Performance Factors and Tradeoffs in Configuring for Multiple Devices

Performance Factors and Tradeoffs in Configuring for Multiple Devices

Configuring a single IDE/ATA or ATAPI device is very simple. You pretty much just need to jumper it, and then connect it with an IDE/ATA cable, and you are usually done. Setting up more than one device is really not much more complicated. Except for some special cases, you just jumper pairs as master and slave (or use cable select) and use both the primary and secondary channels to get support for up to four devices. You can also add additional IDE/ATA channels to get support for up to eight devices, although these involve a bit more work and slightly more risk of resource issues or other complications.

While setting up multiple devices isn't difficult, there are real performance reasons why it makes sense to put some thought into how you decide to allocate different drives to different IDE channels, and which to make master and which slave. As time goes on, the trend in more and more systems is towards more and more drives. As more removable drives are implemented using ATAPI, and as PC users add new hard disks to their systems in greater numbers, many PC users are finding that they have to figure out how to arrange all these drives in a way that makes sense. A common occurrence is a PC user who starts out with a new PC that has just a hard disk and a CD-ROM drive. Over a couple of years, it's easy to add to this a CD-RW drive for backups or making music, and a new hard disk to improve performance or expand capacity. Suddenly, whammo, you're looking at four IDE/ATA devices, and probably only the original two IDE/ATA channels. Some are surprised to find themselves juggling five or even six IDE/ATA drives in a single PC!

The following are some of the issues that you should take into account when configuring multiple IDE/ATA devices, to maximize the performance of your system:

Master/Slave Channel Sharing: By its very nature, each IDE/ATA channel can only deal with one request, to one device, at a time. You cannot even begin a second request, even to a different drive, until the first request is completed. This means that if you put two devices on the same channel, they must share it. In practical terms, this means that any time one device is in use, the other must remain silent. In contrast, two disks on two different IDE/ATA channels can process requests simultaneously on most motherboards. The bottom line is that the best way to configure multiple devices is to make each of them a single drive on its own channel, if this is possible. (This restriction is one major disadvantage of IDE compared to SCSI). An add-in controller like the Promise "Ultra" series is a cheap way of adding extra IDE/ATA channels to a modern PC.
Boot Devices and Boot Order: The boot device of the PC is usually the first hard disk that the operating system "sees" when it boots up. By default, this means the master drive on the primary IDE/ATA channel. If you want to boot from a drive on an add-in controller, you may actually need to set up your BIOS boot sequence to boot from SCSI first, so the add-in controller is seen before the ones on the motherboard. (These controllers often "look" like SCSI controllers to the system.) This is only an issue if you have hard disks on both the existing and the add-in controller; if you have hard disks on the add-in controller but not on the built-in motherboard controllers, this won't be necessary.
Independent Master/Slave Device Timing: Hard disk controllers on modern systems support running the master and slave device at different speeds, if one supports faster transfer modes than the other. Some systems, however, especially older ones, do not. If you are using two devices with radically different maximum transfer rates, and the chipset doesn't support independent timing, you will slow down the faster device to the speed of the slower one.
Hard Disk and ATAPI Device Channel Sharing: There are several reasons why optical drives (or other ATAPI devices) should not be shared on the same channel as a fast hard disk. ATAPI allows the use of the same physical channels as IDE/ATA, but it is not the same protocol; ATAPI uses a much more complicated command structure. Opticals are also generally much slower devices than hard disks, so they can slow a hard disk down when sharing a channel. Finally, some ATAPI devices cannot deal with DMA bus mastering drivers, and will cause a problem if you try to enable bus mastering for a hard disk on a channel they are using.
Older Hard Disks: Older hard disks, typically those three years in age or more, typically only support the older, low-speed PIO modes for transfers (check the drive's specifications to be sure). In this case, the caveats about hard disk performance matter much less, since the drive is relatively slow anyway.
Interface Bus Type: High-speed transfer modes require the use of a local bus (PCI or VLB) hard disk controller. ISA-based controllers, such as regular sound cards that are sometimes used as tertiary IDE channels, are not suitable for use with high-speed modern disks.
IRQ Resource Usage: Each conventional IDE channel requires an IRQ line, and in some PCs these are a scarce commodity. In some cases devices can be combined on a single channel with minimal impact on performance, allowing the reclamation of one or more IRQs for use by other peripherals. Add-in controllers can also provide you with two channels on a single IRQ; see here for more.

For information on how to use these factors to choose configurations for different combinations of IDE/ATA and ATAPI devices, see this page.

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