Backup eMTool and Roll-Forward Logging
Novell Cool Solutions: Feature
By Linda Kennard
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Posted: 29 Apr 2003
When you use the Backup eDirectory Management Tool (eMTool) that ships with Novell eDirectory 8.7, roll-forward logging is off by default. When roll-forward logging is off, it serves no purpose, but when it's on, it can cause problems. How can you use roll-forward logging successfully?
When roll-forward logging is off, Backup eMTool can restore an eDirectory database only up to the last backup file. For example, suppose you have three servers, and you schedule full backups on Friday nights and incremental backups on Monday through Thursday nights. Now suppose one of your servers crashes on Wednesday afternoon.
Without roll-forward logging, you can restore most -- but not all -- of your database. To do so, you first restore the latest full backup and then restore any incremental backups you've run since the last full backup. Unfortunately, this restores your server only up through Tuesday, the time of the last incremental backup. This doesn't restore changes to the database that were made on Wednesday. If the restored server isn't current with your other servers, you've got problems, particularly if this server shares a replica with one or both of your other servers.
This is where roll-forward logging comes into play. When turned on, roll-forward logging continuously records all modifications to the database. (You turn on and configure roll-forward logging by selecting Backup Configuration from the list of eDirectory Maintenance Tools in Novell iManager.) Backup eMTool refers to roll-forward logs to restore your eDirectory database right up to the moment a server crashes.
To return to the previous example, suppose you've already restored your full and incremental backups. The next step would be to replay the roll-forward log for this server. You decide which roll-forward log to replay by opening the last backup file in Notepad and checking out its machine-parsable, humanly-readable header.
This header includes a wealth of information about a backup file, including the type of backup (incremental or full), the server from which this backup was made, and the roll-forward log that was used when the backup was made. Based on this information, you can determine which log to replay. Backup eMTool can then restore all changes to the directory that occurred since the last incremental backup.
Problem and Solution
The problem with roll-forward logging is that when it's on, roll-forward logs fill up the volume storing them. When this volume is full, the eDirectory database stops transactions from occurring.
To avoid this problem, Novell engineer Brian Hawkins, who developed Backup eMTool, recommends that you set up a volume specifically for roll-forward logs. Doing so makes it easier for you to monitor the size of the roll-forward volume. To keep the volume in check, back up and delete the roll-forward logs in this volume every time you run a full backup.
Content for this passage was drawn from "Some Like it Hot: Backing Up Novell eDirectory 8.7," Linda Kennard and Brian Hawkins, Novell Connection, July 2002. You can read the complete article at http://www.novell.com/connectionmagazine/2002/07/backup72.pdf.
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