Data loss presents serious challenges for Oracle users, and while recovery rates are generally high for Oracle systems, you need to quickly identify corruption in order to avoid permanent losses. To ensure high system availability, you should be familiar with the basic symptoms of corruption and have a plan in place to deal with data loss when it occurs.
Logical Block Corruption
Corruption is a major problem for databases, and while Oracle has built-in tools to protect from some common issues, data loss can still occur if best practices are not in use. Input/output systems often introduce errors that prevent Oracle from finding the information that it needs to open and access datafiles; the system presents a partial version of the affected file as a result. Corruption often occurs on older systems as media begins to degrade and as operating environments age.
When block corruption becomes problematic, you may notice dropped information or unreadable datafiles. As failures can occur at any time, you should always keep an up-to-date backup of your databases and follow the best practices outlined by Oracle.
Tools for Identifying and Treating Block Corruption
There are a number of built-in tools to help you identify and treat block issues with Oracle databases. Effectively using these programs will greatly limit your chances of permanent data loss.
Programs like DBVerify allow users to analyze Oracle databases to find file corruption. DBVerify looks for corrupted blocks and can be executed at the operating system level. You can also use dbms_repair.fix_corrupt_object to mark corrupted blocks, which will allow you to make an appropriate treatment decision.
Oracle's Data Recovery Advisor is an especially user-friendly tool that analyzes datafiles to find data failures. It is useful if you find a significant level of corruption, and its GUI interface provides a simple way to treat common issues.
Many system administrators couple Data Recovery Advisor with Oracle Recovery Manager (RMAN) to treat common block corruption issues. RMAN allows you to set corruption thresholds and look for logical corruption in individual datafiles. You can then display the resulting corruption and compare it with your backup set to make a disaster recovery decision.
To recover individual blocks or multiple blocks, you can use the rman> blockrecover command. Alternately, you can use the user-friendly Data Recovery Adviser to treat localized issues. However, as with all logical tools, Data Recovery Advisor and RMAN cannot treat physical media problems. All recovery programs are somewhat limited in this respect, and excessive block corruption may prevent normal functionality.
Physical Media Issues
Physical media issues can contribute to corruption reports, so when error checks result in a large number of damaged blocks, you should check to make sure that your media is healthy. Unfortunately, disk check utilities can contribute to file corruption when used improperly, so exercise care when checking the health of a RAID or other device. When in doubt, back up your Oracle databases before making any changes.
Remember that failed attempts to treat corrupt files may contribute to damage. Use Oracle Secure Backup or another backup program to keep an up-to-date copy of important datafiles and always back up your databases before attempting to treat file corruption. Get help from a professional data recovery provider if your initial repair attempt fails. Most damaged Oracle databases are completely treatable, but your chances of success are much higher if you do not take any steps that could contribute to block corruption.