Tribblix: manual page: ufs.4fs

UFS(4FS) File Systems UFS(4FS)

NAME

ufs - UFS file system

SYNOPSIS

#include <sys/param.h>
#include <sys/types.h>
#include <sys/fs/ufs_fs.h>
#include <sys/fs/ufs_inode.h>

DESCRIPTION

ufs is one of the primary default disk-based file systems for illumos.
The UFS file system is hierarchical, starting with its root directory
(/) and continuing downward through a number of directories. The root
of a UFS file system is inode 2. A UFS file system's root contents
replace the contents of the directory upon which it is mounted.

Subsequent sections of this manpage provide details of the UFS file
systems.

State Flags (fs_state, fs_clean)
UFS uses state flags to identify the state of the file system.
fs_state is FSOKAY - fs_time. fs_time is the timestamp that indicates
when the last system write occurred. fs_state is updated whenever
fs_clean changes. Some fs_clean values are:

FSCLEAN
Indicates an undamaged, cleanly unmounted file system.

FSACTIVE
Indicates a mounted file system that has modified data in
memory. A mounted file system with this state flag indicates
that user data or metadata would be lost if power to the system
is interrupted.

FSSTABLE
Indicates an idle mounted file system. A mounted file system
with this state flag indicates that neither user data nor
metadata would be lost if power to the system is interrupted.

FSBAD Indicates that this file system contains inconsistent file
system data.

FSLOG Indicates that the file system has logging enabled. A file
system with this flag set is either mounted or unmounted. If a
file system has logging enabled, the only flags that it can
have are FSLOG or FSBAD. A non-logging file system can have
FSACTIVE, FSSTABLE, or FSCLEAN.

It is not necessary to run the fsck command on unmounted file
systems with a state of FSCLEAN, FSSTABLE, or FSLOG. mount(2)
returns ENOSPC if an attempt is made to mount a UFS file system
with a state of FSACTIVE for read/write access.

As an additional safeguard, fs_clean should be trusted only if
fs_state contains a value equal to FSOKAY - fs_time, where
FSOKAY is a constant integer defined in the
/usr/include/sys/fs/ufs_fs.h file. Otherwise, fs_clean is
treated as though it contains the value of FSACTIVE.

Extended Fundamental Types (EFT)
Extended Fundamental Types (EFT) provide 32-bit user ID (UID), group ID
(GID), and device numbers.

If a UID or GID contains an extended value, the short variable
(ic_suid, ic_sgid) contains the value 65535 and the corresponding UID
or GID is in ic_uid or ic_gid. Because numbers for block and character
devices are stored in the first direct block pointer of the inode
(ic_db[0]) and the disk block addresses are already 32 bit values, no
special encoding exists for device numbers (unlike UID or GID fields).

Multiterabyte File System

A multiterabyte file system enables creation of a UFS file system up to
approximately 16 terabytes of usable space, minus approximately one
percent overhead. A sparse file can have a logical size of one
terabyte. However, the actual amount of data that can be stored in a
file is approximately one percent less than one terabyte because of
file system overhead.

On-disk format changes for a multiterabyte UFS file system include:

+o The magic number in the superblock changes from FS_MAGIC to
MTB_UFS_MAGIC. For more information, see the
/usr/include/sys/fs/ufs_fs.h file.

+o The fs_logbno unit is a sector for UFS that is less than 1
terabyte in size and fragments for a multiterabyte UFS file
system.

UFS Logging

UFS logging bundles the multiple metadata changes that comprise a
complete UFS operation into a transaction. Sets of transactions are
recorded in an on-disk log and are applied to the actual UFS file
system's metadata.

UFS logging provides two advantages:

1. A file system that is consistent with the transaction log
eliminates the need to run fsck after a system crash or an unclean
shutdown.

2. UFS logging often provides a significant performance improvement.
This is because a file system with logging enabled converts
multiple updates to the same data into single updates, thereby
reducing the number of overhead disk operations.

The UFS log is allocated from free blocks on the file system and is
sized at approximately 1 Mbyte per 1 Gbyte of file system, up to 256
Mbytes. The log size may be larger (up to a maximum of 512 Mbytes),
depending upon the number of cylinder groups present in the file
system. The log is continually flushed as it fills up. The log is
also flushed when the file system is unmounted or as a result of a
lockfs(8) command.

Mounting UFS File Systems

You can mount a UFS file system in various ways using syntax similar to
the following:

1. Use mount(8) from the command line:

# mount -F ufs /dev/dsk/c0t0d0s7 /export/home

2. Include an entry in the /etc/vfstab file to mount the file system
at boot time:

/dev/dsk/c0t0d0s7 /dev/rdsk/c0t0d0s7 /export/home ufs 2 yes -

For more information on mounting UFS file systems, see mount_ufs(8).

INTERFACE STABILITY

Uncomitted

NOTES

For information about internal UFS structures, see newfs(8) and
mkfs_ufs(8). For information about dumping and restoring file systems,
see ufsdump(8), ufsrestore(8), and
/usr/include/protocols/dumprestore.h. If you experience difficulty in
allocating space on the ufs filesystem, it may be due to fragmentation.
Fragmentation can occur when you do not have sufficient free blocks to
satisfy an allocation request even though df(8) indicates that enough
free space is available. (This may occur because df only uses the
available fragment count to calculate available space, but the file
system requires contiguous sets of fragments for most allocations). If
you suspect that you have exhausted contiguous fragments on your file
system, you can use the fstyp(8) utility with the -v option. In the
fstyp output, look at the nbfree (number of blocks free) and nffree
((number of fragments free)) fields. On unmounted filesystems, you can
use fsck(8) and observe the last line of output, which reports, among
other items, the number of fragments and the degree of fragmentation.
To correct a fragmentation problem, run ufsdump(8) and ufsrestore(8) on
the ufs filesystem.

illumos November 29, 2021 illumos