A lot of people have started seeing “asynch descriptor resize” wait event in Oracle 11gR2. Here’s my understanding of what it is. Note that I didn’t spend too much time researching it, so some details may be not completely accurate, but my explanation will at least give you an idea of why the heck you suddenly see this event in your database.
FYI, there’s a short, but incomplete explanation of this wait event also documented in MOS Note 1081977.1
Update: There’s a bug and a patch related to this wait event too.
The “direct path loader” (KCBL) module is used for performing direct path IO in Oracle, such as direct path segment scans and reading/writing spilled over workareas in temporary tablespace. Direct path IO is used whenever you see “direct path read/write*” wait events reported in your session. This means that IOs aren’t done from/to buffer cache, but from/to PGA directly, bypassing the buffer cache.
This KCBL module tries to dynamically scale up the number of asynch IO descriptors (AIO descriptors are the OS kernel structures, which keep track of asynch IO requests) to match the number of direct path IO slots a process uses. In other words, if the PGA workarea and/or spilled-over hash area in temp tablespace gets larger, Oracle also scales up the number of direct IO slots. Direct IO slots are PGA memory structures helping to do direct IO between files and PGA.
- If it’s someting like 1% of your response time, then this is not your problem anyway and troubleshooting this further would be not practical – it’s just how Oracle works :)
- If it’s something like 20% or more of your response time, then it’s clearly a problem and you’d need to talk to Oracle Support to sort out the bug
- If it’s anything in between, make sure you don’t have an IO problem first, before telling that this is a bug. In one recent example I saw direct path reads take over a second on average when this problem popped up. The asynch descriptor resize wait event may well disappear from the radar once you fix the root cause – slow IO (or SQL doing too much IO). Remember, the asynch descriptor resize wait event, at least on Linux, is actually an IO wait event, the process is waiting for outstanding IO completion before the descriptor count increase/decrease can take place.