Diagnosing Locking Problems using ASH – Part 6

“Toto, I’ve a feeling we’re not in Kansas any more.“

What started as a simple write-up of a course demo gone wrong (or right, depending the way you look at these things) has grown arms and legs and staggered away from the original intention to talk about ASH. so I thought it might be worth taking a breather and see where we’ve been so far, before pressing on with another couple of posts.

Part 1 – Introduced a blocking lock problem and showed how it would look in the OEM DB/Grid Control GUI.

Part 2 – Demonstrated that you might not see any ASH samples for the blocking session if it doesn’t generate enough activity to be captured.

Part 3 – Demonstrated that even if the blocking session *has* been active enough to be sampled, you might not have the specific activity that caused the problem.

Part 4 – Reinforced part 3 to show that you might *think* you’re looking at the SQL statement which was holding the blocking lock when you actually aren’t and highlighted JB’s point that there is no way of knowing the SQL statement that was holding a particular lock. I see that Jonathan Lewis has posted an illustration of this although let’s wait to see if Tanel Poder comes up with something 😉

I decided at this point to draw some conclusions as to the limitations of ASH as a tool in diagnosing locking problems after they’ve cleared. In amongst the meandering detail, it’s important to reiterate that objective. There are lots of ways of diagnosing locking problems that are happening right now or that you expect to happen in future.

Part 5 – The same subject cropped up on the Oak Table list and in amongst the suggestions were a couple to use LogMiner. One was from Kyle Hailey and I posted his scripts and output verbatim, but there were a few cut and paste errors and so I was faffing around with the post for a while and eventually added a comment to say I’d fix it later. I’ve now done so, re-running the tests with the correct code and removed the comment from the post. Sadly, none of the corrections fixed the two central problems with this approach

a) Kyle and I have both found LogMiner behaviour to be very flaky indeed and it doesn’t seem to return the results I’d expect, over multiple tests by two different ‘testers’. (See the last post for an example.)

b) I had to change the examples to use UPDATE statements rather than SELECT FOR UPDATEs.

I’ll be looking at these two issues in Parts 7 and 8 (I think) but who knows what turn these posts will take next! Here’s where I think I might be heading

Part 6 – You’re reading it.

Part 7 – Redo log dump confirmation that the LogMiner queries in Part 5 aren’t showing what I’d expect.

Part 8 – Redo log dump confirmation that redo entries won’t help with the original SELECT FOR UPDATE example anyway.

Part 9 – Can I stop now, or should I attempt to reach a double-digit part number for the first (and hopefully last) time?