Unix/Linux Basics: Hard Disks
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
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
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.
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
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
WILL NOT WORK- you'd have to cd off /mnt first.
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