bfs storage blocks

The remainder of the space allocated to the filesystem is taken up by storage blocks, also called data blocks. The size of the storage blocks is 512 bytes. The storage blocks store the root directory and the regular files. For a regular file, the storage blocks contain the contents of the file. For the root directory, the storage blocks contain 16-byte entries. Each entry represents a file and consists of two bytes for the i-number and 14 bytes for the filename.

Managing bfs data blocks

The data or storage blocks for a file are allocated contiguously. The data block after the last data block used in the filesystem is considered the next data block available to store a file. When a file is deleted, its data blocks are released; for the filesystem to reuse them, one of the following must be true:

• the file deleted must be the last file stored in the filesystem

• the system must detect the need for compaction and perform it

Compaction

Compaction is a way of recovering data blocks by shifting files until the gaps left behind by deleted files are eliminated. This operation can be expensive, but it is necessary because of the method used by bfs to store and delete files.

The system recognizes the need for compaction and performs it when either of the following situations occurs:

• The system has reached the end of the filesystem, and there are still free blocks available.

• The system deletes a large file and the file after it on disk is small and is the last file in the filesystem. (Small files are files of no more than ten blocks; large files are files of 500 or more blocks.)