116 Storage Management with DB2 for OS/390
records into the input buffer used by the readingpr ocess (such as a recovery job
or a rollback).
From a performance point of view,it is always best for DB2 to obtain the log
recordsf rom theout put buffer.These accesses are reported by DB2 PM; see Fin
Figure 41on page 118. The next fastest access for DB2 is the active log; see Gin
Figure 41. Access to the archive log is not desirable; it can be delayed for a
considerable length of time.For example, tape drives may not be available, or a
tape mount can berequir ed.A zero value for Ain Figure 41 indicates that the
active logs are sized adequately.
10.5.1 Improving Log Read Performance
In this section we present some considerations on choices to improve log read
performance.
Active Log Size
Active logs shouldbe large enough to avoid reading the archives, especially
during restart, rollback, and recovery. When data is backed out, performance is
optimal if the data is available from the output buffer or from the active log. If the
data is no longeravailable from the active log, the active log is probably too
small. For information about sizing the active log data sets, see 10.5.2, “Active
Log Size” on page 117.
Log Input Buffer
The defaultsize for the input buffer is 60 KB. It is specified in the INPUT BUFFER
field of installation panel DSNTIPL(see Figure 40 on page 115). The default
value is recommended.
Avoid Device Contention
Avoiddevice contention on the log data sets. See the recommendation made in
10.4.5, “Improving Log Write Performance” on page 114.
Archive to Disk or Tape
If the archive log data set resides on disk, it can be shared by many log readers.
In contrast, an archive on tape cannot be shared among log readers. Although it
is always best to avoidreading ar chivesaltogether, if a process must read the
archive, that process is serialized with anyone else who must read the archive
tape volume.For example, every rollback that accesses the archive log must wait
for anyprevious rollback work that accesses the same archive tape volume to
complete.
Archiving to disk offers several advantages:
• Recovery times can be reduced by eliminating tape mounts and rewind
time for archive logs kept on tape.
• Multiple RECOVER utilities can be run in parallel.
• DB2 log data can span a greater length of timethan what is currently kept
in your active log data sets.
• Need for tape drives during DB2 archive log creation is eliminated. If DB 2
needs to obtain a tape drive on which to create the archive logs and it
cannot allocate one, all activitywill stop until DB2 can create the archive
log data sets.