LVCB warnings

Edit online

A warning results if the LVCB contains invalid information.

The logical volume control block (LVCB) is the first block of a logical volume. The size of LVCB is the block size of the physical volumes within the volume group. This area holds important information such as the creation date of the logical volume, information about mirrored copies, and possible mount points in the JFS. Certain LVM commands are required to update the LVCB, as part of the algorithms in LVM. The old LVCB is read and analyzed to see if it is a valid. If the information is valid LVCB information, the LVCB is updated. If the information is not valid, the LVCB update is not performed, and you might receive the following message: 
Warning, cannot write lv control block data.

Most of the time, this message results when database programs bypass the JFS and access raw logical volumes as storage media. When this occurs, the information for the database is literally written over the LVCB. For raw logical volumes, this is not fatal. After the LVCB is overwritten, the user can still:

  • Expand a logical volume
  • Create mirrored copies of the logical volume
  • Remove the logical volume
  • Create a journaled file system to mount the logical volume

There are limitations to deleting LVCBs. A logical volume with a deleted LVCB might not import successfully to other systems. During an importation, the LVM importvg command scans the LVCBs of all defined logical volumes in a volume group for information concerning the logical volumes. If the LVCB does not exist, the imported volume group still defines the logical volume to the new system that is accessing this volume group, and the user can still access the raw logical volume. However, the following typically happens:

  • Any JFS information is lost and the associated mount point is not imported to the new system. In this case, you must create new mount points, and the availability of previous data stored in the file system is not ensured.
  • Some non-JFS information concerning the logical volume cannot be found. When this occurs, the system uses default logical volume information to populate the ODM information. Thus, some output from the lslv command might be inconsistent with the real logical volume. If any logical volume copies still exist on the original disks, the information will not be correctly reflected in the ODM database. Use the rmlvcopy and mklvcopy commands to rebuild any logical volume copies and synchronize the ODM.