Ffs

Intro

The Berkeley FFS was written by Marshall Kirk McKusick for 4.2BSD at UCB. It resides on a disk partition in blocks greater or equal to 4096 bytes (no less). It has a super block that contains filesystem specific data (eg. when the last fsck took place, use dumpfs(8) to display the superblock, this is a very long listing) which is at block 0. It also has backup superblocks in case the first superblock is corrupted (fsck -b to repair a broken superblock and check the filesystem with an alternate superblock), the next superblock is at block 32 and the others are made known at newfs time.

# newfs /dev/rsvnd0a
Warning: cylinder groups must have a multiple of 2 cylinders
Warning: 64 sector(s) in last cylinder unallocated
/dev/rsvnd0a:   65536 sectors in 656 cylinders of 1 tracks, 100 sectors
        32.0MB in 3 cyl groups (250 c/g, 12.21MB/g, 3136 i/g)
super-block backups (for fsck -b #) at:
 32, 25032, 50032,

Here in the example the backups are 32, 25032 and 50032. To find out the alternate superblocks after a filesystem has already been created use newfs -N.

Next (not counting the super-blocks) the filesystem is split into 2 different parts. One part is data and the other is meta-data (inodes). The way inodes are placed on the disk is original, the cylinders on the disk are placed in cylinder groups and each cylinder group holds inodes and data, inodes at the beginning of the cylinder group and data after. The inodes are the reference points for objects and data in the filesystem and they hold information such as:

• permissions
• file link count
• file size
• atime
• mtime
• ctime
• references to blocks of data
• number disk blocks used by file
• chflags
• file owner
• file group

The data an inode holds is also called metadata. The size of a UFS1 inode is 128 bytes. By default an inode exists for every 8192 bytes of filesystem space, this is tweakable with newfs when the filesystem is first created. To save the tweaked filesystem in a backup one could use dumpfs with the -m option to dump the exact newfs command to create a similar filesystem (neat feature).

The inode in FFS does not store the filename, that has to be read out of the data of a directory which contains entries of filenames up to 255 bytes long not including the NUL terminator and also has the inode number stored for reference back to the inode and thus the data of a file. Lookup always starts at the root of the system which is usually inode number 2 in FFS:

$ ls -lid /
2 drwxr-xr-x  17 root  wheel  512 Oct 25 22:39 /
^

This way the path of a program is traversed until the file is found and then the inode of the file is identified. The system uses a caching mechanism to speed up consecutive lookups of filenames in a path.

For more information on FFS see A fast filesystem for UNIX.

UFS2

FreeBSD and NetBSD have a second version of FFS that allows filesystem sizes greater than 1 TB. This is UFS2. The size of an UFS2 inode is 256 bytes and thus UFS1 and UFS2 are not compatible.

Growing an FFS filesystem

If you happen to have a partition that doesn't use up the entire disk, it is possible to grow this filesystem. Before FreeBSD 4.4 this was not possible. Here is the breakdown on how to grow an ffs filesystem.

Soft Updates

What are Soft Updates? A paper outlining its use and implementation is here.

You will want to check out soft-updates if you have ffs, and most likely install it on all partitions except / (example for FreeBSD):

$ less -XF /usr/src/sys/contrib/softupdates/README

To see if you have soft-updates enabled: on FreeBSD:

$ mount 
/dev/ad0s1a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0s1g on /backup (ufs, local, soft-updates)
/dev/ad0s1e on /tmp (ufs, local, soft-updates)
/dev/ad0s1f on /usr (ufs, local, soft-updates)
/dev/ad0s1d on /var (ufs, local, soft-updates)

on OpenBSD

$  mount
/dev/wd0a on / type ffs (local, softdep)
/dev/wd0e on /usr type ffs (NFS exported, local, nodev, softdep)
/dev/wd0d on /var type ffs (local, nodev, nosuid, softdep)
mfs:22707 on /tmp type mfs (asynchronous, local, nodev, nosuid, size=261120 512-blocks)

Side effects of having soft updates on the / filesystem are that a new kernel written to /bsd shortly before a panic will be corrupted requiring the next boot off a backup of the kernel.

Magic Numbers

In the file /usr/include/ufs/ffs/fs.h

#define FS_MAGIC        0x011954        /* the fast filesystem magic number */
#define FS_UFS1_MAGIC   0x011954        /* the fast filesystem magic number */
#define FS_UFS2_MAGIC   0x19540119      /* UFS fast filesystem magic number */

We suspect these magic numbers are the ffs authors birthdays (in hex). ;-)