This is fundamentaly untrue.
A mirror is still a mirror. At most in a RAID 10 you can have two
simultaneous seeks. You are always going to be limited by the seek
time of your drives. It's a stripe, so you have to read from all
members of the stripe to get data, requiring all drives to seek.
There is no advantage to seek time in adding more drives. By adding
more drives you can increase throughput, but the max throughput of the
PCI-X bus isn't that high (I think around 400MB/sec) You can easily
get this with a six or seven drive RAID 5, or a ten drive RAID 10. At
that point you start having to factor in the cost of a bigger chassis
to hold more drives, which can be big bucks.
Alex Turner
netEconomist
On 18 Apr 2005 10:59:05 -0400, Greg Stark <gsstark@mit.edu> wrote:
>
> William Yu <wyu@talisys.com> writes:
>
> > Using the above prices for a fixed budget for RAID-10, you could get:
> >
> > SATA 7200 -- 680MB per $1000
> > SATA 10K -- 200MB per $1000
> > SCSI 10K -- 125MB per $1000
>
> What a lot of these analyses miss is that cheaper == faster because cheaper
> means you can buy more spindles for the same price. I'm assuming you picked
> equal sized drives to compare so that 200MB/$1000 for SATA is almost twice as
> many spindles as the 125MB/$1000. That means it would have almost double the
> bandwidth. And the 7200 RPM case would have more than 5x the bandwidth.
>
> While 10k RPM drives have lower seek times, and SCSI drives have a natural
> seek time advantage, under load a RAID array with fewer spindles will start
> hitting contention sooner which results into higher latency. If the controller
> works well the larger SATA arrays above should be able to maintain their
> mediocre latency much better under load than the SCSI array with fewer drives
> would maintain its low latency response time despite its drives' lower average
> seek time.
>
> --
> greg
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: the planner will ignore your desire to choose an index scan if your
> joining column's datatypes do not match
>