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Planning Volume 2, Control Workstation and Software Environment

Planning for Virtual Shared Disk Communications

When you define virtual shared disks, you specify the SP Switch or other connection method. See the book PSSP: Administration Guide for detailed node and disk connection information. See the |relevant edition of the AIX book System Management Guide: Operating System and Devices to read about the Logical Volume Manager component if you are not already familiar with it.

By design of the Logical Volume Manager, each logical partition maps to one physical partition and each physical partition maps to a number of disk sectors. The Logical Volume Manager limits the number of physical partitions that it can track per disk to 1016. In most cases, not all the 1016 tracking partitions are used by a disk. The default size of each physical partition during a mkvg command is 4 MB, which implies that individual disks up to 4 GB can be included into a volume group.

If a disk larger than 4 GB is added to a volume group (based on usage of the default 4 MB size for the physical partition), the disk addition fails. The warning message provided will be: The physical partition size ofnumber A requires the creation of number B partitions for hdiskX.

The system limitation is 1016 physical partitions per disk. Specify a larger physical partition size in order to create a volume group on this disk. Note that the size of the partition determines the granularity by which logical volumes (and file systems) could be increased in size in a given volume group definition. Moreover, this setting could not be overridden once a volume group is defined. If you intend to dedicate DASD for a database with large tables on external DASD, you should consider using a large partition size.

There are two instances where this limitation is enforced:

  1. You try to use mkvg to create a volume group and the number of physical partitions on a disk in the volume group exceeds 1016. A work-around to this limitation is to select from the physical partition size ranges of: 1, 2, (4), 8, 16, 32, 64, 128, 256 Megabytes and use the mkvg -s option.
  2. The disk that violates the 1016 limitation attempts to join an existing volume group with the extendvg command.

You can recreate the volume group with a larger partition size allowing the new disk to work or create a stand-alone volume group consisting of a larger physical size for the new disk. If the install code detects that the rootvg drive is larger than 4 GB, it will change the mkvg -s value until the entire disk capacity can be mapped to the available 1016 tracks. This install change also implies that all other disks added to rootvg, regardless of size, will also be defined at that physical partition size. For RAID systems, the /dev/hdiskX name used by the Logical Volume Manager might really consist of many non-4 GB disks. In that case, the 1016 requirement still exists. Logical Volume Manager is not aware of the size of the individual disks that really make up /dev/hdiskX. Logical Volume Manager bases the 1016 limitation on the AIX-recognized size of /dev/hdiskX, and not the real physical disks that make up /dev/hdiskX.

In some instances, you will experience a problem adding a new disk to an existing volume group or in creating a new volume group. The warning message provided by Logical Volume Manager will be: Not enough descriptor area space left in this volume group.

Either try adding a smaller PV or use another volume group. On every disk in a volume group, there exists an area called the volume group descriptor area (VGDA). This space allows you to take a volume group to another AIX system and importvg the volume group into the AIX system. The VGDA contains the names of disks that make up the volume group, their physical sizes, partition mapping, logical volumes that exist in the volume group, and other pertinent LVM management information. When you create a volume group, the mkvg command defaults to allowing the new volume group to have a maximum of 32 disks in a volume group. However, as bigger disks have become more prevalent, this 32 disk limit is usually not achieved because the space in the VGDA is used up faster, as it accounts for the capacity on the bigger disks. This maximum VGDA space, for 32 disks, is a fixed size which is part of the LVM design. Large disks require more management mapping space in the VGDA, causing the number and size of available disks to be added to the existing volume group to shrink. When a disk is added to a volume group, not only does the new disk get a copy of the updated VGDA, but all existing drives in the volume group must be able to accept the new, updated VGDA. The exception to this description of the maximum VGDA is rootvg. In order to provide users more free disk space, when rootvg is created, mkvg does not use the maximum limit of 32 disks that are allowed into a volume group. Instead, the number of disks picked in the AIX install menu is used as the reference number by mkvg -d during the creation of rootvg. This limit does not prohibit you from adding more disks to rootvg during post-install. The amount of free space left in a VGDA, and the number size of the disks added to a volume group, depends on the size and number of disks already defined for a volume group. If you require more VGDA space in the rootvg, then use the mksysb and migratepv commands to reconstruct and reorganize your rootvg (the only way to change the -d limitation is recreation of a volume group).

IBM suggests that you do not place user data onto rootvg disks. This separation provides an extra degree of system integrity.

The logical volume control block (LVCB) is the first 512 bytes of a logical volume. This area holds important information such as the creation date of the logical volume, information about mirrored copies, and possible mount points in the journaled file system (JFS). Certain Logical Volume Manager commands are required to update the LVCB, as part of the algorithms in Logical Volume Manager. 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 the following warning message is issued: Warning, cannot write lv control block data

Most of the time, this is a result of database programs accessing raw logical volumes (and bypassing the JFS) as storage media. When this occurs, the information for the database is literally written over the LVCB. Although this might seem fatal, it is not the case. Once the LVCB is overwritten, you can still do the following:

There are limitations to deleting LVCBs. The logical volumes with deleted LVCB's face possible, incomplete importation into other AIX systems. During an importvg, the Logical Volume Manager command scans the LVCB's of all defined logical volumes in a volume group for information concerning the logical volumes. If the LVCB is deleted, the imported volume group will still define the logical volume to the new AIX system, which, is accessing this volume group, and you can still access the raw logical volume. However, any journaled file system information is lost and the associated mount point will not be imported into the new AIX system. You must create new mount points and the availability of previous data stored in the file system is not assured. Also, during this import of logical volume with an erased LVCB, some non-JFS information concerning the logical volume, which is displayed by the lslv command, cannot be found. When this occurs, the system uses default logical volume information to populate the logical volume's ODM information. Therefore, some output from lslv will 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 rmlvcopy and mklvcopy commands to rebuild any logical volume copies and synchronize the ODM.

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