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Don Burleson Blog 







Using the "alter system dump" command

Oracle Database Tips by Donald BurlesonFebruary 25, 2015


Question:  I'm trying to dump a data block from Oracle using this command:  "Alter system dump datafile 15 block min 987600 block max 551016715;"

Is there any performance effect if you issue this statement against a production database?

How do I find and interpret the trace file?  I cannot find the trace file in my dump directory.

Answer:  First, why would you need to look at the internals of an Oracle data block?  Are your trained to understand the structures, you need to have the top secret internals documentation (the data sections or DESECT's),  maps for the proprietary internal structures that make Oracle special.  Oracle corporation does not want anybody to understand how their competitive secrets work, and in 99% of tuning cases, block internals do not matter.  Dumping data blocks is ONLY for Oracle Technical Support and DBA's with at least a decade of full-time experience, IMHO.

>> Is there any performance effect if you issue this statement against a production database?

There can be!  If you are copying a datafile while it competes for access on the same disk, you will get thrashing of the read-write heads, and very slow access speeds.

>> How do I find and interpret the trace file? 

You will always see a message in the alert log, giving the command, date and UDUMP file location.  The trace file should always be in $ORACLE_HOME/admin/udump, and you can do an "ls -alt|head" command to find it quickly.

If you are familiar with the unpublished and undocumented Oracle block DESECT's, then you can use online tools (such as BBED), in conjunction with Oracle Technical Support to view and alter Oracle data blocks. Also, you can view block contents directly from the OS with third-party tools and editors. For example, you can use the strings command to see printable characters with an Oracle data file (.dbf).

Note:  Using BBED will make your database unsupported, unless it is used as part of a Service Request (SR).  Unless you have picked-up the block internals over many years of experience, ALWAYS Open an SR when dumping Oracle data blocks.

If you like Oracle tuning, you might enjoy my book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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