System Administration Guide, Volume 1

Chapter 40 UFS File System Reference

This is a list of the reference information in this chapter.

Default Directories for root (/) and /usr File Systems

The /kernel directory contains only platform-independent objects, including a platform-independent kernel, genunix. See Table 40-3 for a description of /platform and /usr/platform, the platform-dependent directories.

The table below describes all the default directories contained in the root (/) file system.

Table 40-1 Default Directories in the root (/) File System




Root of the overall file system name space 


Primary location for special files 


Symbolic links to physical ap_ids 


Device files for uucp 


Block disk devices 


Frame buffer device files 


Logical volume management meta-disk devices 


File descriptors 


pty slave devices


Raw disk devices 


Raw tape devices 


Entry points for the STREAMS Administrative Driver 


Audio device and audio device control files 


Default swap device 


Serial devices 


Host-specific system administrative configuration files and databases 


Accounting configuration information 


Configuration information for cron


Defaults information for various programs 


Solstice Enterprise AgentsTM configuration files


Configuration information for shared file systems 


Dynamic Host Configuration Protocol (DHCP) configuration files 


Federated Naming Service and x.500 support files 


Binaries organized by file system types for operations required before /usr is mounted


Generic Security Service (GSS) Application Program Interface configuration files 


Configuration files for Internet services 


Scripts for changing between run levels 


Dynamic linking libraries needed when /usr is not available


Logical link control (llc2) driver configuration files


Configuration information for the printer subsystem 


Mail subsystem configuration information 


Configuration information for TI (transport- independent) network services 


NFS server logging configuration file 


OpenWindowsTM configuration files


Configuration information for optional packages 


Scripts for entering/leaving run level 0 


Scripts for entering/leaving run level 1 


Scripts for entering/leaving run level 2 


Scripts for entering/leaving run level 3 


Scripts for bringing the system up in single user mode 


This directory may contain a NIS+ authentication configuration file 


Service access facility files (including FIFOs) 


Basic Security Module (BSM) configuration files 


Default profile scripts for new user accounts 


Trademark files; contents displayed at boot time 


uucp configuration information


Default directory for users' home directories, client file systems, or other shared file systems 


Default directory or mount point for a user's home directory on a standalone system. When AutoFS is running, you cannot create any new entries in this directory. 


Directory of platform-independent loadable kernel modules required as part of the boot process. It includes the generic part of the core kernel that is platform independent, /kernel/genunix. See Table 40-3 for the /platform and /usr/platform directory structure.


Convenient, temporary mount point for file systems 


Default directory or mount point for add-on application packages 


Essential executables used in the booting process and in manual system failure recovery 


Standalone programs 


Temporary files; cleared during boot sequence 


Mount point for the /usr file system. See Table 40-2 for more information.


Directory for varying files, which usually includes temporary, logging, or status files 


System logging and accounting files 


Basic Security Module (BSM) audit files 


Default depository for kernel crash dumps 


cron's log file


Solstice Enterprise AgentsTM (SEA) Desktop Management Interface (DMI) run time components


dtlogin configuration files


FTP server directory 


IPv6 router state files 


System log files 


Line printer subsystem logging information  


Directory where users' mail is kept 


Community service messages (note: not the same as USENET-style news)


NIS+ databases 


NFS server log files 


Network Time Protocol (NTP) server state directory 


Root of a subtree for varying files associated with software packages 


Backup files for vi and ex


Temporary system files that are not needed across system reboots. This is a TMPFS-mounted directory. 


Databases maintained by the software package management utilities 


saf (service access facility) logging and accounting files


Directories for spooled temporary files 


cron and at spool files


Spooling lock files 


Line printer spool files 


Mail queued for delivery 


Spooled packages 


Queued uucp jobs


Files deposited by uucp


Network status monitor files 


Directory for temporary files; not cleared during boot sequence 


uucp log and status files


NIS databases (for backwards compatibility with NIS and unnecessary after full transition to NIS+) 

The table below describes the default directories in the /usr file system.

Table 40-2 Default Directories in the /usr File System




SunOS 4.1 binary compatibility package libraries 


Symbolic link to the /usr/bin directory


Symbolic link to the /usr/openwin directory


Symbolic link to the /var/adm directory


Directory for Automated Security Enhancement Tools (ASET) programs and files 


Location for standard system commands 


C compilation programs and libraries 


Demo programs and data 


Symbolic link to the /usr/share/lib/dict directory, which contains the dictionary file used by the UNIX spell program


Directory or mount point for CDE software 


An empty directory, which is a remnant of the SunOS 4.0/4.1 software 


Header files (for C programs, etc.) 


Directories containing JavaTM programs and libraries


Additional kernel modules 


Implementation architecture-specific binaries and libraries 


Various program libraries, architecture-dependent databases, and binaries not invoked directly by the user 


Commands local to a site 


Symbolic link to the /var/mail directory


Symbolic link to the /usr/share/man directory


Directory for network listener services 


Symbolic link to the /var/news directory


Files pertaining to the Form and Menu Language Interpreter (FMLI) execution environment 


Programs that are being phased out 


Directory or mount point for OpenWindows software 


Perl 5 programs and documentation 


See Table 40-3 for more information


Symbolic link to the /var/preserve directory


Directory for the proc tools


Files for online man page and character processing 


Various files and directories related to system administration 


Executables for system administration 


Statically linked version of selected programs from /usr/bin and /usr/sbin


Architecture-independent sharable files 


Architecture-independent databases 


Source code for kernel, libraries, and utilities 


Programs and libraries related to system and network administration  


Symbolic link to the /var/spool directory


Symbolic link to the share/src directory


Symbolic link to the var/tmp directory


Berkeley compatibility package binaries 


Berkeley compatibility package header files 


Berkeley compatibility package libraries 


Directory for Framed Access Command Environment (FACE) programs 


Directory for POSIX-compliant utilities 

The Platform-Dependent Directories

The table below describes the platform-dependent objects in the /platform and /usr/platform directories.

Table 40-3 The /platform and /usr/platform Directories




Contains a series of directories, one per supported platform that need to reside in the root (/) file system.


Contains platform-dependent kernel components, including the file unix, the core kernel that is platform dependent. See kernel(1M).


Contains platform-dependent objects that do not need to reside in the root (/) file system. It contains objects which replace the contents of /usr/kvm, which has been removed.


Contains platform-dependent objects similar to those found in the /usr/lib directory.


Contains platform-dependent objects similar to those found in the /usr/sbin directory.

The Structure of UFS File System Cylinder Groups

When you create a UFS file system, the disk slice is divided into cylinder groups, which is made up of one or more consecutive disk cylinders. The cylinder groups are then further divided into addressable blocks to control and organize the structure of the files within the cylinder group. Each type of block has a specific function in the file system. A UFS file system has these four types of blocks:

This Block Type ... 

Stores ... 

Boot block 

Information used when booting the system 


Detailed information about the file system 


All information about a file 

Storage or data block 

Data for each file 

This section provides additional information about the organization and function of these blocks.

The Boot Block

The boot block stores the procedures used in booting the system. If a file system is not to be used for booting, the boot block is left blank. The boot block appears only in the first cylinder group (cylinder group 0) and is the first 8 Kbytes in a slice.

The Superblock

The superblock stores much of the information about the file system. A few of the more important things it contains are:

The superblock is located at the beginning of the disk slice, and is replicated in each cylinder group. Because the superblock contains critical data, multiple superblocks are made when the file system is created. Each of the superblock replicas is offset by a different amount from the beginning of its cylinder group. For multiple-platter disk drives, the offsets are calculated so that a superblock appears on each platter of the drive. That way, if the first platter is lost, an alternate superblock can always be retrieved. Except for the leading blocks in the first cylinder group, the leading blocks created by the offsets are used for data storage.

A summary information block is kept with the superblock. It is not replicated, but is grouped with the first superblock, usually in cylinder group 0. The summary block records changes that take place as the file system is used, and lists the number of inodes, directories, fragments, and storage blocks within the file system.


An inode contains all the information about a file except its name, which is kept in a directory. An inode is 128 bytes. The inode information is kept in the cylinder information block, and contains:

The array of 15 disk addresses (0 to 14) point to the data blocks that store the contents of the file. The first 12 are direct addresses; that is, they point directly to the first 12 logical storage blocks of the contents of the file. If the file is larger than 12 logical blocks, the 13th address points to an indirect block, which contains direct block addresses instead of file contents. The 14th address points to a double indirect block, which contains addresses of indirect blocks. The 15th address is for triple indirect addresses, if they are ever needed. The figure below shows this chaining of address blocks starting from the inode.

Figure 40-1 The File System Address Chain in a UFS System


Data Blocks

The rest of the space allocated to the file system is occupied by data blocks, also called storage blocks. The size of these data blocks is determined at the time a file system is created. Data blocks are allocated, by default, in two sizes: an 8-Kbyte logical block size, and a 1-Kbyte fragmentation size.

For a regular file, the data blocks contain the contents of the file. For a directory, the data blocks contain entries that give the inode number and the file name of the files in the directory.

Free Blocks

Blocks not currently being used as inodes, as indirect address blocks, or as storage blocks are marked as free in the cylinder group map. This map also keeps track of fragments to prevent fragmentation from degrading disk performance.

To give you an idea of the appearance of a typical UFS file system, The figure below shows a series of cylinder groups in a generic UFS file system.

Figure 40-2 A Typical UFS File System


Deciding on Custom File System Parameters

Before you choose to alter the default file system parameters assigned by the newfs command, you need to understand them. This section describes each of these parameters:

Logical Block Size

The logical block size is the size of the blocks that the UNIX kernel uses to read or write files. The logical block size is usually different from the physical block size (usually 512 bytes), which is the size of the smallest block that the disk controller can read or write.

You can specify the logical block size of the file system. After the file system is created, you cannot change this parameter without rebuilding the file system. You can have file systems with different logical block sizes on the same disk.

By default, the logical block size is 8192 bytes (8 Kbytes) for UFS file systems. The UFS file system supports block sizes of 4096 or 8192 bytes (4 or 8 Kbytes). 8 Kbytes is the recommended logical block size.

SPARC only -

You can only specify 8192-byte block size on the sun4u platform.

To choose the best logical block size for your system, consider both the performance desired and the available space. For most UFS systems, an 8-Kbyte file system provides the best performance, offering a good balance between disk performance and use of space in primary memory and on disk.

As a general rule, to increase efficiency, use a larger logical block size for file systems where most of the files are very large. Use a smaller logical block size for file systems where most of the files are very small. You can use the quot -c file-system command on a file system to display a complete report on the distribution of files by block size.

Fragment Size

As files are created or expanded, they are allocated disk space in either full logical blocks or portions of logical blocks called fragments. When disk space is needed to hold a data for a file, full blocks are allocated first, and then one or more fragments of a block are allocated for the remainder. For small files, allocation begins with fragments.

The ability to allocate fragments of blocks to files, rather than just whole blocks, saves space by reducing fragmentation of disk space resulting from unused holes in blocks.

You define the fragment size when you create a UFS file system. The default fragment size is 1 Kbyte. Each block can be divided into 1, 2, 4, or 8 fragments, which results in fragment sizes from 8192 bytes to 512 bytes (for 4-Kbyte file systems only). The lower bound is actually tied to the disk sector size, typically 512 bytes.

Note -

The upper bound might equal the full block size, in which case the fragment is not a fragment at all. This configuration might be optimal for file systems with very large files when you are more concerned with speed than with space.

When choosing a fragment size, look at the trade-off between time and space: a small fragment size saves space, but requires more time to allocate. As a general rule, to increase storage efficiency, use a larger fragment size for file systems where most of the files are large. Use a smaller fragment size for file systems where most of the files are small.

Minimum Free Space

The minimum free space is the percentage of the total disk space held in reserve when you create the file system. The default reserve is ((64 Mbytes/partition size) * 100), rounded down to the nearest integer and limited between 1% and 10%, inclusively. Free space is important because file access becomes less and less efficient as a file system gets full. As long as there is an adequate amount of free space, UFS file systems operate efficiently. When a file system becomes full, using up the available user space, only root can access the reserved free space.

Commands such as df report the percentage of space that is available to users, excluding the percentage allocated as the minimum free space. When the command reports that more than 100 percent of the disk space in the file system is in use, some of the reserve has been used by root.

If you impose quotas on users, the amount of space available to the users does not include the free space reserve. You can change the value of the minimum free space for an existing file system by using the tunefs command.

Rotational Delay (Gap)

The rotational delay is the expected minimum time (in milliseconds) it takes the CPU to complete a data transfer and initiate a new data transfer on the same disk cylinder. The default delay is zero, as delay-based calculations are not effective when combined with modern on-disk caches.

When writing a file, the UFS allocation routines try to position new blocks on the same disk cylinder as the previous block in the same file. The allocation routines also try to optimally position new blocks within tracks to minimize the disk rotation needed to access them.

To position file blocks so they are "rotationally well-behaved," the allocation routines must know how fast the CPU can service transfers and how long it takes the disk to skip over a block. Using options to the mkfs command, you can indicate how fast the disk rotates and how many disk blocks (sectors) it has per track. The allocation routines use this information to figure out how many milliseconds it takes to skip a disk block. Then using the expected transfer time (rotational delay), the allocation routines can position or place blocks so that the next block is just coming under the disk head when the system is ready to read it.

Note -

It is not necessary to specify the rotational delay (-d option to newfs) for some devices.

Place blocks consecutively only if your system is fast enough to read them on the same disk rotation. If the system is too slow, the disk spins past the beginning of the next block in the file and must complete a full rotation before the block can be read, which takes a lot of time. You should try to specify an appropriate value for the gap so that the head is located over the appropriate block when the next disk request occurs.

You can change the value of this parameter for an existing file system by using the tunefs command. The change applies only to subsequent block allocation, not to blocks already allocated.

Optimization Type

The optimization type is either space or time.

Number of Files

The number of inodes determines the number of files you can have in the file system: one inode for each file. The number of bytes per inode determines the total number of inodes created when the file system is made: the total size of the file system divided by the number of bytes per inode. Once the inodes are allocated, you cannot change the number without recreating the file system.

The default number of bytes per inode is 2048 bytes (2 Kbytes) if the file system is less than one Gbyte. If the file system is larger than one Gbyte, the following formula is used:

File System Size 

Number of Bytes Per Inode 

Less than or equal to 1 Gbyte 


Less than 2 Gbytes 


Less than 3 Gbytes 


3 Gbytes or greater 


If you have a file system with many symbolic links, they can lower the average file size. If your file system is going to have many small files, you can give this parameter a lower value. Note, however, that having too many inodes is much better than running out of them. If you have too few inodes, you could reach the maximum number of files on a disk slice that is practically empty.

Commands for Creating a Customized File System

This section describes the two commands you use to create a customized file system:

The newfs Command Syntax, Options, and Arguments

The newfs command is a friendlier version of the mkfs command that is used to create file systems. The newfs command is located in the /usr/sbin directory.

The syntax is:

newfs [-Nv] [mkfs_options] raw_device

The table below describes the options and arguments to the newfs command.

Table 40-4 The newfs Command Options and Arguments




Displays the file system parameters that would be used in creating the file system without actually creating it. This option does not display the parameters used to create an existing file system. 


Displays the parameters that are passed to the mkfs command.


Use the following options to set the parameters passed to the mkfs command. The options are listed below in the order they are passed to mkfs. Separate the options with spaces.

-s size

The size of the file system in sectors. The default is automatically determined from the disk label. 

-t ntrack

The number of tracks per cylinder on the disk. The default is determined from the disk label. 

-b bsize

The logical block size in bytes to use for data transfers. Specify the size of 4096 or 8192 (4 or 8 Kbytes). The default is 8192 bytes (8 Kbytes). 

-f fragsize

The smallest amount of disk space in bytes that is allocated to a file. Specify the fragment size in powers of two in the range from 512 to 8192 bytes. The default is 1024 bytes (1 Kbyte). 

-c cgsize

The number of disk cylinders per cylinder group. The default value is calculated by dividing the number of sectors in the file system by the number of sectors in a gigabyte, and then multiplying the result by 32. The default value ranges from 16 to 256.  

-m free

The minimum percentage of free disk space to allow. The default is ((64 Mbytes/partition size) * 100), rounded down to the nearest integer and limited between 1% and 10%, inclusively. 

-r rpm

The speed of the disk, in revolutions per minute. This setting is driver- or device-specific. If the drive can report how fast it spins, mkfs uses this value. If not, the default is 3600. This parameter is converted to revolutions per second before it is passed to mkfs.

-i nbpi

The number of bytes per inode to use in computing how many inodes to create. See the section above for the default values. 

-o opt

Optimization type to use for allocating disk blocks to files: space or time. The default is time.

-a apc

The number of alternate blocks per disk cylinder (SCSI devices only) to reserve for bad block placement. The default is 0. 

-d gap

(Rotational delay) The expected minimum number of milliseconds it takes the CPU to complete a data transfer and initiate a new data transfer on the same disk cylinder. The default is zero. 

-n nrpos

The number of different rotation positions in which to divide a cylinder group. The default is 8. 

-C maxcontig

The maximum number of blocks, belonging to one file, that will be allocated contiguously before inserting a rotational delay. The default varies from drive to drive. Drives without internal (track) buffers (or drives/controllers that don't advertise the existence of an internal buffer) default to 1. Drives with buffers default to 7. 

This parameter is limited in the following way: 

blocksize x maxcontig must be <= maxphys

maxphys is a read-only kernel variable that specifies the maximum block transfer size (in bytes) that the I/O subsystem is capable of satisfying. (This limit is enforced by mount, not by newfs or mkfs.)

This parameter also controls clustering. Regardless of the value of rotdelay, clustering is enabled only when maxcontig is greater than 1. Clustering allows higher I/O rates for sequential I/O and is described in tunefs(1M).


The special character (raw) device file name of the partition to contain the file system. This argument is required. 

Examples--newfs Command Options and Arguments

This newfs example uses the -N option to display file system information, including the backup superblocks.

# newfs -N /dev/rdsk/c0t0d0s0
/dev/rdsk/c0t0d0s0:  37260 sectors in 115 cylinders of 9 tracks, 36 sectors
        19.1MB in 8 cyl groups (16 c/g, 2.65MB/g, 1216 i/g)
superblock backups (for fsck -b #) at:
 32, 5264, 10496, 15728, 20960, 26192, 31424, 36656,

The Generic mkfs Command

The generic mkfs command calls a file system-specific mkfs, which then creates a file system of a specified type on a specified disk slice. Although mkfs can support different types of file systems, in practice you would use it to create UFS or PCFS file systems. To make other types of file systems, you would have to write the software for the file system-specific versions of the mkfs command to use. Normally, you do not run mkfs directly; it is called by the newfs command.

The generic mkfs command is located in /usr/sbin. See mkfs(1M) for a description of the arguments and options.

UFS Direct Input/Output (I/O)

Direct I/O is intended to boost bulk I/O operations. Bulk I/O operations use large buffer sizes to transfer large files (larger than 256 Kbytes).

An example of a bulk I/O operation is downloading satellite data, which writes large amounts of data to a file. Direct I/O data is read or written into memory without using the overhead of the operating system's page caching mechanism.

There is a potential penalty on direct I/O startup. If a file requested for I/O is already mapped by another application, the pages will have to be flushed out of memory before the direct I/O operation can begin.

See directio(3C) for more information.

Direct I/O can also be enabled on a file system by using the forcedirectio option to the mount command. Enabling direct I/O is a performance benefit only when a file system is transferring large amounts of sequential data.

When a file system is mounted with this option, data is transferred directly between a user's address space and the disk. When forced direct I/O is not enabled for a file system, data transferred between a user's address space and the disk is first buffered in the kernel address space.

The default behavior is no forced direct I/O on a UFS file system. See mount_ufs(1M) for more information.

How to Enable Forced Direct I/O on a UFS File System

  1. Become superuser.

  2. Mount a file system with the forcedirectio mount option.

    # mount -F ufs -o forcedirectio /dev/dsk/c0t3d0s7 /datab
  3. Verify the mounted file system has forced direct I/O enabled.

    # mount
    /export/home on /dev/dsk/c0t3d0s7 read/write/setuid/forcedirectio ...