Online Relayout

Online relayout allows you to convert between storage layouts in VxVM, with uninterrupted data access. Typically, you would do this to change the redundancy or performance characteristics of a volume. VxVM adds redundancy to storage either by duplicating the data (mirroring) or by adding parity (RAID-5). Performance characteristics of storage in VxVM can be changed by changing the striping parameters, which are the number of columns and the stripe width.

See Performing Online Relayout for details of how to perform online relayout of volumes in VxVM. Also see Converting Between Layered and Non-Layered Volumes for information about the additional volume conversion operations that are possible.

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^TM) 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:

You can override the default size used for the temporary area by using the tmpsize attribute to vxassist. See the vxassist(1M) manual page for more information.

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. The figure, 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. The size of the volume remains the same but an extra disk is needed to extend one of the columns.

Example of Decreasing the Number of Columns in a Volume

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

• Change a RAID-5 volume to a concatenated, striped, or layered volume (remove parity). See Example of Relayout of a RAID-5 Volume to a Striped Volume. Note that removing parity (shown by the shaded area) decreases the overall storage space that the volume requires.

Example of Relayout of a RAID-5 Volume to a Striped Volume



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• Change a volume to a RAID-5 volume (add parity). See Example of Relayout of a Concatenated Volume to a RAID-5 Volume. Note that adding parity (shown by the shaded area) increases the overall storage space that the volume requires.

Example of Relayout of a Concatenated Volume to a RAID-5 Volume



Click the thumbnail above to view full-sized image.

• Change the number of columns in a volume. See Example of Increasing the Number of Columns in a Volume. Note that the length of the columns is reduced to conserve the size of the volume.

Example of Increasing the Number of Columns in a Volume



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• Change the column stripe width in a volume. See Example of Increasing the Stripe Width for the Columns in a Volume.

Example of Increasing the Stripe Width for the Columns in a Volume



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For details of how to perform online relayout operations, see Performing Online Relayout. For information about the relayout transformations that are possible, see Permitted Relayout Transformations.

Limitations of Online Relayout

Note the following limitations of online relayout:

• Log plexes cannot be transformed.
• Volume snapshots cannot be taken when there is an online relayout operation running on the volume.
• Online relayout cannot create a non-layered mirrored volume in a single step. It always creates a layered mirrored volume even if you specify a non-layered mirrored layout, such as mirror-stripe or mirror-concat. Use the vxassist convert command to turn the layered mirrored volume that results from a relayout into a non-layered volume. See Converting Between Layered and Non-Layered Volumes for more information.
• Online relayout can be used only with volumes that have been created using the vxassist command or the VERITAS Enterprise Administrator (VEA).
• The usual restrictions apply for the minimum number of physical disks that are required to create the destination layout. For example, mirrored volumes require at least as many disks as mirrors, striped and RAID-5 volumes require at least as many disks as columns, and striped-mirror volumes require at least as many disks as columns multiplied by mirrors.
• To be eligible for layout transformation, the plexes in a mirrored volume must have identical stripe widths and numbers of columns. Relayout is not possible unless you make the layouts of the individual plexes identical.
• Online relayout involving RAID-5 volumes is not supported for shareable disk groups in a cluster environment.
• Online relayout cannot transform sparse plexes, nor can it make any plex sparse. (A sparse plex is not the same size as the volume, or has regions that are not mapped to any subdisk.)
• The number of mirrors in a mirrored volume cannot be changed using relayout.
• Only one relayout may be applied to a volume at a time.

Transformation Characteristics

Transformation of data from one layout to another involves rearrangement of data in the existing layout to the new layout. During the transformation, online relayout retains data redundancy by mirroring any temporary space used. Read and write access to data is not interrupted during the transformation.

Data is not corrupted if the system fails during a transformation. The transformation continues after the system is restored and both read and write access are maintained.

You can reverse the layout transformation process at any time, but the data may not be returned to the exact previous storage location. Any existing transformation in the volume must be stopped before doing a reversal.

You can determine the transformation direction by using the vxrelayout status volume command.

These transformations are protected against I/O failures if there is sufficient redundancy and space to move the data.

Transformations and Volume Length

Some layout transformations can cause the volume length to increase or decrease. If either of these conditions occurs, online relayout uses the vxresize(1M) command to shrink or grow a file system as described in Resizing a Volume.