Unix 3.1 The Buffer Cache

THE BUFFER CACHE

The files are stored on the hard drive and the processes can access these files and create new files on the disk. When a process requests for a file the kernel brings the file into the main memory where user process can change, read or access the file.

The kernel read the super block to figure out the details about the hard drive and the inode table to find the meta data information of any file. So the kernel reads the inode into the memory whenever any process want to access the data and write it back onto the hard disk when the process is done using the file.

The kernel could read and write the file directly from the hard disk and put it in memory and vice versa but the response time and throughput will be very low in this case because of disks sow data transfer speed. 

To minimize the frequency of disk usage/access the kernel keeps a buffer to store the recently accessed files and/or frequently accessed files. This buffer is called the buffer cache.

When the process want to read a file the kernel attempts to read this file in the buffer cache, if the data is found in the buffer cache the data/file is sent to the process. If the file is not found in the buffer cache then the file is read from the disk and then kept in the buffer cache so that it can be made available to the process.


To minimize the disk access frequency the kernel may also implement the pre-caching or write delay functionalities.

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