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System Management Concepts:
Operating System and Devices
Developing a Volume Group Strategy
Disk failure is the most common hardware failure in the storage system,
followed by failure of adapters and power supplies. Protection against disk
failure primarily involves the configuration of the logical volumes. See Developing a Logical Volume Strategy for more information. Volume group size can also
play a part.
To protect against adapter and power supply failure, consider a special
hardware configuration for any specific volume group. Such a configuration
includes two adapters and at least one disk per adapter, with mirroring across
adapters, and a nonquorum volume group configuration. The additional expense
of this configuration is not appropriate for all sites or systems. It is
recommended only where high (up-to-the-last second) availability is a priority.
Depending on the configuration, high availability can cover hardware failures that
occur between the most recent backup and the current data entry. High availability
does not apply to files deleted by accident.
When to Create Separate Volume Groups
You might want to organize physical volumes into volume groups separate
from rootvg for the following reasons:
- For safer and easier maintenance.
- Operating system updates, reinstallations, and crash recoveries are safer
because you can separate user file systems from the operating system so that
user files are not jeopardized during these operations.
- Maintenance is easier because you can update or reinstall the operating
system without having to restore user data. For example, before updating,
you can remove a user-defined volume group from the system by unmounting its
file systems. Deactivate it using the varyoffvg command,
then export the group using the exportvg command. After
updating the system software, you can reintroduce the user-defined volume
group using the importvg command, then remount its file
systems.
- For different physical-partition sizes. All physical volumes within the
same volume group must have the same physical partition size. To have physical
volumes with different physical partition sizes, place each size in a separate
volume group.
- When different quorum characteristics are required. If you have a file
system for which you want to create a nonquorum volume group, maintain a
separate volume group for that data; all of the other file systems should
remain in volume groups operating under a quorum.
- For security. For example, you might want to remove a volume group at
night.
- To switch physical volumes between systems. If you create a separate
volume group for each system on an adapter that is accessible from more than
one system, you can switch the physical volumes between the systems that
are accessible on that adapter without interrupting the normal operation
of either (see the varyoffvg, exportvg, importvg, and varyonvg commands).
High Availability in Case of Disk Failure
The primary methods used to protect against disk failure involve logical
volume configuration settings, such as mirroring. While the volume group considerations
are secondary, they have significant economic implications because they involve
the number of physical volumes per volume group:
- The quorum configuration, which is the default, keeps the volume group
active (varied on) as long as a quorum (51%) of the disks is present. For
more information about requirements, see Vary-On Process. In most cases,
you need at least three disks with mirrored copies in the volume group to
protect against disk failure.
- The nonquorum configuration keeps the volume group active (varied on)
as long as one VGDA is available on a disk (see "Changing a Volume Group to Nonquorum Status" in AIX 5L Version 5.2 System Management Guide: Operating System and Devices). With
this configuration, you need only two disks with mirrored copies in the volume
group to protect against disk failure.
When deciding on the number of disks in each volume group, you must also
plan for room to mirror the data. Keep in mind that you can only
mirror and move data between disks that are in the same volume group. If the
site uses large file systems, finding disk space on which to mirror could
become a problem at a later time. Be aware of the implications on availability
of inter-disk settings for logical volume copies (see Inter-Disk Settings for Logical Volume Copies)
and intra-disk allocation (see Choosing an Intra-Disk Allocation Policy for Each Logical Volume) for a logical
volume.
High Availability in Case of Adapter or Power Supply Failure
To protect against adapter or power supply failure, depending on your requirements,
do one or more of the following:
- Use two adapters, located in the same or different chassis. Locating the
adapters in different chassis protects against losing both adapters if there
is a power supply failure in one chassis.
- Use two adapters, attaching at least one disk to each adapter. This protects
against a failure at either adapter (or power supply if adapters are in separate
cabinets) by still maintaining a quorum in the volume group, assuming cross-mirroring (copies for a logical partition cannot
share the same physical volume) between the logical volumes on disk A (adapter
A) and the logical volumes on disk B (adapter B). This means that you copy
the logical volumes that reside on the disks attached to adapter A to the
disks that reside on adapter B and also that you copy the logical volumes
that reside on the disks attached to adapter B to the disks that reside on
adapter A as well.
- Configure all disks from both adapters into the same volume group. This
ensures that at least one logical volume copy remains intact in case an adapter
fails, or, if cabinets are separate, in case a power supply fails.
- Make the volume group a nonquorum volume group. This allows the volume
group to remain active as long as one Volume Group Descriptor Area (VGDA)
is accessible on any disk in the volume group. See Changing a Volume Group to Nonquorum Status for more information.
- If there are two disks in the volume group, implement cross-mirroring
between the adapters. If more than one disk is available on each adapter,
implement double-mirroring. In that case, you create a mirrored copy on a
disk that uses the same adapter and one on a disk using a different adapter.
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