The basic unit of virtual memory and physical memory is the page. Virtual memory attempts to keep a process' most recently referenced virtual pages in physical memory. When a process references virtual pages, they are brought into physical memory from their storage locations on disk. Modified virtual pages can be moved to a temporary location on the disk called swap space if the physical pages (the pages in physical memory) that contain the virtual pages are needed by either a newly referenced virtual page or by a page with a higher priority. Therefore, a process' virtual address space can consist of pages that are located in physical memory, stored temporarily in swap space, and stored permanently on disk in executable or data files. The operating system uses two operations to move virtual pages between physical memory and disk: paging and swapping.

Note

The operating system will terminate a process if it attempts to write a modified virtual page to swap space that is depleted.

Immediate mode is more conservative than deferred mode because each modifiable virtual page is assigned a page of swap space when it is created. If you use the immediate mode of swap space allocation, you must allocate a swap space that is at least as large as the total amount of modifiable virtual address space that will be created on your system. Immediate mode requires significantly more swap space than deferred mode because it guarantees that there will be enough swap space if every modifiable virtual page is modified.

If you use the deferred mode of swap space allocation, you must estimate the total amount of virtual address space that will be both created and modified, and compare that total amount with the size of your system's physical memory. If this total amount is greater than the size of physical memory, the swap space must be large enough to hold the modified virtual pages that do not fit into your physical memory. If your system's workload is complex and you are unable to estimate the appropriate amount of swap space by using this method, you should first use the default amount of swap space and adjust the swap space as needed.

You may receive the following informational messages when you remove the /sbin/swapdefault file and when you boot a system that is using the deferred method:

vm_swap_init:  warning sbin/swapdefault swap device not found
vm_swap_init:  in swap over-commitment mode

If the /sbin/swapdefault file does not exist and you want to enable the immediate method of swap allocation, become the root user and create the file by using the following command syntax:

ln -s../dev/rzxy /sbin/swapdefault

The x variable specifies the device number for the device that holds the primary swap partition, and the y variable specifies the swap partition. Usually, the swap device number is the same as the boot device number, and the primary swap partition is partition b.

You must reboot the system for the new method to take effect.

There are three types of device files:

• Block device files

  Block device files are used for devices whose driver handles I/O in large blocks and where the kernel handles I/O buffering. Physical devices such as disks are defined as block device files. An example of the block device files in the /dev directory follows:

  brw-------  1 root  system     8,   1 Jan 19 11:20 /dev/rz0a
  brw-------  1 root  system     8,   1 Jan 19 10:09 /dev/rz0b

• Character device files

  Character device files are used for devices whose drivers handle their own I/O buffering. Disk, terminal, pseudoterminal, and tape drivers are typically defined as character device files. An example of the character device files in the /dev directory follows:

  crw-rw-rw-  1 root  system    7,    0 Jan 31 16:02 /dev/ptyp0
  crw-rw-rw-  1 root  system    7,    1 Jan 31 16:00 /dev/ptyp1
  crw-rw-rw-  1 root  system    9, 1026 Jan 11 14:20 /dev/rmt1h

• Socket device files

  The printer daemon (lpd) and error logging daemon (syslogd) use the socket device files. An example of the socket device files in the /dev directory follows:

  srw-rw-rw-  1 root  system         0 Jan 22 03:40 log
  srwxrwxrwx  1 root  system         0 Jan 22 03:41 printer

Because disk and tape drivers often handle more than one device, each device file has a major and a minor number. The major number specifies (to the kernel) the driver that handles the device. The minor number is passed to the appropriate driver and tells it the device on which to perform the operation.

For static drivers, use the MAKEDEV command or the mknod command to create device special files. The kmknod command creates device special files for third-party kernel layered products. Refer to the MAKEDEV(8), mknod(8), and kmknod(8) reference pages for more information.

For loadable drivers, the sysconfig command creates the device special files by using the information specified in the driver's stanza entry in the /etc/sysconfigtab database file.

Note

Changing the default disk geometry values may make it impossible for the fsck program to find the alternate superblocks if the standard superblock is lost.

The newfs command has the following syntax:

/sbin/newfs[-N] [fs_options] device [disk_type]

You must specify the unmounted, raw device (for example, /dev/rrz0a).

File systems can become corrupted in many ways, such as improper shutdown procedures, hardware failures, and power outages and power surges. A file system can also become corrupted if you physically write protect a mounted file system, take a mounted file system off line, or if you do not synchronize the system before you shut the system down.

At boot time, the system runs fsck noninteractively, making any corrections that can be done safely. If it encounters an unexpected inconsistency, the fsck program exits, leaves the system in single-user mode, and displays a recommendation that you run the program manually, which allows you to respond yes or no to the prompts that fsck displays.

Note

The Digital UNIX operating system does not support 4-KB block-size file systems. The default block size for Digital UNIX file systems is 8 KB. To access the data on a disk that has 4-KB block-size file systems, you must back up the disk to either a tape or a disk that has 8-KB block-size file systems.

When you boot the system, file systems that are defined in the /etc/fstab file are mounted. The /etc/fstab file contains entries that specify the device and partition where the file system is located, the mount point, and additional information about the file system, such as file system type. If you are in single-user mode, the root file system is mounted read only.

The mount command supports the UFS, AdvFS, NFS, CDFS, and /proc file system types.

The following mount command syntax is for all file systems:

mount[-adflruv] [-ooption] [-ttype] [file_system] [mount_point]

For AdvFS, the file system argument has the following form:

filedomain#fileset

host :remote_directory

remote_directory @host

The following command lists the currently mounted file systems and the file system options. The backslash contained in this example indicates line continuation and is not in the actual display.

# mount -l
/dev/rz2a on / type ufs (rw,exec,suid,dev,nosync,noquota)
/dev/rz0g on /usr type ufs (rw,exec,suid,dev,nosync,noquota)
/dev/rz2g on /var type ufs (rw,exec,suid,dev,nosync,noquota)
/dev/rz3c on /usr/users type ufs (rw,exec,suid,dev,nosync,noquota)
/usr/share/man@tuscon on /usr/share/man type nfs (rw,exec,suid,dev,
nosync,noquota,hard,intr,ac,cto,noconn,wsize=8192,rsize=8192,
timeo=10,retrans=10,acregmin=3,acregmax=60,acdirmin=30,acdirmax=60)
proj_dmn#testing on /alpha_src type advfs (rw,exec,suid,dev,nosync,\
noquota)

The following command mounts the /usr/homer file system located on host acton on the local /homer mount point with read-write access:

# mount -t nfs -o rw acton:/usr/homer /homer

Refer to the mount(8) reference page for more information on general options and options specific to a file system type.

umount[-afv] [-hhost] [-ttype] [mount_point]

If any user process (including a cd command) is in effect within the file system, you cannot unmount the file system. If the file system is in use when the command is invoked, the system returns the following error message and does not unmount the file system:

mount device busy

You cannot unmount the root file system with the umount command.

The tunefs command has the following syntax:

tunefs[-amaxc] [-drotd] [-emaxb] [-mminf] [-oopt] [file_s]

You can use the tunefs command on both mounted and unmounted file systems; however, changes are applied only if you use the command on unmounted file systems. If you specify the root file system, you must also reboot to apply the changes.

You can use command options to specify the dynamic parameters that affect the disk partition layout policies. Refer to the tunefs(8) reference page for more information on the command options.

With the radisk program, you can perform the following tasks on a DSA disk device:

• Clear a forced error indicator

• Set or clear the exclusive access attribute

• Replace a bad block

• Scan the disk for bad blocks

Refer to the radisk(8) reference page for more information.

The scu program allows you to perform the following tasks on SCSI disk devices, in addition to other tasks:

• Format media

• Reassign a defective block

• Reserve and release a device

• Display and set device and program parameters

• Enable and disable a device

• Check available free space by using the df command

• Check disk use by using the du command or the quot command

• Verify disk quotas (if imposed) by using the quota command

You can use the quota command only if you are the root user.

The df command has the following syntax:

df[-eiknPt] [-F fstype] [file] [file_system . . .]

With no arguments or options, the df command displays the amount of free disk space on all of the mounted file systems. For each file system, the df command reports the file system's configured size in 512-byte blocks, unless you specify the -k option, which reports the size in kilobyte blocks. The command displays the total amount of space, the amount presently used, the amount presently available (free), the percentage used, and the directory on which the file system is mounted.

For AdvFS file domains, the df command displays disk space usage information for each fileset.

If you specify a device that has no file systems mounted on it, df displays the information for the root file system.

You can specify a file pathname to display the amount of available disk space on the file system that contains the file.

Refer to the df(1) reference page for more information.

Note

The newfs command reserves a percentage of the file system disk space for allocation and block layout. This can cause the df command to report that a file system is using more than 100 percent of its capacity. You can change this percentage by using the tunefs command with the -minfree flag.

The du command pinpoints disk space allocation by directory. With this information you can decide who is using the most space and who should free up disk space.

The du command has the following syntax:

/usr/bin/du[-aklrsx] [directory . . . | filename . . .]

The du command displays the number of blocks contained in all directories (listed recursively) within each specified directory, file name, or (if none are specified) the current working directory. The block count includes the indirect blocks of each file in 1-kilobyte units, independent of the cluster size used by the system.

If you do not specify any options, an entry is generated only for each directory. Refer to the du(1) reference page for more information on command options.

The following example displays a summary of blocks that all main subdirectories in the /usr/users directory use:

# /usr/bin/du -s /usr/users/*  
440     /usr/users/barnam
43      /usr/users/broland
747     /usr/users/frome
6804    /usr/users/morse
11183   /usr/users/rubin
2274    /usr/users/somer

From this information, you can determine that user rubin is using the most disk space.

The following example displays the space that each file and subdirectory in the /usr/users/rubin/online directory uses:

# /usr/bin/du -a /usr/users/rubin/online 
1	/usr/users/rubin/online/inof/license
2	/usr/users/rubin/online/inof
7	/usr/users/rubin/online/TOC_ft1
16	/usr/users/rubin/online/build
  .
  .
  .
251	/usr/users/rubin/online

Note

As an alternative to the du command, you can use the ls -s command to obtain the size and usage of files. Do not use the ls -l command to obtain usage information; ls -l displays only file sizes.

Note

The character device special file must be used to return the information, because when the device is mounted the block special device file is busy.

Refer to the quot(8) reference page for more information.

Caution

With both hard and soft limits, you can end up with a partially-written file if the quota limit is reached while you are writing to the file.

The quotacheck command verifies that the actual block use is consistent with established limits. You should run the quotacheck command twice: when quotas are first enabled on a file system and after each reboot. The command gives more accurate information when there is no activity on the system.

The quota command displays the actual block use for each user in a file system. Only the root user can execute the quota command.

The repquota command displays the actual disk use and quotas for the specified file system. For each user, the current number of files and the amount of space (in KB) is displayed along with any quotas.

If you find it necessary to change the established quotas, use the edquota command, which allows you to set or change the limits for each user.

Refer to the quotacheck(8), quota(8), and repquota(8) reference pages for more information on disk quotas.

The disk label provides detailed information about the geometry of the disk and the partitions into which the disk is divided. You can change the label with the disklabel command. You must be the root user to use the disklabel command.

There are two copies of a disk label, one located on the disk and one located in system memory. Because it is faster to access system memory than to perform I/O, when the system boots, it copies the disk label into memory. Use the disklabel -r command to directly access the label on the disk instead of going through the in-memory label.

If the partition is not in use, the command continues to execute. In addition to mounting or creating file systems, commands like mount, newfs, fsck, voldisk, mkfdmn, rmfdmn, and swapon also modify the disk label, so that the fstype field specifies how the partition is being used. For example, when you add a disk partition to an AdvFS domain, the fstype field is set to AdvFS.

If the partition is not available, these commands return an error message and ask if you want to continue, as shown in the following example:

# newfs /dev/rrz8c 
WARNING: disklabel reports that rz8c currently
is being used as "4.2BSD" data. Do you want to
continue with the operation and possibly destroy
existing data? (y/n) [n]

Applications, as well as operating system commands, can modify the fstype of the disk label, to indicate that a partition is in use. See the check_usage(3) and set_usage(3) reference pages for more information.