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AIX Version 4.3 System Management Guide: 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").
However, volume group size also plays a part as is explained below.
To protect against adapter and power supply
failure, you need to 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.
It is important that you understand the
material contained in the "Logical Volume
When to Create Separate Volume Groups
You may 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
- 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, deactivating it (using varyoffvg), then
exporting the group (using exportvg). After updating the system software,
you can reintroduce the user-defined volume group (using importvg), 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.
- To have multiple JFS logs or JFS logs dedicated
on one physical volume for the purpose of reducing bottlenecking, especially on
- 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).
- To remove disks from the system while the
system continues to run normally. By making a separate volume group for
removable disks, provided the volume group is not rootvg, you can make removable
disks unavailable and physically remove them during normal operation without
affecting other volume groups.
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
- 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 quorum requirements, see the
section on vary-on process in the "Logical
Volume Storage Overview". 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"). With this configuration, you need
only two disks with mirrored copies in the volume group to protect against disk
When deciding on the number of disks in each
volume group, you also need to 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 and intra-disk allocation for a
High Availability in Case of Adapter or Power
To protect against adapter or power supply
failure, depending on the stringency of your requirements, do one or more of the
- Use two adapters, located in the same or
different cabinets. Locating the adapters in different cabinets protects against
losing both adapters if there is a power supply failure in one cabinet.
- 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 will
remain 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
- 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
Decide on the Size of Physical Partitions
The physical partition size is set when the
volume group is created. The default size is 4MB. The default is designed to
suit most sites and systems but may not be appropriate in every case. You can
choose a partition size as small as 1MB to gain flexibility in sizing but this
requires more partitions. The additional partitions create more overhead for the
Logical Volume Manager (LVM) and are likely to affect performance.
If you make the partitions larger than 4MB, you
lose some sizing flexibility and may also waste space. For example, if you have
20MB partitions, then your JFS log will have to be 20MB when it only needs 4MB.
Some waste may be an acceptable tradeoff if the particular site or system
requires larger partitions.
Note that you may only create and extend
physical partitions in increments that are a factor of their size; for example,
20MB partitions are created or extended in 20MB increments.
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