Nigel Noble's Oracle Blog

10/01/2013 Strange statistic, large transactions, dirty buffers and “direct path read”

Filed under: 11gR2, Performance — Nigel Noble @ 6:27 pm


I recently investigated an IO performance “spike” on a large transactional system and I thought I would cover some interesting issues found. I am going to take the approach of detailing the observations made from our production and test systems and avoid attempting to cover how other versions of Oracle behave. The investigation also uncovers a confusing database statistic which we are currently discussing with Oracle Development so they can decide if this is an Oracle coding bug or a documentation issue.

The initial IO issue

We run a simple home grown database monitor which watches database wait events and sends an email alert if it detects either a single session waiting on a non-idle wait for a long time or the total number of database sessions concurrently waiting goes above a defined threshold. The monitor can give a number of false alerts but can also draw our attention to some more interesting events.

One day the monitor alerted me to two separate databases waiting on “log file sync” waits at exactly the same few seconds and affecting many hundreds of sessions. Both databases share the same storage array so the obvious place to start was to look at the storage statistics. We found a strange period lasting around 10 seconds long when both databases had a large increase in redo write service times and a few seconds when no IO was written at all. The first database we looked at seemed to show increase in disk service times for a very similar work load.  The second system showed a large increase in data file writes (500MB/sec) but without any matching increase in redo log writes. It seems the first database was slowed down by the second database flushing 5GB of data.

Where did 5GB of data file writes come from, and what triggered it?

Looking at the database we knew there were no corresponding redo writes, there were no obvious large sql statements reading or writing. We confirmed these writes were real and not something outside the database.


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