Saturday, April 25, 2009


Some disk blocks are quite boring. Some are marginally interesting. A very few are just super. :-)

The original Unix filesystem was composed of four sections: Boot blocks, which contain the secondary stage bootstrap loader; a super block, which I will describe in a minute; the i-node table, which contains i-nodes, and i-nodes are the complete file except for the data; and data plus indirect blocks.

I-nodes are, as I said, are the complete file except for the data. That is, they describe who owns the file, when it was last accessed and modified, what the permissions are for the file, and the list of blocks which contain the data. Side note: Unix files don't have names. They have (i-node) numbers. Directories are files which contain a number of filename, i-node number pairs; called links.

Data blocks contain the file data. Since i-nodes are of fixed size, obviously there is an upper limit to the number of data blocks which can be listed in the inode. When an inode can no longer contain the list of data blocks, that list is moved to an indirect block, and the i-node is converted to contain a list of indirect blocks. When the inode can no longer contain a list of indirect blocks, the list is moved to a double-indirect block and the inode then contains a list of double-indirect blocks. I notice (/usr/include/linux/sysv_fs.h) that ``traditional'' filesystems now allow triple-indirect blocks, which just goes to show that even tradition is hard to keep up with.

The super block describes the size of the inode-table and the size of the total file system. It also contains a list of some recently freed data blocks, which is used to quickly find a free block when you want to allocate a new one. Similarly there is a (partial) list of free inodes.

I made the point that files don't have names, they have numbers, and that directories provide a map between names and numbers. So how do you find the root of a filesystem? The answer is that it is inode number two. Inode 1 contains a list of bad blocks on the disk.

One of the less desirable features of the traditional filesystem is that it has only one superblock, and if you lose that you are really stuffed. Look: You wouldn't even know where the inode table ended and the data table started. As a remedy to that, when the BSD Fast File System was designed, it was given backup copies of the superblock. So if you lost the main superblock you could mount using one of the backup copies (but you had to know where it was (but it's not to hard to find it)).

Now don't you agree that from a filesystem perspective, data blocks are only marginally interesting, but that first block is just super? ;-)

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