Transaction log files TA-LOG

The transaction log files (the TA-LOG files) are DBH-specific files in which the SESAM/SQL DBH logs information that is important in the event of a system failure and makes it possible to roll back transactions (see section “Transaction logging” ). Specifically, the DBH logs information in the TA-LOG files that guarantees the physical and logical consistency of the data in the database.

This includes:

• after images, i.e. blocks, or rows or parts of rows in a block which have been created through modifications to data

• logical before images, i.e. parts of rows within a database block as they were prior to the first update

• consistency points set by the DBH at each “end transaction” statement.

The DBH creates two TA-LOG files for each DBH session with enabled transaction logging. The TA-LOG files are only ever not created if the system administrator has disabled transaction logging with the relevant DBH option. Storage information pertaining to the TA-LOG files is stored in the DBH's media catalog.

The TA-LOG files use the following standard names:

DBH variant	Standard name	Link name
independent DBH	SESAMcn.TA-LOG1 SESAMcn.TA-LOG2	TALOG1 TALOG2
linked-in DBH	SESLKcn.TA-LOG1 SESLKcn.TA-LOG2	TALOG1 TALOG2

Table 53: Standard names of the TA-LOG files

Using the RECOVER utility function also results in the “SESLKcnst.TA-LOG1” and “SESLKcnst.TA-LOG2” file being created.

Additional information on naming conventions, the default assignment and the access method associated with the TA-LOG files is provided in the table “DBH-specific files”.

TA-LOG files are I/O-intensive; in exceptional cases I/O bottlenecks can occur. TA-LOG files should therefore be located on a separate device which is as fast as possible.

The size of the write unit for the TA-LOG files depends on the disk type. SESAM/SQL uses the maximum possible input/output length (64 to 160 KB). Information on the maximum input/output length in half pages (2KB) can be obtained using the BS2000 command

/SHOW-PUBSET-CONFIGURATION PUBSET=<catid>,INFORMATION=*PUBSET-FEATURES.

A separate transaction log file exists for SQL statements that define and administer schemas and administer storage structures. Its default name is:
:catid:userid.catalogname.spacename.DDLTA.

The file is a log file which is assigned to the space on which the SQL statement operates. It contains the physical before images of the above SQL statements. The file is deleted at the end of the transaction if it was not created b y the user.