I had to write this post because I can never remember which way round Oracle named the two versions of gc buffer busy when it started identifying them separately. There are two scenarios to cover when my session wants my instance to acquire a global cache lock on a block and some other session is already trying to acquire that lock or is holding it in an incompatible fashion:
- The other session is in my instance
- The other session is in a remote instance
One of these cases is reported as “gc buffer busy acquire”, the other as a “gc buffer busy release” and I always have to check which is which. I think I usually get it right first time when I see it but I always manage to convince myself that I might have got it wrong and end up searching the internet for Riyaj Shamsudeen’s blog posting about it.
The “release” is waiting for another instance to surrender the lock to my instance; the “acquire” is waiting for another session in my instance to finish acquiring the lock from the other instance.
I decided to jot down this note so I didn’t have to keep searching for Riyaj’s and also because a little problem on OTN at the moment showed a couple of AWR reports with an unlikely combination of waits for acquire (180,000,000) and release (2,000) waits.
If you’re wondering why this looks odd – if I’m waiting for an acquire someone else in my instance must be waiting for a release. Obviously many sessions could be waiting for one release, and if acquirers time out very rapidly (though they’re not reported as doing so) then the ratio could get very high – but 90,000 acquires per release doesn’t look right.