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.
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