VERITAS Volume Manager 4.1 Migration Guide

Converting LVM to VxVM
General Information Regarding Conversion Speed
Chapter 2 35
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.