Re: Cheap RAID system recommendation?

Rick Denoire <100.17706_at_germanynet.de> wrote:
>Hello *.admin
>
>We are using a Sun's StorEdge A1000 in a Sun E3500 which happens to be
>extremely stow. Before we realized that it was so slow, we planned to
>buy a second one, but now we are looking for a different Raid system.
>The problem is that due to administrative reasons, we are not allowed
>to spend more money than it was once planned. So we are looking for an
>alternative that won't be more expensive that about 30 000 US$.
>
>This is what we need:
>
>- sustained transfer rate > 70 MB/s (not on paper, but *measurable*)

Is that 70MB/s as individual blocks (e.g. typical of many OLTP systems), or 70MB/s streaming (maybe in DSS, if you're lucky, on a newly installed system, with a following wind)?

With a typical 15k RPM drive, you've an average seek time of 3.4ms, and an average rotational delay of 2ms. The actual time to read a block is fairly small.

At 5.4ms/block that's 185 IO/second - so with 8kB blocks, you'll need just over 48 spindles to sustain 70MB/s.

Of course, that's worst case, where every IO involves a seek. If you're lucky it'll only be 90%.

[snip]

>PS: By the way, nowadays some disks are so fast, that they can almost
>reach a sustained transfer rate of 70 MB/s in single mode!

More like 50MB/s on the outer track zone only, half that on the inner tracks. Assuming no seeks at all.

-- 
Andrew Mobbs - http://www.chiark.greenend.org.uk/~andrewm/