Daniel, Not sure what platform or version of Oracle you're on, but I did run into something similar on AIX with Oracle v9.2.0.5. An overview is in MetalinkNote #3639594.8. It's a bug. Check the note to see if your platform.version fits for this issue. Not sure if there's a patch for this - we were in the process of upgrading to 9.2.0.7 and the problem disappered. * "* When PMON cleans up a dead process then it may notice an in-progress "ALTER SYSTEM SET PARAMETER" command and assume (incorrectly) that this was from the failed process. It may then try to clean up the in-progress command and so conflict with the running session performing the ALTER SYSTEM command. This shows up as numerous "PMON failed to acquire latch, see PMON dump" messages." On 8/7/07, daniel.hubler@xxxxxxxxxx <daniel.hubler@xxxxxxxxxx> wrote: > > > Looking for ideas/comments/suggestions.......... > > We changed cursor_sharing from EXACT to SIMILAR last Saturday morning at > 6am. > Did that via an "ALTER SESSION SET....." command. > The environment is large (7TB) but not very busy on Saturday morning. > We had cursor_sharing=similar in multiple test environments, for many > months without issue. > > At 9:15am, we started to generated ora-04031 errors. > By 9:30am, we could no longer sign-on. > We ended up shutting down the middle-tier, and deleting all of the > OS processes on the DB server, that represented connections to the > instance. > At that point, we were able to signon, backout the change, and startup the > middle-tier again. > > In the alert log, we saw the ora-04031 errors, and also many (thousands) > of messages saying > "PMON failed to acquire latch, see PMON dump". > > The PMON trace file show the same entry, repeated thousands of times: > "PMON unable to acquire latch 2c4cb108 library cache > possible holder pid = 1000 ospid=42586708" > > > Any ideas on what happened would be appreciated. > Thanks. > > > > > Dan Hubler > Database Administrator > Aurora Healthcare > daniel.hubler@xxxxxxxxxx