Database archived log storage

If you do not plan to use Tivoli® Storage Manager to maintain Db2® archived log files, you do not need to allocate space for the archived log files on archive storage. Allocate disk storage space for the archived log files instead.

The storage pool where Tivoli Storage Manager maintains the Db2 archived log files must contain enough storage to hold the log files that are needed to recover the database. There are many factors that you should consider when estimating the storage space needed to hold them:
  • How often do you load reports into the system?
  • How often do you add to or update the Content Manager OnDemand system tables (for users, groups, system printers, storage sets, application groups, applications, and folders? In addition, the System Log tables usually gets updated every time someone logs on or off the system, data is stored, queried, retrieved, and printed, and so forth.
  • Do you store application group data in table spaces?
  • What is the size of the database; the table spaces?
  • What is the frequency and type of database backups taken?
  • How long do you need to keep archived log files?
  • What is the compression ratio that the archive storage manager can achieve on archived log files?
If you take full backup images of the database on a regular schedule, such as once a day or once a week, It is recommended that you allocate two times the space that you have estimated for the active log files. However, you must allocate enough space to hold all of the archived log files created between full backups of the database. After a full database backup image is created, archived log files created prior to the backup are no longer needed and can be deleted. The following calculation can be used to estimate the amount of archive storage needed to hold archived log files:
Figure 1. Calculating database archived log file storage
     DB2ArchiveLogSpace = ( 2 * ActiveLogSpace )
                                * compression ratio
The following example illustrates the archive storage required to hold archived log files, when the space allocated for the active log files is 516 MB: 
     DB2ArchiveLogSpace =  ( 2 * 516 MB )
                                 *  .33  =  340 MB

Content Manager OnDemand requires approximately 340 MB of archive storage to hold the archived log files.

If you do not take full backup images of the database, it is recommended that you keep the archived log files indefinitely. Accordingly, you must carefully estimate the amount of archive storage that you will need. For example, a single archived log file requires approximately 1.3 MB of (uncompressed) storage space. Depending on the variables that listed above, you might need to maintain hundreds of archived log files in storage that is managed by Tivoli Storage Manager. It is recommended that you contact Tivoli Storage Manager and database specialists to help plan your storage requirements.

You might need to regularly initialize and load scratch storage volumes into the storage library where Tivoli Storage Manager maintains the archived log files. If Tivoli Storage Manager determines that there is not enough space available in the storage pool, it can request a mount for a scratch storage volume. However, the backup command cannot complete until the mount request is satisfied. If you operate in an unattended environment, this could have an adverse affect on system availability, especially when running an offline backup.