On 4-Apr-07, at 8:46 AM, Andreas Kostyrka wrote:
* Peter Kovacs <maxottovonstirlitz@xxxxxxxxx> [070404 14:40]:
This may be a silly question but: will not 3 times as many disk
drives
mean 3 times higher probability for disk failure? 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.
Actually, the newest research papers show that all discs (be it
desktops, or highend SCSI) have basically the same failure statistics.
But yes, having 3 times the discs will increase the fault probability.
I highly recommend RAID6 to anyone with more than 6 standard SATA
drives in a single array. It's actually fairly probable that you will
lose 2 drives in a 72 hour window (say over a long weekend) at some
point.
Andreas
Thanks
Peter
On 4/4/07, david@xxxxxxx <david@xxxxxxx> wrote:
On Tue, 3 Apr 2007, Geoff Tolley wrote:
Ron wrote:
At 07:07 PM 4/3/2007, Ron wrote:
For random IO, the 3ware cards are better than PERC
Question: will 8*15k 73GB SCSI drives outperform 24*7K 320GB
SATA II
drives?
Nope. Not even if the 15K 73GB HDs were the brand new Savvio
15K
screamers.
Example assuming 3.5" HDs and RAID 10 => 4 15K 73GB vs 12
7.2K 320GB
The 15K's are 2x faster rpm, but they are only ~23% the
density =>
advantage per HD to SATAs.
Then there's the fact that there are 1.5x as many 7.2K
spindles as 15K
spindles...
Oops make that =3x= as many 7.2K spindles as 15K spindles...
I don't think the density difference will be quite as high as
you seem to
think: most 320GB SATA drives are going to be 3-4 platters, the
most that a
73GB SCSI is going to have is 2, and more likely 1, which would
make the
SCSIs more like 50% the density of the SATAs. Note that this
only really
makes a difference to theoretical sequential speeds; if the
seeks are random
the SCSI drives could easily get there 50% faster (lower
rotational latency
and they certainly will have better actuators for the heads).
Individual 15K
SCSIs will trounce 7.2K SATAs in terms of i/os per second.
true, but with 3x as many drives (and 4x the capacity per drive)
the SATA
system will have to do far less seeking
for that matter, with 20ish 320G drives, how large would a
parition be
that only used the outer pysical track of each drive? (almost
certinly
multiple logical tracks) if you took the time to set this up you
could
eliminate seeking entirely (at the cost of not useing your
capacity, but
since you are considering a 12x range in capacity, it's obviously
not your
primary concern)
If you care about how often you'll have to replace a failed
drive, then the
SCSI option no question, although check the cases for hot-
swapability.
note that the CMU and Google studies both commented on being
surprised at
the lack of difference between the reliability of SCSI and SATA
drives.
David Lang
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