[mythtv-users] OT: 160 gig WD - $40

[Jeff]

Monday, March 21, 2005, 3:37:08 PM, Brad Templeton wrote:

> On Mon, Mar 21, 2005 at 07:18:07AM -0500, Jeff wrote:
>> With IDE you only want one drive on a cable (controller
>> port) as only one drive can transfer data at a time. You can
>> get a 2nd IDE controller card (better to get a S/ATA card perhaps
>> at this point) and run 3 HD's using the 4th channel for your CD/DVD
>> drive. Best to get the largest disks you can (but look for 3yr
>> warranties which tend to be more expensive).

> What do benchmarks say about this?  Current systems and drives come with
> 100 megabyte/second or 133 megabyte/second ATA bus speeds.   The 7200
> rpm drives, on the other hand, tend not to be able to sustain data
> rates more than about 60 megabytes/second, and of course rarely are they
> actually doing this, they spend most of their time seeking or idle.

> So, is it really true that you only want one drive on the cable?  Are
> there some benchmarks out there showing the throughput for the 2 on
> one cable vs. 2 on 2 cables?   I am sure there is some loss here, but
> how much is it?

> As you probably know, MythTV tends to rarely need more than 2 megabytes/second
> of disk bandwidth to play or record even an HD stream.  This is so far below
> the capacity of these channels that it seems odd to me if there would
> be much gained by buying additional disk controllers to squeeze out
> a little more bandwidth, but I am curious as to the numbers you may have.

I haven't done any benchmarking. I don't have a configuration which
would allow it as all my drives are bound into a logical volume.

You're correct that the bandwidth of ATA would seem to be large
enough that multiple drives sharing a cable should not be a
significant problem. However there are other considerations.

The ATA protocol is such that only a single command can be issued
at a time. This means that not only do you have to consider the
transfer rate but you also have to consider the latency. I don't know
if disk reads/writes involve one command or two. Additionally there are
issues when you mix ATAPI devices and ATA devices on the same cable. Depending
on the controller you may slow down that cable to the speed of the ATAPI
device so it may not be good to put a hard drive on the same cable as an
optical disk.