Re: Huge Data sets, simple queries

On Tue, Jan 31, 2006 at 09:00:30AM -0800, Luke Lonergan wrote:
> Jim,
>
> On 1/30/06 12:25 PM, "Jim C. Nasby" <jnasby(at)pervasive(dot)com> wrote:
>
> > Why divide by 2? A good raid controller should be able to send read
> > requests to both drives out of the mirrored set to fully utilize the
> > bandwidth. Of course, that probably won't come into play unless the OS
> > decides that it's going to read-ahead fairly large chunks of the table
> > at a time...
>
> I've not seen one that does, nor would it work in the general case IMO. In
> RAID1 writes are duplicated and reads come from one of the copies. You
> could alternate read service requests to minimize rotational latency, but
> you can't improve bandwidth.

(BTW, I did some testing that seems to confirm this)

Why couldn't you double the bandwidth? If you're doing a largish read
you should be able to do something like have drive a read the first
track, drive b the second, etc. Of course that means that the controller
or OS would have to be able to stitch things back together.

As for software raid, I'm wondering how well that works if you can't use
a BBU to allow write caching/re-ordering...
--
Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461