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 Structure of UFS File System Cylinder Groups"

• "Deciding on Custom File System Parameters"

• "Commands for Creating a Customized File System"

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.

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

The Platform-Dependent Directories

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

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 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:

• Size and status of the file system

• Label (file system name and volume name)

• Size of the file system logical block

• Date and time of the last update

• Cylinder group size

• Number of data blocks in a cylinder group

• Summary data block

• File system state: clean, stable, or active

• Path name of the last mount point

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.

Inodes

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 type of the file:

  • Regular

  • Directory

  • Block special

  • Character special

  • Symbolic link

  • FIFO, also known as named pipe

  • Socket

• The mode of the file (the set of read-write-execute permissions)

• The number of hard links to the file

• The user ID of the owner of the file

• The group ID to which the file belongs

• The number of bytes in the file

• An array of 15 disk-block addresses

• The date and time the file was last accessed

• The date and time the file was last modified

• The date and time the file was created

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:

• Block size

• Fragment size

• Minimum free space

• Rotational delay

• Optimization type

• Number of files

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.

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.

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.

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.

• Space - When you select space optimization, disk blocks are allocated to minimize fragmentation and disk use is optimized.

• Time - When you select time optimization, disk blocks are allocated as quickly as possible, with less emphasis on their placement. When there is enough free space, it is relatively easy to allocate disk blocks effectively, without resulting in too much fragmentation. The default is time.

  You can change the value of the optimization type parameter for an existing file system using the tunefs command.

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:

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:

• newfs

• mkfs

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.

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 ...