Planning for Volume Management

Volume management manages disk space to keep as much data as possible available to the system. This is accomplished by partitioning or restricting data according to your disk space plan.

The following guidelines will help you develop your own disk space plan.

How will disk space be used? (need nonsystem volumes?)
What files should exist on the system volume? on nonsystem volumes?
What types of disk hardware are available (removable, capacity)?
How should data be restricted?
Who needs access to these volumes? (the account structure?)
Are volume classes needed?
What future expansion is planned?

Restricting Data

At the heart of your disk space plan is data restriction. Who can access a file? Which files should reside on a volume set? How should files be restricted?

Restricting files to a volume set, a volume class, or a volume gives you greater control over data access and increases data availability. There are three levels of restricting data.

Volume set (least restrictive).
Volume class (more restrictive).
Volume (most restrictive).




	NOTE: There may be a performance lag for being more restrictive, especially for large, active files such as databases. If many users are accessing a large file and the file is restricted to one volume, the disk drive may become a bottleneck for file access.

The default restriction is volume class DISC, on the home volume set of the group where the file resides. This means that the file extents are placed on any volume that is part of the volume class DISC within the volume set.

Volume Set Restriction

The default volume set restriction is the least restrictive. Since a file cannot span volume sets, if the master volume fails, access to the entire volume set is denied. If a volume in the volume set fails, access to the file is denied.

Figure 2-4 Volume Set File Restriction

Volume Class Restriction

With the exception of the volume class DISC, any volume class restriction must be specified at file creation time. A file is placed only on the volumes within that volume class. If a volume in the class fails, then access to the file is denied.

Figure 2-5 Volume Class File Restriction

Volume Restriction

Volume restriction is the most restrictive/granular level of protection. Volume restriction must be specified at file creation time. The file extents are placed on only one volume. If the volume fails, access to the file is denied.

Figure 2-6 Volume File Restriction

Disk Storage Space

There are two types of disk storage space, permanent and transient. Several commands in VOLUTIL (ALTERVOL, NEWSET, NEWVOL, SHOWUSAGE) allow you to set up, view, or change the allocations for each type of space.

Permanent space is disk space used for permanent structures such as files (permanent and temporary), the file label table, the free space map, and the directory on both system and nonsystem volume sets. Transient space is used for temporary structures such as stacks, heaps, and operating system data structures and is only allocated on the system volume set.

Each volume in the system volume set is given a percentage for permanent and transient space when it is defined with the VOLUTIL NEWVOL command. This value relates to the maximum amount of disk space of that type that can be allocated on the volume. For example, if you specify a permanent space value of 75% on an HP7937 disk drive (which has a total size of 2,232,192 sectors) only 1,674,144 sectors or 75% of the drive, would be available for permanent structures. This example could also be used for transient disk space.

It is possible that the total percentage for permanent and transient space will be greater than 100%. For example, the volume can be set up for 100% permanent and 100% transient. In this case, the entire drive is available for either type of storage.

The system generally uses LDEV 1 for transient space. Therefore, it is not necessary to reserve a large amount of transient space on every system volume. Since nonsystem volume sets do not have transient space,they should be set to 100/100 (permanent/transient).

The VOLUTIL SHOWUSAGE command displays disk space usage of a volume. The main purpose of this command is to help the user locate the 60,000 sectors of contiguous disk space needed in order to perform successful system updates.

The DISCFREE utility can be used to view disk space allocations for each volume. Refer to the MPE/iX Utilities Manual (32650-90081) for more information.

Volume Management Recommendations

In summary, the following volume management recommendations should guide you in designing your disk space plan.

Volume management proves particularly useful if you have four or more disk drives or a multisystem environment.
You should have most of the account structure on nonsystem volume sets for higher data availability. There is little difference in the time to access nonsystem volumes versus system volumes.
Keep volume sets small. Having multiple volume sets with only a few members is similar to having multiple volume classes on one volume set, except that volume sets can be moved and backed up separately.
Keep volume classes small. If a disk fails, it is less likely to prevent access to data.
Use nonsystem volumes to reduce the risk that a disk failure will affect access to data.