IBM Tivoli Storage Manager, Version 7.1

How the server selects volumes with collocation enabled

Volume selection depends on whether collocation is by group, by node, or by file space.

Table 1 shows how the Tivoli® Storage Manager server selects the first volume when collocation is enabled for a storage pool at the client-node, collocation group, and file-space level.

Table 1. How the server selects volumes when collocation is enabled
Volume Selection Order	When collocation is by group	When collocation is by node	When collocation is by file space
1	A volume that already contains files from the collocation group to which the client belongs	A volume that already contains files from the same client node	A volume that already contains files from the same file space of that client node
2	An empty predefined volume	An empty predefined volume	An empty predefined volume
3	An empty scratch volume	An empty scratch volume	An empty scratch volume
4	A volume with the most available free space among volumes that already contain data	A volume with the most available free space among volumes that already contain data	A volume that contains data from the same client node
5	Not applicable	Not applicable	A volume with the most available free space among volumes that already contain data

When the server must continue to store data on a second volume, it uses the following selection order to acquire more space:

An empty predefined volume
An empty scratch volume
A volume with the most available free space among volumes that already contain data
Any available volume in the storage pool

When collocation is by client node or file space, the server tries to provide the best use of individual volumes and minimizes file mixing from different clients or file spaces on volumes. This configuration is depicted in Figure 1, which shows that volume selection is horizontal, where all available volumes are used before all available space on each volume is used. A, B, C, and D represent files from four different client nodes.

Remember:

If collocation is by node and the node has multiple file spaces, the server does not attempt to collocate those file spaces.
If collocation is by file space and a node has multiple file spaces, the server attempts to put data for different file spaces on different volumes.

Figure 1. Using all available sequential access storage volumes with collocation enabled at the node or file space level

All the files from node A are on single volume. That volume holds only the files from node A, and the volume is partially empty. The files from node B are on another volume, and that volume is partially empty. The files from node C are on a third volume, which is partially empty. The files from node D fill two volumes.

Collocation can be by filespace group or node group. When collocation is by node group (node collocation group), the server tries to collocate data from nodes that belong to the same collocation group. A filespace collocation group uses the same methods as a node collocation group, but can use up more space because of the granularity of filespace sizes. As shown in Figure 2, data for the following groups of nodes was collocated:

Group 1 consists of nodes A, B, and C
Group 2 consists of nodes D and E
Group 3 consists of nodes F, G, H, and I

Whenever possible, the Tivoli Storage Manager server collocates data that belongs to a group of nodes on a single tape, as represented by Group 2 in the figure. Data for a single node can also be spread across several tapes that are associated with a group (Group 1 and 2). If the nodes in the collocation group have multiple file spaces, the server does not attempt to collocate those file spaces.

Figure 2. Using all available sequential access storage volumes with collocation enabled at the group level

Data from nodes A, B, and C is stored on two volumes, one of which is partially empty. The data from nodes D and E is stored in another volume, which is partially empty. Data from nodes F, G, H, and I are stored on two other volumes, one of which is partially empty.

Remember: Normally, the Tivoli Storage Manager server always writes data to the current filling volume for the operation that is running. Occasionally, however, you might notice more than one filling volume in a collocated storage pool. Having more than one filling volume in a collocated storage pool can occur if different server processes or client sessions try to store data into the collocated pool at the same time. In this situation, Tivoli Storage Manager allocates a volume for each process or session that needs a volume so that both operations complete as quickly as possible.

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