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Old 10-22-2005, 02:33 AM   #1
danimalz
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IDE cabling question


(This is related to another post, moderators, but not a double post)

Most PCs, especially older ones come standard with two IDE controllers, cabling. Normally, one is used for HDD's and the other for CD's (in my experience).

I've heard that if you mix a CD with a HDD on a single cable, then your performance suffers greatly. Is this true under linux?

Whats the speed (and other) implications of changing from this:

ide0========/dev/hda1=========/dev/hda2
ide1========/dev/hdc1

..to this:

ide0 ========/dev/hda1========/dev/hdc1
ide1========/dev/hda2=========

Actually, I could live without the cdrom, i suppose....

What say?
 
Old 10-22-2005, 03:06 AM   #2
spooon
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I think you misunderstand the meaning of hda/hdb/hdc/hdd; they refer to the location on the IDE bus, not to what kinds of drives they are or anything. This is how it works:
Primary IDE ======== /dev/hdb (slave) ======== /dev/hda (master)
Secondary IDE ======== /dev/hdd (slave) ======== /dev/hdc (master)
And the number after the drive refers the partition number on that drive. So something like "/dev/hda1" refers to a specific partition, not a drive.
 
Old 10-22-2005, 03:18 AM   #3
CWizard
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Think of it this way, the drives on one channel share bandwidth. So it is not good to put two drives on the same channel that are going to simultaniously read/write. IDE can only read or write, so it has implications if you are copying between drives on a single IDE channel. If one of the drives is idle, it won't hurt the other's performance, though.


It comes down to how the drives are used. Eg having IDE0=(system,cd) IDE1=(data1,data2)
wouldn't be that bad, since the pairs would selldom do heavy duty at the same time.
 
Old 10-22-2005, 03:28 AM   #4
danimalz
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Ahh. that makes sense - thank you!

I guess i should state the issue more clearly.

Here's the thing: Multiple HDDs (Hard Disk Drives) in the same machine will peform better if they do not share the same cable. That's why if you do IDE RAID (as opposed to SCSI), you put the discs on separate cables/controllers.

I have two IDE controllers/cables in my server. Right now, i have two HDDs on one cable. The other cable supports a single CDROM.

I would like to take advantage of separating the HDDs - on on each cable/controller - to increase performance (and possibly do RAID).

But....

Now I have heard that you should never put a HDD on the same cable as a CDROM, because the IDE controller will default to the slowest speed of the attached devices on its cable. So you'd end up with a HDD severely throttled down to the data transfer speed of the CDROM if it's on the same cable, so i've heard.

So if i want to keep the CDROM, then it is better to keep it the way it is.

Okay, now I am confuewewswwsd.
 
Old 10-22-2005, 04:18 AM   #5
CWizard
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Quote:
Originally posted by danimalz
Now I have heard that you should never put a HDD on the same cable as a CDROM, because the IDE controller will default to the slowest speed of the attached devices on its cable. So you'd end up with a HDD severely throttled down to the data transfer speed of the CDROM if it's on the same cable, so i've heard.
Hmm... I can't say it's that way or the other. I have often sharen HD with CD drives on the same channel without too much trouble. The best way to test the theory is to run hdparm -tT /dev/hdc without the CD (eg as /dev/hdd), and then with it, and see if it is slower.
 
Old 10-22-2005, 08:32 AM   #6
saikee
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Fact : All DVD/CD drive runs at 33Mb/s. Current IDE hard disks do 133Mb/s (ATA 133). The hard disks that still work at 33Mb/s should have only about 2Gb. Since then the both the capacity and throughput has been increased, the latter to 66, 100 and the current 133 Mb/s commonly known as ultra DMA modes. The original DMA needs only a 40-connector cable.

One can link a CD or DVD drive with any IDE hard disk in an 80-connector cable. I believe the system look after itself. Degradation in performance has not been noticeable in my experience. I believe the 40 extra cables of an ultra DMA cable are for grounding purpose to achieve the stability at higher speeds. The motherboard could default to the slower DMA if a 80-connector cable is not provided.

This is the same as the wireless cards with a router. One can have a 11g wireless router communicating with one computer with a 11b wireless card (11Mbps speed) and another with 11g wireless card (54 Mbps). The two computers can operate at different transmission speeds with a common router.

I have never noticed any performance gain from operating a hdd alone.

One can check all the DMA modes as they are reported on a boot up screen of a desktop machine.

RAID 0 operates by spliting the data between two drives and hence the transmission rate is theoretically halved. This is not from the cable connection. SCSI disks were the common devices for doing RAID long before Sata came to the scene. One standard SCSI cable permits 8 devices can be connected serially.

Last edited by saikee; 10-22-2005 at 08:33 AM.
 
  


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