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System Management Concepts: Operating System and Devices
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. However, volume group
size also plays 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.
It is important that you
understand the material contained in the Logical Volume Storage Overview .
You might want to organize
physical volumes into volume groups separate from rootvg for the following
- For safer and easier
- 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
- 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
bottlenecks, especially on server machines.
- 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
- 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.
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 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 (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.
To protect against adapter or
power supply failure, depending on your requirements, do one or more of the
- 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.
The physical partition size is set
when the volume group is created. The default size is 4 MB. The
default is designed to suit most sites and systems but might not be
appropriate in every case. You can choose a partition size as small as
1 MB 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 4 MB, you lose some sizing flexibility and might also waste space.
For example, if you have 20 MB partitions, then your JFS log will have to be
20 MB when it only needs 4 MB. Some waste may be an acceptable tradeoff
if the particular site or system requires larger partitions.
Note that you can only create and
extend physical partitions in increments that are a factor of their size;
for example, 20 MB partitions are created or extended in 20 MB
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