If you want to improve your hard disk performance, you can get some benefit by using several disks. Suppose that your system engages in a series of disk operations like the following:
Read File 1 Read File 2 Read File 1 Write File 2 Read File 3 Write File 2 Write File 3
Note that, in this example, the even and odd file accesses are interleaved. Now, suppose that the even files are located early on the disk, whereas the odd files are located late on the disk. The result of this string of disk operations will be a series of long head seeks, which slows access to the files. If instead the even files are on one physical disk and the odd files are on another physical disk, then there will be little in the way of head seek operations, and overall performance will improve.
You can't guarantee that every other access uses a different physical disk, but you can improve the odds by using partitions on separate disks for a single Linux installation. For example, you might put your root or /usr directory on /dev/hda and your /home directory on /dev/hdb. This arrangement guarantees that accesses to Linux system files won't disrupt ongoing accesses to users' files, and vice versa. Precisely how you should break up partitions across disks varies with your needs and configuration, including what other OSs are present and the speeds of your hard disks. As a general rule of thumb, you should aim to equalize drive access across your hard disks. Putting little-used directories like /boot on one drive and everything else on another won't help your system's speed much, but putting frequently-accessed directories like /usr and /home on separate drives generally does help. Precisely what directories are frequently used varies from one installation to another.
Splitting your installation across drives tends to help performance when your drives are similar in speed. If one hard disk is vastly slower than another, splitting Linux across those disks will have a detrimental effect on overall system speed compared to restricting Linux to the faster drive. In such a situation, you might want to relegate seldom-accessed directories to the slow drive, simply to make room on the fast drive for your more important files.
Splitting Linux across two or more disks can be slightly more beneficial in a SCSI system than in an EIDE system, because of SCSI's capacity to support multiple simultaneous transfers. This is a speed boost in addition to the boost provided by reducing the need for head movements, however. The basic principle is beneficial for EIDE drives as well as SCSI drives—it's just more beneficial when used with SCSI drives. You can gain some of the same benefit by placing your EIDE hard drives on separate channels.
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