure rate or risk of failure. The impact from a
> statistical stand point is something less than "double" the risk. In
> order to ascertain what the actual risk increase is, a number of
> factors, such as MTBF (Mean Time Between Failure) rate, must be
> considered.
>
> See, link for forumla/light
> reading:http://www.itl.nist.gov/div898/handbook/apr/section1/apr182.htm
>
> In reality, the additional risk is insignificant when virtually all
> drives have a MTBF rate of more than 100,000 hours and you will
> probably have your system for less than 5 years. (Hint: there is 8,760
> hours in a year)".
>
>