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Roll-Back Recovery Roll-back recovery is another TurboIMAGE/XL recovery option. Roll-back recovery provides rapid recovery of database data integrity following a "soft" system crash (that is, system failure or loss of working memory). The roll-back feature is invoked through the DBRECOV utility and requires only the current database log files to restore data integrity. Note that ILR is not needed for roll-back recovery.
NOTE For roll-back recovery, the user log file and the database must be on the same volume set when logging to disk. When logging to tape, the database and the log file must be on the system volume set.
A database backup copy is not required for roll-back recovery. Regular backup of the database is recommended, however, and is always required for roll-forward recovery in the event of a more serious problem (for example, a disk head failure, or problems occurring while roll-back recovery is in progress). With roll-back enabled prior to a system failure, a record of each user transaction in the sequence of occurrence is available to determine which transactions were incomplete at the time of failure. When invoked, the roll-back recovery feature will "roll back," or undo, any incomplete database transactions in the log file following a soft system crash.
NOTE A database must be enabled for roll-back before roll-back recovery can be performed.
The following diagram illustrates the transactions of three different users at the time of a system failure:In the above illustration, the first user has completed one transaction (T1) and aborted another (T4) prior to the system failure. Users two and three have each completed two transactions, and each has one incomplete transaction at the time of failure. Individual database transactions T1, T2, T3, T5, and T6 were completed and are properly reflected in the database following system failure. Transactions T7 and T8, however, were incomplete at the time of system failure, causing an incomplete modification of data to be reflected in the database. These incomplete transactions (T7 and T8) are then rolled back (undone) to their beginning, returning all affected data in the database to their original state before T7 and T8 began execution. When transaction T4 is aborted, TurboIMAGE/XL completes the transaction by issuing an abnormal end (DBABEND). This transaction is then seen as completed by the roll-back feature and is not normally rolled back. If the aborted transaction is also to be rolled back, the following DBRECOV command must be issued before issuing the DBRECOV >RUN command (refer to the discussion of DBRECOV in chapter 8): >CONTROL NOABORTS The above command string causes the aborted transaction to be treated as an incomplete transaction during roll-back recovery. When >CONTROL NOABORTS is used, recovery must be performed at system startup time before anyone modifies the database. Refer to "Record Numbers" under the DBRECOV >CONTROL discussion in chapter 8 for considerations when using the >CONTROL command. Enabling the Roll-Back Feature To enable the roll-back feature complete the following sequence: 1. Set the logid and build a log file (if logging to disk) as shown in steps 2 through 6 of "Logging Preparation" earlier in this chapter. 2. For each particular database, disable access and enable the roll-back and recovery features by entering the following DBUTIL commands. You can also enable the MUSTRECOVER option if you wish to use it. >>DISABLE database name FOR ACCESS >>ENABLE database name FOR ROLLBACK,RECOVERY >>ENABLE database name FOR MUSTRECOVER Enable any other appropriate flags as discussed in step 7 under "Logging Preparation." If the logid was not set and/or the log file was not built before the >>ENABLE command is issued for roll-back recovery, a warning message that the logid or the log file is non-existent prints on the screen. Enable any other appropriate flags as discussed in step 7 under "Logging Preparation." Enabling for roll-back or MUSTRECOVER automatically enables the database for logging. 3. Make a backup copy of the database as discussed in step 8 under "Logging Preparation." 4. Start the logging process and enable user access to the appropriate databases as shown in "Logging Maintenance," earlier in this chapter. After the logid has been set and the log file built, DBUTIL automatically enables logging when roll-back is enabled. Note that ILR is not needed for roll-back recovery; however, if ILR is enabled together with roll-back recovery, logging and ILR must be disabled separately. When roll-back recovery is enabled, DBUTIL sets a roll-back flag in the root file to indicate that roll-back recovery is enabled for the database. DBUTIL also reserves six words in the root file for the roll-back time stamp (three words for the previous time stamp and three words for the current time stamp). The roll-back time stamp is updated and logged in the log file and in the root file when the database is first opened. Roll-back recovery then uses the time stamp during recovery to verify the correct log file for each database. Note that if the database and the log file are not on the same volume set, an error is issued.
CAUTION In the event of a system failure, do not restart user logging before running DBRECOV. Log records may have been lost due to the system failure. If user logging is resumed without a recovery, the resulting discontinuous log file would cause invalid results in the event of a subsequent recovery. To prevent access to the database after a system failure without recovery, enable the MUSTRECOVER feature when you enable the roll-back feature.
Disabling the Roll-Back Feature To disable the roll-back feature, make sure that no users are accessing the database involved. Disable the roll-back feature by entering the following DBUTIL command string: >>DISABLE database name FOR ROLLBACK When roll-back is disabled, DBUTIL resets the roll-back flag and roll-back time stamp, but not the logging flag; therefore, logging is still enabled.
CAUTION DO NOT DISABLE ROLL-BACK IF ROLL-BACK RECOVERY MUST BE USED LATER. Disabling roll-back resets the logging time stamp. After the logging time stamp is reset, roll-back recovery cannot be performed with the current log file on the named database. The data in the database is considered correct and therefore cannot be rolled back.
When the >>DISABLE command is issued, DBUTIL prompts a warning to remind you that the time stamp will be erased and prompts for a response as follows: WARNING: ROLLBACK time stamp will be erased. Please type Y to confirm your disable command>> If you enter Y, DBUTIL continues to disable roll-back. If Y is not entered, the >>DISABLE command is not performed. Performing Roll-Back Recovery To complete the transaction roll-back process following a system failure, perform the following steps: 1. Following a start recovery operation to bring up the system, locate the applicable log file media to be used for transaction roll-back. If logging to tape, the correct tape needs to be mounted. If using the MPE/iX CHANGELOG feature and there are multiple log file tapes, this will be the first tape of the series. If logging to disk, TurboIMAGE/XL automatically locates the first log file (for example, LOGF001) in the given logging cycle by checking the beginning of the root file for the logid. Note that the system boot-up process writes a crash record to the last volume of the user log set. If a start recovery operation is not performed, this will not occur, causing DBRECOV to issue a warning. 2. Make a database backup copy in case a system failure occurs during the roll-back recovery process. 3. Enter the following MPE/iX command string: :RUN DBRECOV.PUB.SYS 4. Enter the >CONTROL NOSTORE command of DBRECOV to allow recovery to proceed whether or not the DBSTORE flag is set. 5. Enter the following DBRECOV command (where database name is the name of individual databases to be rolled back): >ROLLBACK database name [,database name2,....,database nameN] 6. Enter all other desired DBRECOV commands (>FILE, >CONTROL with other options except NOSTORE shown above, and >PRINT). Note that the >FILE command's optional parameter rmode is not used with the roll-back feature. Refer to chapter 8 for more information. 7. If you want to recover the databases, enter the DBRECOV >RUN command. Otherwise, enter the >EXIT command, and DBRECOV terminates, and no recovery takes place. >RUN After the >RUN command is given, DBRECOV asks you to mount the log tape (if the log file medium is tape). Continue the roll-back process as directed by messages returned to both the console and the terminal screen. 8. If the CHANGELOG command or GETLOG AUTO option was used during logging and the logging file medium is tape, the following message appears on the terminal screen and the console: Reply CONtinue on console when logfile is ready When the required log file is available, enter the response CON to the console request from DBRECOV. Note that the response is in a form similar to REPLY for a tape mount used when storing or restoring tapes; that is, you need to enter the Process Identification Number (PIN). After the >RUN command is given, the DBRECOV program recovers the specified databases, creates specified user-recovery files, and terminates. The DBRECOV program could be terminated alternatively without any recovery taking place with an >EXIT command. For recovery to succeed, you must have access to the log file. This implies either knowing the maintenance word and having system manager (SM) or operator (OP) capability, or being the creator of the log identifier with read access to the log file if it resides in a different logon group and account. If the log file is on tape, you must know the volume identifier.
NOTE If the operator is unsure of the volume identifier, it is displayed on the console when the tape is mounted.
If the database creator and the creator of the log identifier are not the same, and if the disk log file and the database are in two separate accounts, follow the steps listed below for recovery to proceed: 1. Assign a maintenance word to the database. 2. Logon as the creator of the log identifier. 3. Fully qualify the database name when issuing the >ROLLBACK command. 4. Specify the maintenance word. MPE/iX Cleanup Mode and Roll-Back Recovery. In the event of a system failure and subsequent start recovery operation, MPE/iX attempts to clean up any user log files that were open at the time of the failure. The cleanup procedure involves writing any records left in the system log file disk buffer to the user log file. Note that for roll-back recovery without this cleanup, transactions are not lost during a start recovery operation because they are not held in the memory buffer. You should not cancel (from the console) this cleanup procedure if the log file is on tape. Canceling the start recovery operation for tape log files will cause rollback recovery to fail. The advantage of the cleanup procedure is that fewer user log records written just prior to the failure are lost. For tape files, the disadvantage is the time it takes for the tape to be rewound and sequentially scanned until the end-of-file is detected so that the remaining records can be appended to the end. The TurboIMAGE/XL recovery program DBRECOV does not require the cleanup to be performed. If it is not performed, however, DBRECOV most likely will report a sequence or checksum error when the discrepancy caused by the failure is encountered. This would cause DBRECOV to assume the end-of-file has been reached. DBRECOV Abort Message If DBRECOV aborts before recovery completes, the following information is printed: Abort occurred on database: dbname dbgroup dbaccount Total database open count: #open Current open count: #open Process user is: dbuser dbgroup dbaccount Running program: progname Log file name: logfilename Logging ID: logid Log file record number: nnnnnnnn Transaction type: xx Transaction date/time from log record: day, month, dd, year, time Last successful transaction #: nn First log record # of last successful transaction: nn Record the information, set the log file, locate the database store, and contact your HP support representative.
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