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<html>
<head><title>The BFS filesystem structure</title></head>
<body>
<center><h1>The BFS filesystem structure</h1></center>
The UnixWare Boot FileSystem (BFS) is a filesystem used in SCO UnixWare.
It contains all files necessary for UnixWare boot procedures (such as
<tt>unix</tt>).
Because the object of the bfs filesystem type is to allow quick and
simple booting, BFS was designed as a contiguous flat filesystem. It
is not intended to support general users. The only directory bfs
supports is the root directory. Users can create only regular files;
no directories or special files can be created in the bfs filesystem.<p>
A BFS filesystem consists of three parts:
<ul>
<li> Superblock
<li> Inodes
<li> Data area
</ul>
Each block on disk is 512 bytes long, blocks are numbered from zero. Most
data structures use "offset from begining of disk". Divide this number to
get block number.<p>
<b>NOTE:</b> Operations on a BFS filesystem in SCO UnixWare severely limited.
For example, it is not possible to have two files open for writing
simultaneously. These restrictions do not
apply to operations involving only the reading of files.<p>
You can read a BFS filesystem from your Linux box. See
<A href="http://www.penguin.cz/~mhi/fs/bfs/">BFS Linux module home page</a>.
<p>
<h2>The BFS superblock</h2>
The superblock is at the begining of disk, block 0.
<table border=1>
<tr><th>Type
<th>Name
<th>Description
<tr><td>32bit int
<td>magic
<td>Magic number (0x1BADFACE)
<tr><td>32bit int
<td>start
<td>Start of data blocks (in bytes)
<tr><td>32bit int
<td>size
<td>Size of filesystem (in bytes)
<tr><td>4x 32bit int
<td>sanity words
<td>Sanity words are used to recover filesystem after interrupted
<A href="#compaction">compaction</a>. They are usually 0xFFFFFFFF.
</table>
<h2>BFS inodes</h2>
The inode contains all the information about a file except its name.
Filenames are kept in the root directory, the only directory in the
BFS filesystem. An inode is 64 bytes long. Inode table starts at
block number 1 and fills the space between superblock and first data
block (usually root directory). First inode has number 2.
<table border=1>
<tr><th>Type
<th>Name
<th>Description
<tr><td>32bit int
<td>inode number
<td>Inode number, often contains "garbage" in high 16 bits.
<tr><td>32bit int
<td>first block
<td>First block of file. Next block is n+1, n+2, ... n+x.
<tr><td>32bit int
<td>Last block
<td>Last block of file
<tr><td>32bit int
<td>offset to eof
<td>Disk offset to end of file (in bytes)
<tr><td>32bit int
<td>Attributes
<td>File attributes (1 = regular file, 2 = directory)
<tr><td>32bit int
<td>mode
<td>File mode, rwxrwxrwx (only low 9 bits used)
<tr><td>32bit int
<td>uid
<td>File owner - user id
<tr><td>32bit int
<td>gid
<td>File owner - group id
<tr><td>32bit int
<td>nlinks
<td>Hard link count
<tr><td>32bit int
<td>atime
<td>Access time
<tr><td>32bit int
<td>mtime
<td>Modify time
<tr><td>32bit int
<td>ctime
<td>Create time
<tr><td>4x 32bit int
<td>spare
<td>Unused, should be zero
</table>
The number of inodes is defined when mkfs is used to create the filesystem.
<p>
<h2>BFS storage blocks</h2>
The remainder of the space allocated to the filesystem is taken up by
data blocks. 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.
<table border=1>
<tr><th>Type
<th>Name
<th>Description
<tr><td>16bit int
<td>inode
<td>File inode number
<tr><td>14 8bit characters
<td>name
<td>File name
</table>
The root directory *MUST* begin with two entries "." and "..", both with
inode number 2 (root directory).
<p>
<h2>Managing BFS data blocks</h2>
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.<p>
<A name="compaction"><h2>Compaction</h2></a>
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.
You need to perform compaction when either of the following situations occurs:
<ul>
<li> The system has reached the end of the filesystem, and there are
still free blocks available.
<li> 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.)
</ul>
<h2>Related links</h2>
<A href="http://www.penguin.cz/~mhi/fs/bfs/">BFS Linux module</a><br>
<A href="http://www.sco.com/">SCO homepage</a><br>
<A href="http://www.penguin.cz/~mhi/fs/">Filesystems HOWTO</a><br>
<hr>
<center><i>Copyright (c) 1999 Martin Hinner,
<A href="mailto:mhi@penguin.cz">mhi@penguin.cz</a></i></center>
</body>