The original advantage of a shared index database was that it saved a significant amount of main storage for buffers and some control blocks. However, when VSAM was enhanced with shared resources, the savings in storage became less significant. Now the possible disadvantages of using shared index databases generally outweigh the small amount of space they save.
Commands sometimes operate differently depending on whether they are issued for the first of the secondary indexes or for subsequent secondary indexes. The first secondary index is the first database name specified in the DBDUMP statement of the shared secondary index DBDGEN. This first database is the real database. Other secondary index databases are physically part of the real database but they are logically distinct.
The first column in Table 209 lists the issuing command, the second column lists where the command is issued, the third column lists the affects of the command that was issued, and the fourth column provides additional comments.
Issuing the Commands... | On the... | Affects... | Comments |
---|---|---|---|
/STOP /LOCK |
First secondary index | Only the named database | If no applications
are scheduled on any shared secondary indexes that cause the authorization
of the real database by DBRC, the commands
have the same effect as the /DBRECOVERY command on the first secondary
index.
When a /DISPLAY command is issued on the shared secondary index database, the subsequent secondary indexes are shown as stopped or locked only if the /STOP, /LOCK, or /DBRECOVERY command was issued. To undo the /STOP or /LOCK command, issue a /START or /UNLOCK command on the first secondary index. |
/STOP /LOCK |
Subsequent secondary indexes | Only the named database | To undo the /STOP or /LOCK command, issue a /START or /UNLOCK command on the named database. |
/DBDUMP | First secondary index | All databases sharing the secondary index data set | The /DBDUMP command
quiesces activity on all the indexes in the shared database. The
database is then closed and reopened for input only.
To undo the /DBDUMP command, issue a /START command on the first secondary index. |
/DBDUMP | Subsequent secondary indexes | Only the named database | The secondary index
is available for read only.
To undo the /DBDUMP command, issue a /START command on the named database. |
/DBRECOVERY | First secondary index | All databases sharing the secondary index data set | The /DBRECOVERY command
quiesces activity on all the indexes in the shared database. The
database is then closed and stopped.
When the DISPLAY command is issued on the shared secondary index database, the subsequent secondary indexes are shown as stopped or locked only if the /STOP, /LOCK, or /DBRECOVERY command was issued. To undo the /DBRECOVERY command, issue a /START command on the first secondary index. |
/DBRECOVERY | Subsequent secondary indexes | Only the named database | This command is the
same as the /STOP command for the named database. However, the /DBRECOVERY
command works immediately, but the /STOP command allows current
work to quiesce.
To undo the /DBRECOVERY command, issue a /START command on the named database. |