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How online relayout works

Online relayout allows you to change the storage layouts that you have already created in place without disturbing data access. You can change the performance characteristics of a particular layout to suit your changed requirements. You can transform one layout to another by invoking a single command.

For example, if a striped layout with a 128KB stripe unit size is not providing optimal performance, you can use relayout to change the stripe unit size.

File systems mounted on the volumes do not need to be unmounted to achieve this transformation provided that the file system (such as Veritas File System) supports online shrink and grow operations.

Online relayout reuses the existing storage space and has space allocation policies to address the needs of the new layout. The layout transformation process converts a given volume to the destination layout by using minimal temporary space that is available in the disk group.

The transformation is done by moving one portion of data at a time in the source layout to the destination layout. Data is copied from the source volume to the temporary area, and data is removed from the source volume storage area in portions. The source volume storage area is then transformed to the new layout, and the data saved in the temporary area is written back to the new layout. This operation is repeated until all the storage and data in the source volume has been transformed to the new layout.

The default size of the temporary area used during the relayout depends on the size of the volume and the type of relayout. For volumes larger than 50MB, the amount of temporary space that is required is usually 10% of the size of the volume, from a minimum of 50MB up to a maximum of 1GB. For volumes smaller than 50MB, the temporary space required is the same as the size of the volume.

The following error message displays the number of blocks required if there is insufficient free space available in the disk group for the temporary area:

tmpsize too small to perform this relayout (nblks minimum required)

You can override the default size used for the temporary area by using the tmpsize attribute to vxassist.

See the vxassist(1M) manual page.

As well as the temporary area, space is required for a temporary intermediate volume when increasing the column length of a striped volume. The amount of space required is the difference between the column lengths of the target and source volumes. For example, 20GB of temporary additional space is required to relayout a 150GB striped volume with 5 columns of length 30GB as 3 columns of length 50GB. In some cases, the amount of temporary space that is required is relatively large. For example, a relayout of a 150GB striped volume with 5 columns as a concatenated volume (with effectively one column) requires 120GB of space for the intermediate volume.

Additional permanent disk space may be required for the destination volumes, depending on the type of relayout that you are performing. This may happen, for example, if you change the number of columns in a striped volume.

Example of decreasing the number of columns in a volume shows how decreasing the number of columns can require disks to be added to a volume.

Example of decreasing the number of columns in a volume

Example of decreasing the number of columns in a volume

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Note that the size of the volume remains the same but an extra disk is needed to extend one of the columns.

The following are examples of operations that you can perform using online relayout:

Example of relayout of a RAID-5 volume to a striped volume

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Note that removing parity decreases the overall storage space that the volume requires.

Example of relayout of a concatenated volume to a RAID-5 volume

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Note that adding parity increases the overall storage space that the volume requires.

Example of increasing the number of columns in a volume

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Note that the length of the columns is reduced to conserve the size of the volume.

Example of increasing the stripe width for the columns in a volume

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See "Performing online relayout" on page 278.

See "Permitted relayout transformations" on page 279.