3    Planning Disk Space for a Custom Installation

This chapter applies only to custom installations.

The custom installation procedure lets you select the disks and disk partitions on which to install the required Digital UNIX root (/), /usr, and /var file systems and swap areas.

This chapter contains information to help you determine whether you can use the default partition table and default file system layout or if you should customize the partition table and file system layout.

Before beginning disk planning exercises, you should know what the file systems will be used for and understand the concepts associated with allocating a file system to a disk partition.

The goal of this chapter is to provide the information necessary for you to perform the user actions shown in Table 3-1.

Table 3-1: Summary of User Actions

3.1    Commonly Used Terms

The following terms are used throughout this chapter:

• custom partition table - is any partition table that differs from the default partition table. Disks preinstalled with Digital UNIX typically use a custom partition table as does any other disk whose partition has been modified. Customizing a partition table lets you divide or partition the disk in a fashion best suited to your needs. See partition table.

• default file system layout - the default file system layout in Digital UNIX consists of separate root (/) and /usr file systems and a single swap area, swap1. Furthermore, they are all installed on the same disk in the a, g, and b partitions, respectively. See file system layout.

• default partition table - the default disk partition specification as obtained from the /etc/disktab file or, in the absence of an entry in that file, the disk driver itself. The default disk partition table varies with disk type because it depends upon the size of the disk itself. A default disk partition is designed to fit the default file system layout, therefore, consider using the default partition table when using the default file system layout. Otherwise, there is no particular reason to use this partition table. See partition table.

• disk label - a disk label contains information about the disk such as the disk type, physical parameters, and partition sizes. Without a disk label, a disk is not bootable.

• file system - is a hierarchical structure consisting of directories and files. Each hierarchy starts with the / (root) directory. There is a one-to-one correspondence between file systems and partitions. For example, the a partition of the disk that contains the root file system contains all the files and directories in the root file system.

• file system layout - is the location of the basic Digital UNIX file systems and swap areas: / (root), /usr, /var, swap1, and swap2 on the disk.

• partition - logical divisions (groups of sectors) of a disk that are labeled a through h. Each partition may differ in size and can overlap other partitions. The c partition usually represents the entire disk. Partitions are sometimes known as slices.

• partition table - a component of a disk label that specifies how a physical disk is divided or partitioned into what appears to be several virtual disks. Disks in Digital UNIX support up to eight partitions per disk. Each partition is identified by a letter from a through h.

• system disk - the disk containing the root file system is known as the system disk.

3.2    Using the Default Disk Partitions and the Default File System Layout

As shown in Appendix G, every supported disk has a preset default partition table. With the exception of the RZ55 disk type, the default partition table and default file system layout is designed so that the entire Digital UNIX base operating system can fit on the disk when the root (/) and /usr file systems and swap space are installed on the same disk. The default file system layout is:

• The root file system is on the a partition

• The /usr file system is on the g partition and var is a directory under /usr.

• The swap area is on the b partition.

The design of the default file system layout along with the default disk partition sizes allows the entire Digital UNIX operating system to fit on a supported disk. Therefore, you do not have to perform any disk planning exercises if you are comfortable using the default disk partition sizes and default file system layout because all disk and partition selection decisions have already been made for you.

Also remember that the custom installation procedure calculates the amount of free space remaining in the root, /usr, and /var file systems as you select optional software subsets. This information helps you decide whether the disk partitions you chose are large enough to hold the software subsets you want to install. This file system status is useful during the installation procedure because you can go back and change your disk and partition selections if the partitions are running out of space.

During a text-based custom installation, if a customized disk partition table exists on the disk chosen for the root file system, you have the option to use either the default disk partitions or the existing customized partitions. If neither the default nor existing partitions are suitable, you can exit the installation process to invoke the UNIX shell and use the disklabel command to modify disk partitions.

3.3    When to Perform Disk Planning Exercises

You should perform disk planning exercises if you plan to:

• Install the associated products located on the CD-ROM labeled Digital UNIX V4.0 Associated Products Volume 1

• Use the system as a remote installation services (RIS) server or dataless management services (DMS) server

If your plans include any of the previous, it is suggested you read through the disk planning information in this chapter.

Throughout this chapter you are encouraged to complete worksheets to calculate the size of a file system. These worksheets will help you:

• Identify available disks

• Plan the swap space

• Plan the var area as a directory under the /usr file system or as a separate file system

• Plan the /usr file system

• Decide whether to use the default partition table an existing customized partition table (if there is one), or change the partition table if necessary

It will be helpful to keep in mind the following equations:

• One block equals -1/2 kilobyte (kB) or 512 bytes

• One megabyte (MB) equals 1024 kB (1,048,576 bytes), or 2048 blocks

If you need to determine the size in MB of the default disk partitions for a disk, divide the size in blocks by 2048. Appendix G shows disk partition sizes in megabytes; as shown by the disklabel command, a disk label shows partition sizes in blocks.

The file system planning information in Section 3.4, Section 3.9, Section 3.8, and Section 3.7 will help you complete the file system summary worksheet in Section 3.10. If you perform the planning exercises and fill in the requested information, this worksheet provides the file system layout information that is required during a custom installation.

3.4    Choosing the Disk for the root File System

There are two requirements for the disk that contains the root file system:

1. The disk you choose for the root file system must be one of the supported disks shown in Table 3-2.

2. The root file system is always located on partition a of the disk you choose. Partition a must be at least 98,304 blocks (48 MB) and must start at block 0 (zero), the beginning of the disk. Digital recommends selecting a disk where the size of partition a is 64 MB (which is the default size on most supported disks).

Table 3-2: Supported Disks for root File System

Table Notes:

1. Software device names prefixed with ra identify Digital Storage Architecture (DSA) disk drives.

2. Software device names prefixed with re identify SCSI disks employing RAID technology.

3. Software device names prefixed with rz identify Small Computer System Interface (SCSI) disk drives.

Refer to the Digital UNIX Software Product Description (SPD) for a list of all supported disks on all processors. A printed copy of the SPD is included in the Digital UNIX Software Distribution Kit. Files containing the SPD are located on the CD-ROM labeled Digital UNIX V4.0 Operating System Volume 1 in the directories /DOCUMENTATION/POSTSCRIPT or /DOCUMENTATION/TEXT.

The unit number for the disk that contains the root file system must be in the range 0 to 255 for ra type devices, in the range 0 to 47 for re type devices (SCSI disks employing RAID technology), and in the range 0 to 511 for rz type devices. The installation procedure automatically displays the device name (with unit numbers) for each disk connected to your system.

The installation procedure prevents you from installing the Digital UNIX operating system on a disk that is too small. Refer to Section 5.8.1 for restrictions about putting the root file system on RZ55 and RZ24L disks. The RZ25 disk is not large enough to hold all mandatory and optional software subsets.

3.5    Recording Disk Partition Sizes

The purpose of this section is for you to review the disks connected to your system, select the disks you want to use during the installation, and then record information about those disks in the worksheet shown in Table 3-3.

• If you are installing Digital UNIX for the first time (possibly on a brand new AlphaServer or AlphaStation), do the following:

  1. At the console mode prompt (>>>), enter the console command show device to determine the disks on your system.

  2. Review the entries under the DEVTYPE column; disks are identified by the word DISK. Disk types are displayed under the DEVNAM column where you will see entries such as RZ26, RZ26L, RZ73, RZ58, and so forth.

  3. Use your system's hardware documentation and the disk partition information in Appendix G to determine the device name and the size of each disk partition.

  4. Decide which disks you will use, and record the device names and partition sizes in the worksheet shown in Table 3-3.

• If you are installing Digital UNIX on a system that is already running a version of Digital UNIX, the disks may have a customized disk partition table. To check the disk layout, you have to examine the existing disk label. A disk label contains information about the disk such as the disk type, physical parameters, and partition sizes. Without a disk label, a disk is not bootable. To check the disk label on an already installed system, do the following:

  1. Follow the instructions in Section 3.5.2 to review an existing disk label.

  2. Record the disk label information in the worksheet shown in Table 3-3.

3.5.1    Completing the Disk Configuration Worksheet

The disk configuration worksheet provides space for five disks, which is the maximum number of disks that can be used and configured during an installation. Your system may have many more disks, but even if you choose a separate disk for the root, /usr, and /var file systems and a separate disk for the swap1 and swap2 areas, a total of five disks are used. You can configure and use the other disks connected to your system after the installation. Refer to the Digital UNIX Software Product Description (SPD) for the number of disks that can be configured on each type of system.

Table 3-3: Disk Configuration Worksheet

3.5.2    Reviewing Existing Disk Labels to Determine Disk Layout

If you are installing Version 4.0 on a system that is already running Digital UNIX, use the disklabel command to look at the existing disk partition layout and sizes. A disk label looks similar to the following:

type: SCSI
disk: RZ26L
label:
flags:
bytes/sector: 512
sectors/track: 57
tracks/cylinder: 14
sectors/cylinder: 798
cylinders: 2570
sectors/unit: 2050860
rpm: 3600
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0		# milliseconds
track-to-track seek: 0	# milliseconds
drivedata: 0

 

8 partitions:
#     size   offset   fstype  [fsize bsize  cpg]
a:  131072        0   4.2BSD    1024  8192  16  # (Cyl.    0 - 164*)
b:  262144   131072     swap                    # (Cyl.  164*- 492*)
c: 2050860        0   unused    1024  8192      # (Cyl.    0 - 2569)
d:  552548   393216   unused    1024  8192      # (Cyl.  492*- 1185*)
e:  552548   945764   unused    1024  8192      # (Cyl. 1185*- 1877*)
f:  552548  1498312   unused    1024  8192      # (Cyl. 1877*- 2569)
g:  819200   393216   4.2BSD    1024  8192  16  # (Cyl.  492*- 1519*)
h:  838444  1212416   unused    1024  8192      # (Cyl. 1519*- 2569)

The procedure to display a disk label differs upon the following:

• If your system is already running a version of Digital UNIX, as superuser or root, enter the disklabel command using the following syntax:

  disklabel -r disk

  Enter a command similar to the following to read the disk label for disk rz0:

  #  disklabel -r rz0
  

• If you have already started a text-based installation procedure from the Digital UNIX Version 4.0 CD-ROM or from a RIS server, choose option 3 from the first menu to access the UNIX Shell option. If you are past the first menu, enter the history command to go back to the first menu. Then, invoke the disklabel command from the UNIX Shell. Refer to Section 3.5.3 for more information about running disklabel from the UNIX Shell.

• If you have already started a graphical custom installation procedure from the Digital UNIX Version 4.0 CD-ROM or from a RIS server, view the current disk partition information by clicking on the Partition Disks... pushbutton to open the Disk Configuration application.

3.5.3    Using the disklabel Command in the UNIX Shell to View Disk Labels

If you are invoking the disklabel command from the UNIX Shell, you must make the device special file for the device. This example shows how to make the device special file for a SCSI device, rz1:

#  cd /dev
#  ./MAKEDEV rz1
#  disklabel -r rz1

When you execute the disklabel command, the existing disk partition layout is read and displayed. If you want to display the default disk label for rz1 (or any other device), enter the following command:

#  disklabel -p rz1

Refer to Section 7.8 for more information about using the disklabel command in the UNIX Shell environment to customize disk partitions.

3.6    File System Overhead

When calculating the available disk space for the root, /usr, and /var file systems, the installation procedure uses the following approximations for file system overhead based on the file system type selected for a particular file system:

• UNIX File System (UFS): 4%

• Advanced File System (AdvFS): 5%

That is, 4% of a file system (for UFS) and 5% of a file system (for AdvFS) is allocated for file system housekeeping and is not available to the partition for holding software. Additionally, UFS file systems are created with a default 10% minfree value, the minimum free space threshold, which holds back 10% of a UFS file system from non-root users. Privileged users still have access to this free space. Refer to the newfs(8) and tunefs(8) reference pages for a description of the minfree value.

In addition to the space set aside for file system overhead, additional space is reserved for kernel build considerations as follows:

• root file system: 13 MB

• /usr file system: 20 MB

This allows room for the kernel to be built in /usr and subsequently copied to the root file system.

During an installation, the free space shown during software subset selection includes these overhead requirements; you do not have to calculate this yourself.

If you plan to use the POLYCENTER Advanced File System (AdvFS) as the file system type and you install the optional AdvFS Utilities, which are available on a separate CD-ROM distribution and require a special license, modifying file system space is simplified. After the installation, the AdvFS utilities let you add or remove volumes from the AdvFS file systems with no changes to the directory structure and with no user interruption. There is no need to overallocate file system space for AdvFS file systems.

With the exception of the root file system, AdvFS file system size can be modified at any time (with the addvol command). Increases or decreases to file system size are transparent to the user.

3.7    Planning the /usr File System

The /usr directory contains the majority of the operating system files, including libraries, executable programs, and documentation. The directory structure contains directories such as /usr/sys, /usr/adm, and /usr/bin. These directories contain required system files and UNIX command binary files that require a considerable amount of space in the /usr file system.

During the installation procedure, you allocate the /usr file system either by accepting the default partition or by specifying another partition. If you choose the default allocation, the installation procedure uses the g partition of the disk that contains the root file system.

To determine the size of the /usr file system, consider the following:

• Software subsets you plan to install on /usr

• The number of accounts (users) and the amount of space needed by each user

• Size of the /var area if it is on the same partition as /usr

• File system overhead as described in Section 3.6.

Over time, you probably will add files to the /usr file system. Because of this, the file system can run out of space. Be sure to allow for future growth on the /usr file system.

If you plan to use the POLYCENTER Advanced File System (AdvFS) as the file system type and install the AdvFS Utilities (available with a separate license), you do not need to greatly overallocate space for the /usr file system. AdvFS file system space can be dynamically increased without changing directory structures and without system interruption. Refer to System Administration for more information about the AdvFS file system.

Section 3.7.1 to Section 3.8.7 briefly describe how these items affect the size of the /usr file system. As you work through each section, complete the worksheet in Table 3-4.

3.7.1    Software Subsets Within the /usr File System

The /usr file system must be large enough to accommodate the software subsets that will reside within it. A software subset is a collection of executable files and data files needed to perform a specific function or to provide a particular class of services; for example, you need the System Accounting Utilities software subset to perform system accounting.

Appendix C contains software subset descriptions along with the dependent software subsets and kernel configuration file options related to each software subset. Appendix F contains a table of software subset sizes.

The mandatory software subsets are always installed. The optional software subsets are not required for the operating system to be fully functional; you can choose none, some, or all of the optional software subsets, depending on your requirements and available disk space.

You may want to consider allocating space for other associated or layered products that are available for Digital UNIX. When planning space requirements for /usr, allow additional space if you will be adding products in the future. Refer to the specific layered product's Release Notes for the exact block size of the application.

Determine which subsets you will install, add their sizes together, and include any additional space that will be required for large applications in the near future. Enter the total on the first line of Table 3-4.

3.7.2    Space for Users' Accounts and Files

The custom installation does not provide an area for user accounts and files; you need to set up this area after the installation.

However, you should consider the amount of space needed for user files when planning your system. If you plan to place users' home directories on /usr, you should reserve at least 10 MB of disk space for each user on the system. For example, if there are 10 users, you should reserve a minimum of 100 MB of disk space.

Note

Digital recommends creating a separate file system (on another disk) for users' home directories and mounting the new file system perhaps under the /usr file system. Mounting user's home directories in another file system ensures that the directories will not be overwritten during future full installations.

If you intend to set quotas on the user area, multiply the quota for each user by the number of users to determine the amount of user space. Refer to System Administration for information on disk quotas.

Enter the amount of space needed for the users directory on the second line of Table 3-4.

3.7.3    Completing the /usr Worksheet

Add the values in column 3 of Table 3-4 to determine the total space requirements for the /usr file system.

Table 3-4: /usr Worksheet

Refer to Table 3-3 to identify partitions that are large enough for the /usr file system.

In the following table, record the disk number (for example 3), device name (for example, rz0), and partition where you plan to allocate the /usr file system:

Disk Number	Device Name	Partition
_______________	________________	________________

3.8    Planning the /var Area

The /var area contains volatile, machine-specific directories and directories such as tmp and adm.

You can allocate the /var area either as a file system on its own partition or in a directory under the /usr file system. Depending on system use, the /var area can potentially use large amounts of space in the /usr/var directory. If system use is heavy, you might want to create a separate /var file system.

If you choose the default file system layout, the installation procedure places the var area as a directory in the /usr directory.

To determine the size of the var area, consider the following:

• Crash dump space

• Error logger files

• System accounting files

• Size of the /var/adm/ris directory, if your system is to be used as a Remote Installation Services (RIS) server

• Size of the /var/adm/dms directory if your system is to be used as a Dataless Management Services (DMS) server

• Space required for mail, print, and uucp spooling

As you read through each of the following sections, complete the worksheet in Table 3-5.

If you plan to use the POLYCENTER Advanced File System (AdvFS) as the file system type for /var along with the AdvFS Utilities (available with a separate license), you do not need to greatly over allocate space for the /var file system. AdvFS file system space can be dynamically increased without changing directory structures and without system interruption. Refer to System Administration for more information about AdvFS.

3.8.1    Crash Dump Space

Two disk areas are used when the system produces a crash dump.

As described in Section 3.9, the first area is located in the swap partition and is used to hold the crash dump until the system is rebooted. This area must be large enough to hold a single crash dump.

The second area is where the savecore utility copies the crash dump and a copy of the kernel, /vmunix, when the system is rebooted. This area is located in the /var/adm/crash directory. The disk partition that contains /var/adm/crash must be at least large enough to hold one crash dump and one copy of /vmunix which is 7 to 10 MB in size, but can be made as large as resources permit if you want to retain multiple crash dumps.

The crash dump partition must be as large as the size of physical memory on systems configured for full dumps, and can be somewhat smaller on systems configured for partial dumps.

If you want to retain multiple crash dumps, estimate the size of this partition by multiplying the total size required for a single crash dump and a copy of /vmunix by n, where n is the number of crash dumps to retain.

The Kernel Debugging guide contains a chapter devoted to managing crash dumps and crash dump files. This chapter includes information about how crash dumps are written, choosing partial or full dumps, deciding how much space to reserve for both crash dumps and crash dump files, and much more.

To determine the size and to record the location of the crash dump space, provide the following information:

1. The memory size in MB for your system is _________.
   If you do not know the amount of memory on your system, do one of the following:

   • As superuser or root, enter the following command:

     #  uerf | grep -i memory
     

   • If your system is at the console mode prompt (>>>), enter the following command:

     >>>  show mem
     

2. You need ________ memory to accommodate your crash dump partition. Refer to Table 3-3 to identify the partitions that are large enough for the crash dump space.

3. Enter the amount of space needed for the /var/adm/crash directory on the first line of Table 3-5.

3.8.2    Error Logger and syslog Files

The var area requires room to accommodate the log files produced by both syslog and the binary error logger. These log files are a record of system events and errors in ASCII text (syslog) and binary formats.

The syslog utility collects information regarding such system activities as mail, system startup, shutdown, rebooting, root account logins, time daemon, printer subsystem, and syslog itself. Summary information on hardware errors is also logged. The amount of data logged is related to system activity and the number of users.

The binary error logger records information on hardware errors and system startup.

If you are creating a new system, estimate your total requirements at about 500 kB per week. There is no limit to how large the /var/adm/binary.errlog and the /var/adm/syslog files can grow, so they might eventually fill their partition. If you plan to back up or remove these log files once a month, you need to plan your total requirements at about 2 MB.

Enter the amount of space needed for the error logger on the second line of Table 3-5.

3.8.3    System Accounting Files

The /var/adm directory in the var area contains data files generated by administrative programs such as acct and wtmp. The data that these programs generate can vary widely from system to system and over time. For example, if you create a /var/adm/acct file, it can grow by 50 kB a day for a large system and by 5 kB a day for a workstation.

As a general guideline for system accounting, you should allot 10 kB per day for workstations and 100 kB per day for larger systems. If you back up or remove the system accounting file once a month, you should plan for accounting files that occupy about 300 kB for workstations and 3 MB for large systems. Refer to System Administration for more information on the space requirements for system accounting.

Enter the amount of space needed for system accounting on the third line in Table 3-5 that is located in Section 3.8.6.

3.8.4    Size of the /var/adm/ris Directory

The information in this section applies only if you are setting up the system to be a Remote Installation Services (RIS) server.

If you are planning to set up your system as a RIS server, you can transfer software subsets from the distribution media to the /var/adm/ris directory in the var area.

You must reserve enough space in the /var/adm/ris directory in the var area for the software you want to install in each RIS environment. Refer to Appendix C for a description of each software subset and the names of other subsets or kernel configuration file options related to its operation. Refer to Appendix F for the subset sizes.

Fill in the fourth line on the worksheet in Table 3-5 with the amount of space needed for the /var/adm/ris directory in the var area. If you plan to mount a separate partition on the /var/adm/ris directory after the installation, enter a 0 (zero) for item 4 in Table 3-5 that is located in Section 3.8.6.

Refer to Sharing Software on a Local Area Network for more information about RIS and setting up the network kit.

3.8.5    Size of the /var/adm/dms Directory

The information in this section applies only if you are setting up this system to be a Dataless Management Services (DMS) server.

If you want the system to serve a dataless environment, you should decide whether you want /var on a separate file system or whether you want to reserve a partition to mount under /var/adm/dms.

In a dataless management environment, the dataless server's environment file systems are located in /var/adm/dms/dmsn.alpha. Each environment must have at least the Digital UNIX mandatory subsets installed as well as other optional software subsets. Space must be reserved for associated or layered products plus an additional 10% for file system administration tasks and file system information. Refer to Appendix F for software subset sizes. For more information about the size requirements of a dataless environment, refer to Sharing Software on a Local Area Network. A worksheet in that guide is provided to help you calculate the amount of space required for a single /var/adm/dms file system.

Fill in the fifth line on the worksheet in Table 3-5 with the amount of space needed for the /var/adm/dms environments in the var area. If you plan to mount a separate partition on the /var/adm/dms area after the installation, enter a 0 (zero) for item 5 in Table 3-5 which is located in Section 3.8.6.

3.8.6    Completing the var Worksheet

Add the values in column 3 of Table 3-5 to determine the space requirements for the var area.

Table 3-5: var Worksheet

3.8.7    Placing the var Area in the /usr File System

If you plan to place the var area on the same partition as /usr, you must add the total size of the var area from the worksheet in Table 3-5 to the total of /usr.

If appropriate for your system, enter the amount of space needed for /var on the third line of Table 3-4.

3.9    Planning the Swap Space

Virtual memory is implemented in the Digital UNIX operating system by transparently moving pages back and forth between physical memory and swap space. The amount of virtual address space that can be created is limited only by the amount of swap space. This section discusses some of the factors to consider when configuring swap space on your system. System Tuning and Performance Management provides additional information about optimizing the use of swap space.

The custom installation procedure lets you configure two swap areas: a primary swap partition named swap1 and an optional swap partition named swap2. Additional swap partitions can be configured after the installation is complete by using the procedures described in System Administration.

During a custom installation, you are asked to choose which disk partition to use for swap1. The default choice is partition b of the system disk.

Note

Digital recommends a minimum of 128 MB of swap space. On supported disks, the default size of the b partition is 128 MB.

To optimize the use of your swap space, spread out your swap space across multiple devices and use the fastest disks for swap devices. To ensure the best performance, place swap areas on different disks instead of placing multiple swap areas on the same disk. The amount of swap space you allocate also depends on the virtual memory requirements of the applications you plan to install.

If you want to calculate the true amount of swap space your system needs, an effective strategy to determine how much disk space to set aside for swapping is to compare the aggregate modifiable virtual address space needs of the processes that you plan to run with the size of your system's physical memory. Modifiable virtual address space holds data elements and structures that are modified during process execution, such as heap space, stack space, and data space. If you expect the aggregate need for modifiable virtual address space to be greater than your system's physical memory, consider allocating at least as much swap space as the size of your system's physical memory.

Although you cannot choose swap strategy modes during the installation procedure, there are two strategies for swap allocation: immediate and over-commitment. The swap strategy mode used for Digital UNIX systems is immediate mode which means that swap space is allocated when modifiable virtual address space is created. This mode requires more swap space than over-commitment mode because it guarantees that there will be enough swap space if every modifiable virtual page is modified. Refer to System Administration for more information about swap allocation strategies and how to switch from one swap allocation mode to the other after the installation.

Also keep in mind that by default, crash dumps are temporarily stored on the swap partition. This area is used to hold the crash dump until the system is rebooted and must be large enough to hold a single crash dump. This area is referred to as the crash dump partition. In the event of a system crash, the kernel writes the contents of physical memory to the swap partition. The amount of information written, and hence the size of the crash dump, depends on several factors:

• If the system is configured to produce full dumps as described in the System Administration guide, the size of the crash dump will be the same as the size of the system's physical memory.

• If the system is configured to produce partial dumps, the crash dump might be considerably smaller.

The factor that determines the size of a partial crash dump is the amount of physical memory in use at the time of the crash by various kernel data structures that define the state of the system. The more tasks and threads that are active, the more kernel data structures that will be in use, and the larger the resulting partial crash dump.

Be prepared to add more swap space later if the system issues warning messages that indicate that swap space is approaching exhaustion. On systems where the balance between modifiable virtual address space usage and available physical memory is more even, less swap space is required.

Refer to the worksheet in Table 3-3 to identify partitions that are large enough for use as swap partitions. Record the location of the swap space in the following table:

Swap Space	Size in MB	Device Name	Partition
swap1	_______	________	________
swap2	_______	________	________

3.10    Finalizing the File System Layout

After you determine how much space each file system needs, determine whether you can accept the default disk partition table. If you need to customize the disk partition table, do one of the following:

• If you are using the text-based interface, choose the UNIX Shell option from the installation menu to use the disklabel command to modify the disk partitions. Section 3.5.3 describes how to use the disklabel command in the UNIX Shell. Refer to Appendix G for information on the default disk partition layout and sizes. When you are finished modifying the default disk partitions, enter the restart command to start the installation procedure again. Once in installation setup, be sure not to select a default installation because doing so overwrites your customized disk partitions.

• If you are using the graphical user interface to perform a custom installation, in the Installation Setup window, click on Partition Disks... to access the Disk Configuration application to reconfigure only those disks that will be used during the installation process. The utility can also be launched from the SysMan Configuration Checklist to reconfigure disks that were not used during the installation procedure.

The following guidelines apply if you modify the partition table for the disk that contains the root file system:

• The root file system is always located on partition a. Partition a must be a minimum of 98,304 blocks (48 MB), however Digital recommends 131,000 blocks (64 MB).

• Partition a must start at block 0 (zero), the beginning of the disk.

If the modified partition does not meet these requirements, the custom installation requires that you choose the default partition table or select a different disk.

Enter the values that you determined in the previous sections in Table 3-6. This table will provide you with the complete file system layout and space requirements for your system.
docroff: ignoring superfluous symbol sys-disk-table

Table 3-6: File System Worksheet

After completing the worksheet, verify the disk partition table. If you are installing a system for the first time, refer to Table 3-3 and Appendix G for the default partition table for your disk or disks.

Compare the disk partition table with the total var and /usr values in Table 3-6. If the space required consumes more than 75 percent of the available disk space, not including the file system overhead, consider expanding the partition. After determining your disk space needs, use either the disklabel command (if you are using the text-based installation interface) or use the Disk Configuration application (if you are using the graphical installation interface) to modify the default partition table before beginning the installation.