VERITAS Volume Manager 5.0 Migration Guide (September 2006)

43Converting LVM to VxVM
General information regarding conversion speed
Selected Volume Groups have been restored.
Hit RETURN to continue.
Rollback other LVM Volume Groups? [y,n,q,?] (default: n)
General information regarding conversion speed
The speed of the process of converting an existing LVM volume group to a similar VxVM
disk group is largely dependent upon the size of the volume group being converted, as
well as on the complexity of the volumes within that volume group.
Factors affecting conversion speed include:
■ Size of volume groups. The larger the volume groups, the larger the LVM metadata
on each disk. A copy must be made of the LVM metadata for each physical disk.
Some areas are greater than 2MB; therefore, a 50-disk volume group requires 50
2MB reads and writes (i.e., 100 large I/Os) to complete.
■ Individual size of a logical volume in a volume group, and the complexity of the
logical volume layout. For example, for a system with 50 9GB drives, a simple 50GB
logical volume of the first 5 1/2 disks can be created. But a 50GB striped logical
volume that takes the first 1GB of all 50 disks can also be created. The first and
simple logical volume takes less time to convert than the striped volume. However,
for the striped volume, 50 disks need to be checked. Also, the complexity of
reproducing the VxVM commands to set up the striped volumes requires more
VxVM commands to be generated to represent more smaller sub-disks representing
the same amount of space.
Another factor in converting stripes is that stripes create more work for the converter.
In some cases, stripes require 1GB volume, although only the metadata is being
changed. In other cases, where there are more physical disks in one volume than
another, there is more metadata to deal with. The converter has to read every physical
extent map to ensure there are no holes in the volume; if holes are found, the
converter maps around them.
■ Number of volumes. While it takes longer to convert one 64GB volume than one
2GB volume, it also takes longer to convert 64 1GB volumes than one 64GB volume,
providing that the volumes are of similar type.
■ Mirrored volumes. Mirrored volumes typically do not take more time to convert than
simple volumes. Volumes that are mirrored and striped at the same time would take
longer, but LVM currently does not allow this.
Currently, after conversion, mirrored volumes are not automatically synchronized
because a large mirror could take hours to complete.
For example, in tests, a 150 GB volume group consisting of 20 simple logical
volumes takes approximately 35-40 minutes to convert. In contrast, the same volume
group (150 Gb) consisting of mirrored volumes that need to be synchronized can take
30-40 hours to convert.