lecture17 - Directories and Security Directories...

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Directories and Security Directories Directories are collections of files. Their primary function is to impose a naming system on files and organize them relative toeach other .Directories provide a mechanism for collecting the names of files together so that a user and find the file(s) they’re interested in. Theyare an important piece of metadata. Flat Directories The simplest kind of directory is a simple flat list of filenames and the information needed to find the respective file contents. These are rarely used today ,although theystill do exist. The Palm Pilot has a flat file name space. IBM MVS, which will nevercompletely die, has a flat file name space. Flat file systems are not often used because theyprovide no inherent organization to the files and are difficult to makeefficient for large collections of files. Name space collisions have tobeavoided in a flat space, so generally some naming convention is fol- lowed to prevent them. Acommon one is to prepend filenames with the user’sname. For example, a col- lection of my files might have names like: DISK1.FABER.HOME.PROFILE DISK1.FABER.PROJECT1.GRADES Problems with this include enforcing the conventions (what if other users choose $ to separate parts of the file name?) and the inefficiencyofholding all the system’sfiles in one big list. Consider scanning the sys- tem directory to print all the files that I own. Either the whole table will have tobesearched, or it will have to be kept sorted. Keeping a large list sorted adds overhead, and scanning a large table linearly is not effi- cient. Also updating such a centralized structure will require synchronization - all file creations and dele- tions will have toenforce exclusion on the table, or we have tointroduce a very fine-grained locking mech- anism. Basically ,asingle directory doesn’ tscale easily to manyusers, both in terms of technical operation and user behavior. Hierarchical Directories Anatural organization of files is into a hierarchy. That is files can be seen as being in related classes and each class has a directory.
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This note was uploaded on 02/28/2008 for the course CS 402 taught by Professor Tedfaber during the Spring '05 term at USC.

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lecture17 - Directories and Security Directories...

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