illumos: 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