For instance it is quite normal to keep the system software in one partition

For instance it is quite normal to keep the system

This preview shows page 67 - 68 out of 102 pages.

For instance, it is quite normal to keep the system software in one partition and user data in another partition. That way, when one makes a back-up of the disk, user data can easily be kept separate from system data. The separation becomes a hardware matter. Partitions are supported on MS-DOS, Macintosh, BSD UNIX, AmigaDOS etc. Remarkably there are versions of system 5 UNIX which do not support partitions. BSD UNIX partitions are a good example, and since we are focussing on UNIX we shall discuss its partitions in more detail. BSD UNIX uses a special convention for the partitions on each disk. Each disk may have up to eight logical partitions which are labelled from a to h . Partition Usage a root and boot partition b swap partition c the whole disk d anything e anything f anything g anything h anything Each partition is assigned a separate logical device and each device can only write to the cylinders which are defined as being its own. Partitions can overlap, because they are just limits. Thus, if we read from logical device c, which is defined as the whole disk, we could, in principle read from the whole disk, whereas if we use logical device b we may only read from the swap partition. To use a partition we have to create a filesystem on it. This involves reserving space workspace for the operating system and suitable markers for navigating over the surface of the disk.. Since partitions are defined for convenience, it does not matter that they overlap. What is important is that the filesystems on two partitions do not overlap! This is extremely important. If two filesystems overlap, they will destroy each other! In BSD UNIX, partitions are created by editing a table which is downloaded into the device driver. In Sun's SunOS and Solaris operating systems, a special command format is used to make partitions. The newfs command is used to create a filesystem. Once a partition has been created, it has to be mounted in order to be reachable from the directory structure of the filesystem. The mount action is analagous to the opening of a file. On the Macintosh and Amiga operating systems, new disks are immediately sensed by the system and are mounted. In the Macintosh case (which has only a pictoral graphic user interface) new partitions or disks are mounted on the desktop at the root level. Under AmigaDOS, each new partition becomes a logical device and is given a logical device name which identifies the disk. If the Workbench (graphical user interface) is running, the disks appear together with their device names on the workbench in the same way as the Macintosh. Otherwise they appear in the mountlist . In UNIX a partition is mounted using the command mount . For example a command like mount /dev/sd0g /user-data would mount partition g on disk number zero onto the directory /user-data . The result would be that all files on that partition would appear under the directory /user-data . A prerequisite for mounting a UNIX partition is that the partition must contain a filesystem.
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