Disk Quotas

By: Jesse Miller

To prevent people from hogging too much disk space, multiuser operating systems often provide a mechanism for enforcing disk quotas. The idea is that the system administrator assigns each user a maximum allotment of files and blocks, and the operating system makes sure that the users do not exceed their quotas.

When a user opens a file, the attributes and disk addresses are located and put into an open file in main memory. Among the attributes is an entry telling who the owner is. Any increase in the file`s size will be charged to the owner`s quota.
A second table contains the quota record for every user with a currently open file, even if the file was opened by someone else.

It is an extract from a quota file on disk for the users whose files are currently open. When all the files are closed, the record is written back to the quota file.

When a new entry is made in the open file table, a pointer to the owner`s quota record is entered into it, to make it easy to find the various limits. Every time a block is added to a file, the total number of blocks charged to the owner is incremented, and a check is made against both the hard and soft limits.

The soft limit may be exceeded, but the hard limit may not. An attempt to append to a file when the hard block limit has reached will result in an error. Analogous checks also exist for the number of files.

When a user attempts to log in, the system checks the quota file to see if the user has exceeded the soft limit for either number of files or number of disk blocks. If either limit has been violated, a warning is displayed, and the count of warnings remaining is reduced by one. If the count ever gets to zero, the user has ignored the warning time for a number of (1) sessions; he is not permitted to log in. Getting permission to log in again will require some discussion with the system administrator.

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This method has the property that users may go above their soft limits during a login session, provided they remove the excess before logging out. The hard limits may never exceed the defined quota. Quotas are extensively used when the volume of information kept on the server is defined.

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