Veritas™ Volume Manager Administrator's Guide

Concatenation, spanning, and carving

Concatenation maps data in a linear manner onto one or more subdisks in a plex. To access all of the data in a concatenated plex sequentially, data is first accessed in the first subdisk from the beginning to the end. Data is then accessed in the remaining subdisks sequentially from the beginning to the end of each subdisk, until the end of the last subdisk.

The subdisks in a concatenated plex do not have to be physically contiguous and can belong to more than one VM disk. Concatenation using subdisks that reside on more than one VM disk is called spanning.

Figure: Example of concatenation shows the concatenation of two subdisks from the same VM disk.

If a single LUN or disk is split into multiple subdisks, and each subdisk belongs to a unique volume, it is called carving.

Figure: Example of concatenation

The blocks n, n+1, n+2 and n+3 (numbered relative to the start of the plex) are contiguous on the plex, but actually come from two distinct subdisks on the same physical disk.

The remaining free space in the subdisk, disk01-02, on VM disk, disk01, can be put to other uses.

You can use concatenation with multiple subdisks when there is insufficient contiguous space for the plex on any one disk. This form of concatenation can be used for load balancing between disks, and for head movement optimization on a particular disk.

Figure: Example of spanning shows data spread over two subdisks in a spanned plex.

Figure: Example of spanning

The blocks n, n+1, n+2 and n+3 (numbered relative to the start of the plex) are contiguous on the plex, but actually come from two distinct subdisks from two distinct physical disks.

The remaining free space in the subdisk disk02-02 on VM disk disk02 can be put to other uses.