HP-UX System Administrator's Guide: Logical Volume Management (762803-001, March 2014)

will lose time while you restore data from backup media, and you will lose any data changed
since your last backup
• Initializing from scratch: If you do not mirror or back up a logical volume, be aware that you
will lose data if the underlying hard disk fails. This can be acceptable in some cases, such as
a temporary or scratch volume.
4.7.1.2 Using LVM Online Disk Replacement (LVM OLR)
LVM online disk replacement (LVM OLR) simplifies the replacement of disks under LVM. You can
temporarily disable LVM use of a disk in an active volume group without deactivating the volume
group or removing the logical volumes on the disk.
The LVM OLR feature uses a new option (–a) in the pvchange command. The –a option disables
or re-enables a specified path to an LVM disk, as used to halt LVM access to the disk under “Step
6: Replacing a Bad Disk (Persistent DSFs)” (page 137) or “Step 7: Replacing a Bad Disk (Legacy
DSFs)” (page 145). For more information, see the pvchange(1M) manpage.
4.7.1.3 Mirroring Critical Information, Especially the Root Volume Group
By making mirror copies of the root, boot, and primary swap logical volumes on another disk, you
can use the copies to keep your system in operation if any of these logical volumes fail.
For details on mirroring the boot disk (which contains the root, boot, and primary swap logical
volumes), see “Mirroring the Boot Disk” (page 93).
There are three corollaries to the mirroring recommendation:
1. Use the strict allocation policy for all mirrored logical volumes. Strict allocation forces mirrors
to occupy different disks. Without strict allocation, you can have multiple mirror copies on the
same disk; if that disk fails, you will lose all your copies. To control the allocation policy, use
the –s option with the lvcreate and lvchange commands. By default, strict allocation is
enabled.
2. To improve the availability of your system, keep mirror copies of logical volumes on separate
I/O busses if possible. With multiple mirror copies on the same bus, the bus controller becomes
a single point of failure—if the controller fails. If you create physical volume groups and set
the allocation policy to PVG-strict, LVM helps you avoid inadvertently creating multiple mirror
copies on a single bus. For more information about physical volume groups, see lvmpvg(4).
3. Consider using one or more free disks within each volume group as spares. If you configure
a disk as a spare, then a disk failure causes LVM to reconfigure the volume group so that the
spare disk takes place of the failed one. That is, all the logical volumes that were mirrored on
the failed disk are automatically mirrored and resynchronized on the spare, while the logical
volume remains available to users. You can then schedule the replacement of the failed disk
at a time of minimal inconvenience to you and your users. Sparing is particularly useful for
maintaining data redundancy when your disks are not hot-swappable, since the replacement
process may have to wait until your next scheduled maintenance interval. For information
about disk sparing, see “Increasing Disk Redundancy Through Disk Sparing” (page 30)
NOTE: Version 2.x volume groups do not support disk sparing.
4.7 Disk Troubleshooting and Recovery Procedures 125