Observe many copy threads blocking there when use pt-online-schema-change

Polyglot Projects Percona Toolkit
1

The other day, i use pt-online-schema-change to add a column and index to a table,the table is about 1 Minion rows and the .ibd file size is about 5G Bytes.
I tried several times each time i got an error says the process is aborted duo to the critical load is reached(which is threads_running=60, i always use this and never failed before)

I found something strange using show full process list: there are so many copy threads there like:
REPLACE INTO db._some_table_new …
running for a long time(tens and hundreds of seconds) and in a status of ‘update’.

As i know the pt-online-schema-change uses only one copy threads and i use this tool many times and each time i can only observe one thread chunking and copying but this time everything changed, there were so many copy threads blocking there.

I searched the documents in percona.com and can not find any information about the option about multi-threaded copying.

The table i altered this time does be accessed very frequently, but i think this will not cause a problem as long as there is a single copying thread. Could anyone give me some help,thanks!

2

pt-online-schema-change is a Perl program with no multi-thread capabilities.
The row copy process use INSERT IGNORE … not REPLACE.

The reason why you see many threads running UPDATES could be because the original table is receiving too many INSERT/DELETE/UPDATE and the triggers created for pt-online-schema-change to work, are trying to update those rows into the new table (these queries are par of the triggers created by osc and they are REPLACE).

Regards

3

Yes, you are right! There are caused by triggers. I can not use pt-online-schema-change to finish the alter, finally i use the build-in online DDL function to finish this(no running thread boost).Thank you!

4

I check the process lists on that time, i found a lot of lock waits there, one REPLACE INTO wait for another on AUTO_INC table lock,may be pt-online-schema-change is not suitable for table having a very heavy access on it.