Here I traced the session and got the following lines in the trace file. Here you can see that only UNDO blocks are read and there is no request for data block. Is this due to commit clean out? We kill the session and rerun the job then it goes through. To avoid this commit clean out we need to find which process did modify the blocks in the node3 so that it can be run on the other nodes.
We are not able to find it. For commit cleanouts:- undo header blocks transaction table entries and undo blocks associated with the transaction table control block changes must be read. The transaction table in the undo header block will indicate the transaction state. But, if the transaction table slot is overwritten for that transaction and so commit SCN is not immediately available , then Database code will follow undo chain of the transaction table control blocks to create a prior version of transaction table recursively, in a quest to find commit SCN of the transaction.
For consistent reads:- undo chain of a transaction a transaction that is either active or commit SCN of that transaction is greater than query SCN must be followed for consistent read buffer generation.
Stats names can be little different between versions Check the session level stats to identify that:. As an additional check, review the block contents by dumping the blocks. Check if the undo records are indeed application table changes.
You may have to dump the blocks to see contents. But, this can negatively affect commit cleanouts since the code is seemingly trying to find exact commit SCN by rolling back the transaction table header changes numerous times. Meaning, few processes have updated objects heavily in one node,and sessions in other nodes are reading the blocks, but can not use the blocks since there is a pending transaction or that commit SCN is later then query env SCN , and so rolling back the transactions to create a Consistent Read copies.
You could solve this problem with two approaches. If node 3 is modifying blocks heavily by a either schedule those intensive updates during less active period, so that, you can avoid these CR storm b Modify the application affinity such a way that programs modifying the objects aggressively and programs reading the blocks aggressively are executed in the same node.
Hello Riyaj, Thanks a lot for your wonderful explanation. I will run the query provided by you to check session level stats when this problem occurs again. This will confirm if waits are for data block consistent reads or for transaction table consistent reads. I will also dump the blocks next time when this issue occurs again I hope it is better to dump the data block here UNDO block when this issue is happening.
Maybe typos? Step 3 buffer cache flushing was actually done in node2 not node1 and step4 is re-reading block in node 2 not node 1 … or am I misunderstanding the sequence? There is some typo. Node 1 and node 2 got interchanged while explaining the sequence. I got the same doubt and was searching for the comment pointing it out. You are commenting using your WordPress. You are commenting using your Google account.
You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Click image for details:. SCN — What, why, and how? Then the sequence of operation is simplified : Foreground session calculates the master node of the block; Requests a LMS process running in the master node to access the block.
If the block is in a consistent state meaning block version SCN is lower or equal? Since the block has uncommitted changes, LMS process can not send the block immediately. Then the CR block is sent to the foreground process. LMS is a light weight process Global cache operations must complete quickly, in the order of milli-seconds, to maintain the overall performance of RAC database.
Why do we need this new event? Test case Of course, a test case would be nice, Just any regular table will do and my table structure have just two columns number, varchar2 with rows or so. When I reread the block again in node 1, here is an approximate sequence of operations that occurred: For SELECT statement in step 4, foreground sent a block request to LMS process in node2; LMS process in node 2 did not find the block in the buffer cache since we flushed the buffer cache.
On the other hand, if your hard drive has only one head, then it really is bouncing back and forth like crazy. On the Disk Offset graph, flushes are marked with vertical red lines. You can see the points at which everything on the disk came to a stop. Bruce Dawson has a nice picture of the disk offset graph. Comments are closed. They are probably trained well in that!
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Subscribe to: Post Comments Atom. This error means that you are trying to perform some operation in the database which requires encryption wallet to be open, but wallet is Tablespace Growth History and Forecast for 10g and 11g.
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