The quorum is one of the mechanisms that the LVM uses to ensure that a volume group is ready to use and contains the most up-to-date data.
A quorum is a vote of the number of Volume Group Descriptor Areas and Volume Group Status Areas (VGDA/VGSA) that are active. A quorum ensures data integrity of the VGDA/VGSA areas in the event of a disk failure. Each physical disk in a volume group has at least one VGDA/VGSA. When a volume group is created onto a single disk, it initially has two VGDA/VGSA areas residing on the disk. If a volume group consists of two disks, one disk still has two VGDA/VGSA areas, but the other disk has one VGDA/VGSA. When the volume group is made up of three or more disks, then each disk is allocated just one VGDA/VGSA.
A quorum is lost when enough disks and their VGDA/VGSA areas are unreachable so that a 51% majority of VGDA/VGSA areas no longer exists. In a two-disk volume group, if the disk with only one VGDA/VGSA is lost, a quorum still exists because two of the three VGDA/VGSA areas still are reachable. If the disk with two VGDA/VGSA areas is lost, this statement is no longer true. The more disks that make up a volume group, the lower the chances of quorum being lost when one disk fails.
When a quorum is lost, the volume group varies itself off so that the disks are no longer accessible by the LVM. This prevents further disk I/O to that volume group so that data is not lost or assumed to be written when physical problems occur. Additionally, as a result of the vary-off, the user is notified in the error log that a hardware error has occurred and service must be performed.
There are cases when it is desirable to continue operating the volume group even though a quorum is lost. In these cases, quorum checking can be turned off for the volume group. This type of volume group is referred to as a nonquorum volume group. The most common case for a nonquorum volume group occurs when the logical volumes have been mirrored. When a disk is lost, the data is not lost if a copy of the logical volume resides on a disk that is not disabled and can be accessed. However, there can be instances in nonquorum volume groups, mirrored or nonmirrored, when the data (including copies) resides on the disk or disks that have become unavailable. In those instances, the data might not be accessible even though the volume group continues to be varied on.
Friday, July 11, 2008
Volume groups
A volume group is a collection of 1 to 32 physical volumes of varying sizes and types.
A big volume group can have from 1 to 128 physical volumes. A scalable volume group can have up to 1024 physical volumes. A physical volume can belong to only one volume group per system; there can be up to 255 active volume groups.
When a physical volume is assigned to a volume group, the physical blocks of storage media on it are organized into physical partitions of a size you specify when you create the volume group. For more information.
When you install the system, one volume group (the root volume group, called rootvg) is automatically created that contains the base set of logical volumes required to start the system, as well as any other logical volumes you specify to the installation script. The rootvg includes paging space, the journal log, boot data, and dump storage, each in its own separate logical volume. The rootvg has attributes that differ from user-defined volume groups. For example, the rootvg cannot be imported or exported. When performing a command or procedure on the rootvg, you must be familiar with its unique characteristics.
You create a volume group with the mkvg command. You add a physical volume to a volume group with the extendvg command, make use of the changed size of a physical volume with the chvg command, and remove a physical volume from a volume group with the reducevg command. Some of the other commands that you use on volume groups include: list (lsvg), remove (exportvg), install (importvg), reorganize (reorgvg), synchronize (syncvg), make available for use (varyonvg), and make unavailable for use (varyoffvg).
Small systems might require only one volume group to contain all the physical volumes attached to the system. You might want to create separate volume groups, however, for security reasons, because each volume group can have its own security permissions. Separate volume groups also make maintenance easier because groups other than the one being serviced can remain active. Because the rootvg must always be online, it contains only the minimum number of physical volumes necessary for system operation.
You can move data from one physical volume to other physical volumes in the same volume group with the migratepv command. This command allows you to free a physical volume so it can be removed from the volume group. For example, you could move data from a physical volume that is to be replaced.
A volume group that is created with smaller physical and logical volume limits can be converted to a format which can hold more physical volumes and more logical volumes. This operation requires that there be enough free partitions on every physical volume in the volume group for the volume group descriptor area (VGDA) expansion. The number of free partitions required depends on the size of the current VGDA and the physical partition size. Because the VGDA resides on the edge of the disk and it requires contiguous space, the free partitions are required on the edge of the disk. If those partitions are allocated for a user's use, they are migrated to other free partitions on the same disk. The rest of the physical partitions are renumbered to reflect the loss of the partitions for VGDA usage. This renumbering changes the mappings of the logical to physical partitions in all the physical volumes of this volume group. If you have saved the mappings of the logical volumes for a potential recovery operation, generate the maps again after the completion of the conversion operation. Also, if the backup of the volume group is taken with map option and you plan to restore using those maps, the restore operation might fail because the partition number might no longer exist (due to reduction). It is recommended that backup is taken before the conversion and right after the conversion if the map option is used. Because the VGDA space has been increased substantially, every VGDA update operation (creating a logical volume, changing a logical volume, adding a physical volume, and so on) might take considerably longer to run.
A big volume group can have from 1 to 128 physical volumes. A scalable volume group can have up to 1024 physical volumes. A physical volume can belong to only one volume group per system; there can be up to 255 active volume groups.
When a physical volume is assigned to a volume group, the physical blocks of storage media on it are organized into physical partitions of a size you specify when you create the volume group. For more information.
When you install the system, one volume group (the root volume group, called rootvg) is automatically created that contains the base set of logical volumes required to start the system, as well as any other logical volumes you specify to the installation script. The rootvg includes paging space, the journal log, boot data, and dump storage, each in its own separate logical volume. The rootvg has attributes that differ from user-defined volume groups. For example, the rootvg cannot be imported or exported. When performing a command or procedure on the rootvg, you must be familiar with its unique characteristics.
You create a volume group with the mkvg command. You add a physical volume to a volume group with the extendvg command, make use of the changed size of a physical volume with the chvg command, and remove a physical volume from a volume group with the reducevg command. Some of the other commands that you use on volume groups include: list (lsvg), remove (exportvg), install (importvg), reorganize (reorgvg), synchronize (syncvg), make available for use (varyonvg), and make unavailable for use (varyoffvg).
Small systems might require only one volume group to contain all the physical volumes attached to the system. You might want to create separate volume groups, however, for security reasons, because each volume group can have its own security permissions. Separate volume groups also make maintenance easier because groups other than the one being serviced can remain active. Because the rootvg must always be online, it contains only the minimum number of physical volumes necessary for system operation.
You can move data from one physical volume to other physical volumes in the same volume group with the migratepv command. This command allows you to free a physical volume so it can be removed from the volume group. For example, you could move data from a physical volume that is to be replaced.
A volume group that is created with smaller physical and logical volume limits can be converted to a format which can hold more physical volumes and more logical volumes. This operation requires that there be enough free partitions on every physical volume in the volume group for the volume group descriptor area (VGDA) expansion. The number of free partitions required depends on the size of the current VGDA and the physical partition size. Because the VGDA resides on the edge of the disk and it requires contiguous space, the free partitions are required on the edge of the disk. If those partitions are allocated for a user's use, they are migrated to other free partitions on the same disk. The rest of the physical partitions are renumbered to reflect the loss of the partitions for VGDA usage. This renumbering changes the mappings of the logical to physical partitions in all the physical volumes of this volume group. If you have saved the mappings of the logical volumes for a potential recovery operation, generate the maps again after the completion of the conversion operation. Also, if the backup of the volume group is taken with map option and you plan to restore using those maps, the restore operation might fail because the partition number might no longer exist (due to reduction). It is recommended that backup is taken before the conversion and right after the conversion if the map option is used. Because the VGDA space has been increased substantially, every VGDA update operation (creating a logical volume, changing a logical volume, adding a physical volume, and so on) might take considerably longer to run.
Viewing BOS installation logs using SMIT
You can use the SMIT fast path to view some logs in the /var/adm/ras directory.
To view some logs in the /var/adm/ras directory, you can use the following SMIT fast path:
smit alog_show
The resulting list contains all logs that are viewable with the alog command. Select from the list by pressing the F4 key
Viewing BOS installation logs
Information saved in BOS installation log files might help you determine the cause of installation problems.
To view BOS installation log files, type cd /var/adm/ras and view the files in this directory. One example is the devinst.log, which is a text file that can be viewed with any text editor or paged
Viewing BOS installation logs with the alog command
You can use the alog command to view some logs in the /var/adm/ras directory.
To view some logs in the /var/adm/ras directory, type:
alog -o -f bosinstlog
To view some logs in the /var/adm/ras directory, you can use the following SMIT fast path:
smit alog_show
The resulting list contains all logs that are viewable with the alog command. Select from the list by pressing the F4 key
Viewing BOS installation logs
Information saved in BOS installation log files might help you determine the cause of installation problems.
To view BOS installation log files, type cd /var/adm/ras and view the files in this directory. One example is the devinst.log, which is a text file that can be viewed with any text editor or paged
Viewing BOS installation logs with the alog command
You can use the alog command to view some logs in the /var/adm/ras directory.
To view some logs in the /var/adm/ras directory, type:
alog -o -f bosinstlog
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