Online Relayout




	NOTE: You may need an additional license to use this feature.

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 SystemTM) 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 for more information.

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, Figure 1-27 “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.

Figure 1-27 Example of Decreasing the Number of Columns in a Volume

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 Figure 1-28 “Example of Relayout of a RAID-5 Volume to a Striped Volume” below. Note that removing parity (shown by the shaded area) decreases the overall storage space that the volume requires.

Figure 1-28 Example of Relayout of a RAID-5 Volume to a Striped Volume

Change a volume to a RAID-5 volume (add parity). See Figure 1-29 “Example of Relayout of a Concatenated Volume to a RAID-5 Volume” below. Note that adding parity (shown by the shaded area) increases the overall storage space that the volume requires.

Figure 1-29 Example of Relayout of a Concatenated Volume to a RAID-5 Volume

Change the number of columns in a volume. See Figure 1-30 “Example of Increasing the Number of Columns in a Volume” below. Note that the length of the columns is reduced to conserve the size of the volume.

Figure 1-30 Example of Increasing the Number of Columns in a Volume

Change the column stripe width in a volume. See Figure 1-31 “Example of Increasing the Stripe Width for the Columns in a Volume” below.

Figure 1-31 Example of Increasing the Stripe Width for the Columns in a Volume

For details of how to perform online relayout operations, see “Performing Online Relayout”.

Permitted Relayout Transformations

The tables below give details of the relayout operations that are possible for each type of source storage layout

Table 1-1 Supported Relayout Transformations for Unmirrored Concatenated Volumes

Relayout to	From concat
concat	No.
concat-mirror	No. Add a mirror, and then use vxassist convert instead.
mirror-concat	No. Add a mirror instead.
mirror-stripe	No. Use vxassist convert after relayout to striped-mirror volume instead.
raid5	Yes. The stripe width and number of columns may be defined.
stripe	Yes. The stripe width and number of columns may be defined.
stripe-mirror	Yes. The stripe width and number of columns may be defined.

Table 1-2 Supported Relayout Transformations for Layered Concatenated-Mirror Volumes

Relayout to	From concat-mirror
concat	No. Use vxassist convert, and then remove unwanted mirrors from the resulting mirrored-concatenated volume instead.
concat-mirror	No.
mirror-concat	No. Use vxassist convert instead.
mirror-stripe	No. Use vxassist convert after relayout to striped-mirror volume instead.
raid5	Yes.
stripe	Yes. This removes a mirror and adds striping. The stripe width and number of columns may be defined.
striped-mirror	Yes. The stripe width and number of columns may be defined.

Table 1-3 Supported Relayout Transformations for RAID-5 Volumes

Relayout to	From raid5
concat	Yes.
concat-mirror	Yes.
mirror-concat	No. Use vxassist convert after relayout to concatenated-mirror volume instead.
mirror-stripe	No. Use vxassist convert after relayout to striped-mirror volume instead.
raid5	Yes. The stripe width and number of columns may be changed.
stripe	Yes. The stripe width and number of columns may also be changed.
stripe-mirror	Yes. The stripe width and number of columns may also be changed.

Table 1-4 Supported Relayout Transformations for Mirrored-Concatenated Volumes

Relayout to	From mirror-concat
concat	No. Remove unwanted mirrors instead.
concat-mirror	No. Use vxassist convert instead.
mirror-concat	No.
mirror-stripe	No. Use vxassist convert after relayout to striped-mirror volume instead.
raid5	Yes. The stripe width and number of columns may be defined. Choose a plex in the existing mirrored volume on which to perform the relayout. The other plexes are removed at the end of the relayout operation.
stripe	Yes.
stripe-mirror	Yes.

Table 1-5 Supported Relayout Transformations for Mirrored-Stripe Volumes

Relayout to	From mirror-stripe
concat	Yes.
concat-mirror	Yes.
mirror-concat	No. Use vxassist convert after relayout to concatenated-mirror volume instead.
mirror-stripe	No. Use vxassist convert after relayout to striped-mirror volume instead.
raid5	Yes. The stripe width and number of columns may be changed.
stripe	Yes. The stripe width or number of columns must be changed.
stripe-mirror	Yes. The stripe width or number of columns must be changed. Otherwise, use vxassist convert.

Table 1-6 Supported Relayout Transformations for Unmirrored Stripe, and Layered Striped-Mirror Volumes

Relayout to	From stripe, or stripe-mirror
concat	Yes.
concat-mirror	Yes.
mirror-concat	No. Use vxassist convert after relayout to concatenated-mirror volume instead.
mirror-stripe	No. Use vxassist convert after relayout to striped-mirror volume instead.
raid5	Yes. The stripe width and number of columns may be changed.
stripe	Yes. The stripe width or number of columns must be changed.
stripe-mirror	Yes. The stripe width or number of columns must be changed.

Transformations are not supported for the following objects:

Log plexes.
Volume snapshot when there is an online relayout operation running on the volume.

Also note the following limitations:

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.
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.)

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”.

Online Relayout

Technical documentation

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How Online Relayout Works

Permitted Relayout Transformations

Transformation Characteristics

Transformations and Volume Length