Oooops, I revived the never-ending $7K thread. :)
Well part of my message is to first relook at the idea that SATA is
cheap but slow. Most people look at SATA from the view of consumer-level
drives, no NCQ/TCQ -- basically these drives are IDEs that can connect
to SATA cables. But if you then look at the server-level SATAs from WD,
you see performance close to server-level 10K SCSIs and pricing also close.
Starting with the idea of using 20 consumer-level SATA drives versus 4
10K SCSIs, the main problem of course is the lack of advanced queueing
in these drives. I'm sure there's some threshold where the number of
drives advantage exceeds the disadvantage of no queueing -- what that
is, I don't have a clue.
Now if you stuffed a ton of memory onto a SATA caching controller and
these controllers did the queue management instead of the drives, that
would eliminate most of the performance issues.
Then you're just left with the management issues. Getting those 20
drives stuffed in a big case and keeping a close eye on the drives since
drive failure will be a much bigger deal.
Greg Stark 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.
>
