I am loading data into Mysql DB(8.0.16) through pentaho(8.3) jobs, from the last 2months process was going fine(jobs usually complete in 1hr), Since last night the process is going very slow(executing for more than 12hrs), I have checked the processlist the queries are taking for ever for executing(even the count on small volume tables). am unable to figureout the issue.
any suggestions to findout the bottlenecks.
Related
I was trying to run analyze command on a table out of 900 tables in mysql 5.7.30. Its stuck my all db process-list and connections spike immediate and lot of commands found with state "Waiting for table flush" even our max_connection parameter reaches at 2500. We are running the analyze table command from last 3 years but from last 1 month we notice this issue 4th time. If we didn't analyze our tables then we see severe performance issues and lot of queries enter into state "statistics". Whats your thoughts on it
You most definitely shouldn't be running ANALYZE regularly or automatically. It sounds like you were dodging the bullet of queries stuck in the waiting for able flush state purely because the load on your servers was sufficiently low that you didn't notice it before. You should only ever run this on a table sparingly when you have clear, definitive evidence that the index statistics on that table are sufficiently detached from reality to cause the query optimiser to regularly come up with egregiously poor execution plan.
We've set up a database replication about a week ago, and we are having an issue with keeping it in sync.
The setup is a master-master replication with MariaDB 10.1.35/MySQL 5.5.5. Only one database is being used to make calls on. The other database will only be used as a backup. I will refer to this one as the slave. And it's the slave we're having issues with. The replication is statement based.
The first 24 hours went fine. The next day, the slave was more and more behind, up until almost 24 hours. When we checked 24 hours later, the slave was back on track again, being behind on the master by just a few seconds.
Now again, it's starting to get behind more and more (over 5 hours of data now).
It's still syncing, so the replication itself is working. However, some queries just take way too long on the slave, which is delaying everything.
All queries are being executed quite fast, except for one UPDATE query. It's this one which stays in the processlist for 5, 10 and sometimes even 20 or 30 seconds. The query is being handled in less than a second on the master, and also when we execute this query manually on the slave, it's doesn't take longer than a second. So, we don't think it's related to the query itself. The structure of both databases/tables are exactly the same. The storage engine of the table is InnoDB.
At this point, we have no clue what could be causing this delay. Inserts are being processed instantly.
There's one difference in the processlist when the query is being executed on the slave; the command stays on 'Connect', while the command says 'Execute' on the master. Is this normal behaviour?
If I should provide more information, please let me know. It's clear that a slave only handles one query at a time and thus it can get behind if there are a lot of queries on the master, but it should not be necassary for that query to take up to 30 seconds, while it takes less than one when being executed manually.
Thank you.
P.S. We already optimized the table (OPTIMIZE) but unfortunately that didn't make a difference.
If the database is overloaded you'll get an increased number of queries running. Occasional spikes are OK for very short period of time. Too many active threads indicate that:
MySQL is taking too much time to process you requests.
You are continuously retrieving/updating large datasets.
Make sure that queries are tuned to use indexes. Execute SHOW FULL PROCESSLIST of find queries that are getting locked continuously. Try
isolating long running queries by enabling the slow query log.
any help please
Lately we are seeing some queries in mysql(master) logs but no idea why they are shown there:
Queries are select/update table where id = <some integer>.
There is index on id
table size is below 100 000
Rows scanned are in hundreds (sometimes < 100)
Server is running on extremely good hardware
there are no joins involved
We do not see any heavy activity running on database at that time
tables are innodb
the same queries generally don’t even take 50ms, but sometimes all the execution of these queries take about 4-8 seconds
One observation was all the similar "non-slow-but-weirdly-taking-high-time" queries take almost the same amount of time for some duration . I.e. queries like stated in the top will all take about 4.35 seconds with variation of 0.05 seconds.
Does the network latency/packets-drop affect mysql query timing?
show processlist;
show global status like '%onnect%';
show global status like '%open%';
Is anything backed up? Is it waiting in a queue? waiting for file handles? What are your max_connections, open-files-limit, thread_concurrency ?
One side question: does the network latency/packets-drop affect mysql query timing?
Yes, the timeout must occur before the query is resent by the client
Do you see these problems locally or over the network? If the latter, then obviously packet drops can affect your performance if you are measuring from the client.
Is it running in a virtual machine that can affect the performance?
Disk problems?
How is serialization set up? Can it be a contention problem by many processes accessing the same row?
You may want to enable the query/slow query logs to see if there is any sort of pattern that causes this.
Mysql slow log is not representative source to learn about your slow queries. If something makes server work slow all queries usually go to slow log.
E.g. if you have some slow blocking select on MyISAM a lot of PK updates will go to slow log.
You need to search for other slow queries or server problems. What about load average on this particular machine? Isn't mysql displaced into swap memory? Other applications? Queries per second?
I have a service that sits on top of a MySQL 5.5 database (INNODB). The service has a background job that is supposed to run every week or so. On a high level the background job does the following:
Do some initial DB read and write in one transaction
Execute UMQ (described below) with a set of parameters in one transaction.
If no records are returned we are done!
Process the result from UMQ (this is a bit heavy so it is done outside of any DB
transaction)
Write the outcome of the previous step to DB in one transaction (this
writes to tables queried by UMQ and ensures that the same records are not found again by UMQ).
Goto step 2.
UMQ - Ugly Monster Query: This is a nasty database query that joins a bunch of tables, has conditions on columns in several of these tables and includes a NOT EXISTS subquery with some more joins and conditions. UMQ includes ORDER BY also has LIMIT 1000. Even though the query is bad I have done what I can here - there are indexes on all columns filtered on and the joins are all over foreign key relations.
I do expect UMQ to be heavy and take some time, which is why it's executed in a background job. However, what I'm seeing is rapidly degrading performance until it eventually causes a timeout in my service (maybe 50 times slower after 10 iterations).
First I thought that it was because the data queried by UMQ changes (see step 4 above) but that wasn't it because if I took the last query (the one that caused the timeout) from the slow query log and executed it myself directly I got the same behavior only until I restated the MySQL service. After restart the exact query on the exact same data that took >30 seconds before restart now took <0.5 seconds. I can reproduce this behavior every time by restoring the database to it's initial state and restarting the process.
Also, using the trick described in this question I could see that the query scans around 60K rows after restart as opposed to 18M rows before. EXPLAIN tells me that around 10K rows should be scanned and the result of EXPLAIN is always the same. No other processes are accessing the database at the same time and the lock_time in the slow query log is always 0. SHOW ENGINE INNODB STATUS before and after restart gives me no hints.
So finally the question: Does anybody have any clue of why I'm seeing this behavior? And how can I analyze this further?
I have the feeling that I need to configure MySQL differently in some way but I have searched and tested like crazy without coming up with anything that makes a difference.
Turns out that the behavior I saw was the result of how the MySQL optimizer uses InnoDB statistics to decide on an execution plan. This article put me on the right track (even though it does not exactly discuss my problem). The most important thing I learned from this is that MySQL calculates statistics on startup and then once in a while. This statistics is then used to optimize queries.
The way I had set up the test data the table T where most writes are done in step 4 started out as empty. After each iteration T would contain more and more records but the InnoDB statistics had not yet been updated to reflect this. Because of this the MySQL optimizer always chose an execution plan for UMQ (which includes a JOIN with T) that worked well when T was empty but worse and worse the more records T contained.
To verify this I added an ANALYZE TABLE T; before every execution of UMQ and the rapid degradation disappeared. No lightning performance but acceptable. I also saw that leaving the database for half an hour or so (maybe a bit shorter but at least more than a couple of minutes) would allow the InnoDB statistics to refresh automatically.
In a real scenario the relative difference in index cardinality for the tables involved in UMQ will look quite different and will not change as rapidly so I have decided that I don't really need to do anything about it.
thank you very much for the analysis and answer. I've been searching this issue for several days during ci on mariadb 10.1 and bacula server 9.4 (debian buster).
The situation was that after fresh server installation during a CI cycle, the first two tests (backup and restore) runs smoothly on unrestarted mariadb server and only the third test showed that one particular UMQ took about 20 minutes (building directory tree during restore process from the table with about 30k rows).
Unless the mardiadb server was restarted or table has been analyzed the problem would not go away. ANALYZE TABLE or the restart changed the cardinality of the fields and internal query processing exactly as stated in the linked article.