VERITAS Volume Manager 3.5 Migration Guide (September 2004)

Chapter 2

Converting LVM to VxVM

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.

NOTE If you convert mirrored volumes, you must synchronize them in a separate step.