Describes the transaction log files (TLOGs) used in IMS database Backward and Forward recovery.
TLOG File Properties
You specify the properties for TLOG files using the following environment variables:
- When set to 0 (zero), forces the flushing of TLOG buffers to disk on COMMIT only (default). When set to 1, forces the flushing
of TLOG buffers to disk on all database I/O.
- When set to 0 (zero), disables compression of the Backward Recovery TLOG file. When not compressed, the file contains only
the information required to recover databases. The default is enabled.
Note: When enabled,
Enterprise Server reclaims space in the file when information becomes obsolete.
- When set to 1, forces the frequent flushing of TLOG and database buffers to disk.
Warning: Depending upon the number of database updates, the frequency of commits, and other computer activity, use of ES_IMS_TLOG_FLUSH
could result in significant performance degradation.
Note: This environment variable is deprecated. Use ES_IMS_DB_TLOG_WRITETHRU to flush TLOG buffers and ES_IMS_DB_COMMIT_FLUSH to
flush database buffers.
- Maximum size of the IMS DB transaction logging file in bytes. The default is 4GB or h"FFF00000".
- The location of IMS DB transaction logging files. The default is the
Enterprise Server system directory.
Note: Once created, you can move log files, but do not rename them. Before attempting to recover a database, all log files must
reside in the directory specified by ES_IMS_TLOG_PATH.
- Threshold size of the IMS DB transaction logging file. When reached, the current TLOG file is closed and a new TLOG file
created for subsequent transactions. The default is 2GB or h"7FFFFFFF".
TLOG File(s) used to Recover Databases
The actual TLOG file or files used to recover an IMS database depends on whether or not Forward recovery is enabled or disabled.
- Forward Recovery Enabled
- For both Forward and Backward recovery, use the most recent Forward recovery TLOG file.
Note: TLOG files created when Forward Recovery is enabled are not backed up because they are never overwritten.
- Forward Recovery Disabled
- For Backward recovery, use the default log file,
IMSDB.TLOG, which is always backed up before a new one is created.
TLOG Naming Convention
With the exception of the
IMSDB.TLOG file created and used when Forward recovery is disabled,
Enterprise Server generates and renames TLOG files using this naming convention:
serverName is the name of the server on which the database is stored, and
YYMMDDhhmmssmm represents two digits for each: year, month, day, hour, minute, second, and millisecond.
Depending on the size of the database and the volume of transactions, recovery log files can become quite large. To better
manage large-volume logs,
Enterprise Server occasionally creates extension logs. The name of an extension log file is the same as the originating log file, plus an appended
extension number such as 001, 002, etc.
During a recovery process,
Enterprise Server uses log file time stamps to determine which log file or files are required for the restoration, and excludes out-of-date
log files from the recovery. This minimizes the time required to complete the recovery.
TLOG File Contents
You can view the contents of a TLOG file using the
cobfhtlog utility as follows:
- Start an
Enterprise Developer command prompt. If you need instructions, see
To start an
Enterprise Developer command prompt.
- Change to the directory specified as your ES_IMS_TLOG_PATH environment variable.
filename is the name of a TLOG file.
The contents of the file are written to the screen. The following example represents a snippet of the contents of a TLOG file:
data len: 00019
data : 100000006 00000006
sequence no: 00000000000000000006
date time: 17081415252517
sequence no: 00000000000000000007
date time: 17081415252523
sequence no: 00000000000000000008
date time: 17081415252523
Note: In the above example, the
date time value for
commit comp represents a point in time you can use with the Recovery Utility and Recovery function PITR parameter. See
Recovery function and
Recovery Utility for details.