Ffs: Difference between revisions
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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. | 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 parts. One for a long index (sequence of [[inode]]s) and the other for data. 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]], disk blocks used by file, chflags, file owner and file group. 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]]. | Next (not counting the super-blocks) the filesystem is split into 2 parts. One for a long index (sequence of [[inode]]s) and the other for data. 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]], disk blocks used by file, chflags, file owner and 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]]. | ||
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: | 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: | ||
Revision as of 06:41, 28 October 2005
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) 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 parts. One for a long index (sequence of inodes) and the other for data. 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, disk blocks used by file, chflags, file owner and 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.
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.
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.
