536 Stripes In recent years some UNIX systems particularly Hewlett Packard have

536 stripes in recent years some unix systems

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5.3.6 Stripes
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In recent years some UNIX systems (particularly Hewlett Packard) have experimented with disk striping . Disk striping is a way of increasing the disk transfer rate up to a factor of , by splitting files across different disks. Instead of saving all the data from a given file on one disk, it is split across many. Since the heads can now search independently, the speed of transfer is, in principle, increased manifold. The disadvantage with disk striping is that, if one of the disks becomes damaged, then the data on all disks is lost. Thus striping needs to be combined with a reliable form of backup in order to be successful. Figure 5.10: Disk striping: files are spread in parallel over several disks. 5.4 Disk Filesystems A filesystem is a high level interface to the disk, which allows users of a system to give names to files, organize files in directories and separate off special areas using partitions. A filesystem is said to be created on a disk by running a special program. On many systems this is identified with formatting the disk and involves writing address data to the surface as well as reserving system workspace on the disk. 5.4.1 Hierachical filesystems and links The most popular type of filesystem interface is the hierachical one. Earlier operating systems like MTS did not have a directory structure. Each user had a separate login area, but the login area was not able to hold subdirectories. The hierachical file structure is a very convenient way of organizing data in directories, sub-directories and so on. But this rigid preoccupation with a hierachical ordering is not always the most appropriate one. Look at the diagram below. Figure 5.11: Links - deviations from a strict hierachical filesystem. /usr/local/bin/prog.exe is a link to /usr/local/mysoftware/prog.exe and /local is a link to /usr/local Suppose are in the directory /usr/local/mysoftware , which contains a complete package of software that we have obtained in all of its sub-directories. Since the package is a unit, we would like to keep all of its files together and preserve that unity - but it might also be necessary for some of the files in the package to be installed in special places, elsewhere in the file tree. For example, the executable binaries might have to be placed in /usr/local/bin , and some configuration files for the system might have to be placed in a special directory where the operating system can find them. The conflict of interest can be solved by introducing links . Links are objects which appear in the file system and look just like files. In fact they are pointers to other files which are elsewhere in the strict hierarchy. Links enable one file to appear to exist at two or more places at the same time. A link is not a copy of a file, it is an alias for the true route to the file through the hierachical system , but for all intents and purposes it looks like another instantiation of the file. The Macintosh filesystem refers to such links as `aliases'.
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