Collocation is a process in which the server attempts to keep files belonging to a single client node or to a single file space of a client node on a minimal number of sequential access storage volumes. You can set collocation for each sequential access storage pool when you define or update the pool.
To have ADSM collocate data in a storage pool by client node, set collocation to YES. To have ADSM collocate data in a storage pool by client file space, set collocation to FILESPACE. By using collocation, you reduce the number of volume mount operations required when users restore, retrieve, or recall many files from the storage pool. Collocation thus improves access time for these operations. Figure 18 shows an example of collocation by client node with three clients, each having a separate volume containing that client's data.
Figure 18. Example of Collocation Enabled
When collocation is disabled, the server attempts to use all available space on each volume before selecting a new volume. While this process provides better utilization of individual volumes, user files can become scattered across many volumes. Figure 19 shows an example of collocation disabled, with three clients sharing space on a volume.
Figure 19. Example of Collocation Disabled
With collocation disabled, when users want to restore, retrieve, or recall a large number of files, media mount operators may be required to mount more volumes to recover user data. The system default is to not use collocation.
Table 17 shows the effects of collocation on operations.
Table 17. Effect of Collocation on Operations
| Operation | Collocation Enabled | Collocation Disabled |
|---|---|---|
| Backing up, archiving, or migrating client files | More media mounts to collocate files. | Usually fewer media mounts are required. |
| Restoring, retrieving or recalling client files | Large numbers of files can be restored, retrieved, or recalled more quickly because files are located on fewer volumes. | Multiple mounts of media may be required for a single user because files
may be spread across multiple volumes.
More than one user's files can be stored on the same sequential access storage volume. For example, if two users attempt to recover a file that resides on the same volume, the second user will be forced to wait until the first user's files are recovered. |
| Storing data on tape | The server attempts to use all available tape volumes to separate user files before it uses all available space on every tape volume. | The server attempts to use all available space on each tape volume before using another tape volume. |
| Media mount operations | More mount operations when user files are backed up, archived, or
migrated from client nodes directly to sequential access volumes.
More mount operations during reclamation and storage pool migration. More volumes to handle because volumes are not fully used. | More mount operations required during restore, retrieve, and recall of client files. |
| Tip: | If you use collocation, but want to reduce the number of media mounts and use
space on sequential volumes more efficiently, you can do the following:
|
When collocation at the client node level is enabled for a storage pool (COLLOCATION=YES) and a client node backs up, archives, or migrates files to the storage pool, the server attempts to select a volume using the following selection order:
When collocation at the file space level is enabled for a storage pool (COLLOCATION=FILESPACE) and a client node backs up, archives, or migrates files to the storage pool, the server attempts to select a volume using the following selection order:
When the server needs to continue to store data on a second volume, it uses the following selection order to acquire additional space:
Through this selection process, the server attempts to provide the best use of individual volumes while minimizing the mixing of files from different clients or file spaces on volumes. For example, Figure 20 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.
Figure 20. Using All Available Sequential Access Storage Volumes with Collocation Enabled
When collocation is disabled, the server attempts to use all available space in a storage volume before it accesses another volume. When storing client files in a sequential access storage pool where collocation is disabled, the server selects a volume using the following selection order:
When the server needs to continue to store data on a second volume, it attempts to select an empty volume. If none exists, the server attempts to select any remaining available volume in the storage pool.
Figure 21 shows that volume utilization is vertical when collocation is disabled. In this example, fewer volumes are used because the server attempts to use all available space by mixing client files on individual volumes.
Figure 21. Using All Available Space on Sequential Volumes with Collocation Disabled
After you define a storage pool, you can change the collocation setting by updating the storage pool. The change in collocation for the pool does not affect files that are already stored in the pool.
For example, if collocation had been off for a storage pool and you turn it on, from then on client files stored in the pool are collocated. Files that had previously been stored in the pool are not moved to collocate them. As volumes are reclaimed, however, the data in the pool tends to become more collocated. You can also use the MOVE DATA command to move data to new volumes to increase collocation, if you are able to afford the processing time and volume mount activity this would cause.
There are special considerations when using collocation on copy storage pools. Primary and copy storage pools perform different recovery roles. Normally you use primary pools to recover data to clients directly, and you use copy storage pools to recover data to the primary pools. In a disaster where both clients and the server are lost, the copy storage pool volumes will probably be used directly to recover clients. The types of recovery scenarios that are of most concern to you will help to determine whether to use collocation on your copy storage pools.
Another consideration is that collocation on copy storage pools will result in more partially filled volumes and potentially unnecessary offsite reclamation activity.
Collocation typically results in a partially filled sequential volume for each client or client file space. This may be acceptable for primary storage pools because these partially filled volumes remain available and can be filled during the next migration process. However, for copy storage pools this may be unacceptable because the storage pool backups are usually made to be taken offsite immediately. If you use collocation for copy storage pools, you will have to decide between:
or
With collocation disabled for a copy storage pool, typically there will be only a few partially filled volumes after storage pool backups to the copy storage pool are complete.
Consider carefully before using collocation for copy storage pools. Even if you use collocation for your primary storage pools, you may wish to disable collocation for copy storage pools. Collocation on copy storage pools may be desirable when you have few clients, but each of them has large amounts of incremental backup data each day.
See "Collocation on Sequential Access Storage Pools" for more information about collocation.