Unix/Linux Basics: Hard Disks

Hardware

The most common hard drives today are IDE or SCSI. IDE drives are generally less expensive (though not by much) and generally not as fast (again, not by much). IDE systems are limited to two drives per controller, which means that you probably cannot have more than 4 IDE drives in one system. With SCSI, you can have 15 (or 7 with older controllers).

IDE drives will generally be installed inside your computer- there is not usually any way to install an IDE drive externally. SCSI drives are often installed externally.

IDE or SCSI drives can be configured as RAID systems. Because of the number of drives you can have, larger RAID systems are usually SCSI.

Calulate the size of a hard drive by multiplying heads * cylinders * sectors * 512 - the result is bytes. Your actual storage capacity will be less due to formatting and file system overhead.

The speed of a hard drive is affected by many things, but RPM is one of the easiest to judge for otherwise similar equipment- the higher the RPM, the faster the drive. Note the "otherwise similar", though: an IDE drive isn't necessarily faster than a SCSI just because the RPM is higher.

Cache on drives speeds up writes. As controllers and OS's themselves also may maintain disk related caches, you need to keep straight what cache is being talked about.

Software

MSDOS and Windows assign drives as letters: C: is the boot disk, D: is the next disk or CDROM, etc.

Unix doesn't do that. In Unix, you mount a filesystem (a drive needs to have a filesystem to be useful) at some directory on your booted or "root" drive (or any other already mounted filesystem). To get to the stuff on the second drive, you just "cd" there, which is a much smarter way to do it, because it's all transparent: if your application lives under /usr/lib/whatsthis, it doesn't matter if "whatsthis" is a directory on your first drive, your second, or your 25th: it's just /usr/lib/whatsthis.

Actually, for a long, long time MSDOS has had a way to "mount" drives, but they called it "JOIN". For example, you could "JOIN D: C:\NEWDRIVE", and then if you "CD \NEWDRIVE" you'd be sitting on D:. Hardly anybody ever used this, but it has been there just the same (there's a Unix "join" also, but that's a database like command that merges files based on a common key- see "man join").

You also mount cdrom drives: a CDROM is treated just like any other hard drive.

You can't unmount a drive or a cd if someone is using it- and sitting in a mounted directory counts as using. So,

mount -r /dev/cd0 /mnt
cd /mnt
umount /dev/cd0
 

WILL NOT WORK- you'd have to cd off /mnt first.

If you are looking for some more information on this, try

man mount
 

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