[Gllug] A SATA Raid card worth having?

[Daniel P. Berrange]

On Fri, Jan 27, 2006 at 01:40:41AM +0000, Andy Farnsworth wrote:
> Russell Howe wrote:
> 
> ><snip>
> >
> >With SCSI, the bus speed is more important, since you can have perhaps
> >15 devices on a single chain (perhaps more? I'm not too up to date with
> >my SCSI knowledge). If you say your SCSI interface is 300Mbyte/s (U320,
> >say?), that's 20Mbyte/s per device.
> > 
> >
> This is why you use 15 drives on a SCSI chain :)  Each drive can easily 
> supply the 20+ Mb/s throughput so you can keep your 320 Mb/s pipe fully 
> saturated.  Actually, if you go with 15,000 RPM drives you should get 
> almost double the throughput of 7200 drives, at least as long as you 
> stay on a cylinder, once you cause the head to move, it is not 
> appreciably faster on the 15,000 RPM drives, however latency is halved.  
> This means that in most cases you can keep 320U SCSI saturated with 
> about 4 drives on the channel.

Aren't you gonna hit the bandwidth limit on the connection from RAM
over PCI bus to the SCSI controller at around 150 MB/s, making the
theoretical 320 MB/s of 15 drivers on 1 SCSI chain unachievable ? 

Regards,
Dan.
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