18 Storage Management with DB2 for OS/390
2. The failing BSDS must be redefined, or alternatively,an existing spare BSDS
copy must be renamed.
3. The BSDS is rebuilt from the good copywith an IDCAMS REPRO.
3.6.2 Active Logs
The activelog data sets are used for data recovery and to ensure data integrity in
case of software or hardwareerrors. DB2 uses active log data sets to record all
updates to user and system data.
The active log data sets are open as long as DB2 is active. Activelog data sets
arereus ed when thetotal active log space is used up, but only after the active log
(to be overlaid)has been copied to an archive log.
DB2 supports dual active logs. It iss tronglyrecommended to make use of dual
active logs forall DB2 production environments.
Sizing Active Logs
The amount of space dedicated to each individual active log data set is not
criticalfor the DB2 administrator. Traditionally,the active logs have been sized for
practical reasons,for example, to make best use of the archive log device (tape
cartridge or disk volume).
It is the overallsize of all active log data sets that is important for the DB2 DBA:
this size plays a critical role in the backup and recovery strategy.
The number ofactive log data sets, multiplied by the space of each activelog,
defines an amount of log information mostreadily available: the capacity of the
active log. Thiscapacity defines the time period that has the best recovery
performance and the highest data availability service level. The reason is that the
DB2 RECOVER utility generallyperforms better with an active log than with an
archivelog. S ee 10.5.2, “ActiveLog Size”on page 117 for more details.
Impact of Log Size on Backup and Recovery Strategy
The relationship betweenthe different types of log data sets is shown in Figure 5
on page 19. This figure shows a timeline that begins when a DB2 subsystem is
first started (Start Time) and proceeds until the currenttime (Current Time).
During this whole time, logdata has been generated; this is shown by the DB2
LOG bar.
The log data setshave limited capacity and cannot cover the total timeper iod.
The amount of DB2 log in the active log data sets (the active log capacity) is
shown as the time period from Time 2 to the Current Time. The oldest still
availablearchive log corresponds to Time 1. Because the whole log is not
available,recoveries are only possible throughout the period from Time 1 to
Current Time. The time period from Time 2 to Current Time corresponds to the
period with most efficient recoveriesbecause, generally, the active log is
allocated on fasterdevices. The archive log usually overlaps withthe active log
for am inimumof the last pair of active log data sets not yet archived up to some
time after Time 2 and before Current Time. If the data needed forRECOVER or
RESTARThas been archived, but is still availableon an active log data set not
yet reused, DB2 accesses the active log.
A good backup and recovery strategyconsiders: