I am working on a project, in which I have used mongoDb as database.
I am working on the cron, in which there was high mongoDb input/output operations, i.e. there was around 10k/s i/o operations on mongoDb including get, insert and update queries.
My system specifications are:-
RAM:- 384GB
OS:- ubunutu 22.04
During my entire cron, there was no change into the schema of the database, there was only get and update queries are running. In which around 95% of the queries are Get query.
My problem is that, mongoDb becomes slow after a certain period of time, to find out the problem, I start analysing slow query logs, and I have found that there was high time of schemaLock into the db queries.
{“t”:{“$date”:“2023-04-05T12:23:26.702+00:00”},“s”:“I”, “c”:“COMMAND”, “id”:51803, “ctx”:“conn4409”,“msg”:“Slow query”,“attr”:{“type”:“command”,“ns”:“canadaloctest1867_bk_db.users”,“command”:{“find”:“users”,“filter”:{“_id”:1,“status”:{“$exists”:true,“$ne”:9}},“limit”:1,“projection”:{“status”:1},“singleBatch”:true,“lsid”:{“id”:{“$uuid”:“c00762d0-98b6-41ab-adac-163039f60c0f”}},“$db”:“canadaloctest1867_bk_db”},“planSummary”:“IXSCAN { _id: 1 }”,“keysExamined”:1,“docsExamined”:1,“cursorExhausted”:true,“numYields”:1,“nreturned”:1,“queryHash”:“D7A7C611”,“planCacheKey”:“1FD4C0F8”,“reslen”:147,“locks”:{“FeatureCompatibilityVersion”:{“acquireCount”:{“r”:2}},“ReplicationStateTransition”:{“acquireCount”:{“w”:2}},“Global”:{“acquireCount”:{“r”:2}},“Database”:{“acquireCount”:{“r”:2}},“Collection”:{“acquireCount”:{“r”:2}},“Mutex”:{“acquireCount”:{“r”:1}}},“storage”:{“data”:{“bytesRead”:133345,“timeReadingMicros”:126},“timeWaitingMicros”:{“handleLock”:120764,“schemaLock”:8429126}},“protocol”:“op_msg”,“durationMillis”:8654}}
According to the mongoDb documentation, schemaLock will happen if we are modifying the schema.
I have two questions:-
- Why this schemaLock happens, if there was no change into the schema in the entire transaction?
- What can be the possible solution to prevent this situation?