# Observe many copy threads blocking there when use pt-online-schema-change **URL:** \ **Category:** Percona Toolkit\ **Created:** [August 18, 2017, 10:24am UTC](https://forums.percona.com/t/observe-many-copy-threads-blocking-there-when-use-pt-online-schema-change/5819 "2017-08-18T10:24:20Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![lujinke](https://avatars.discourse-cdn.com/v4/letter/l/bb73d2/32.png) [@lujinke](https://forums.percona.com/u/lujinke)\ **Post date:** [August 18, 2017, 10:24am UTC](https://forums.percona.com/t/observe-many-copy-threads-blocking-there-when-use-pt-online-schema-change/5819/1 "2017-08-18T10:24:20Z")
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](http://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! ---
**Author:** ![carlos.salguero](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/carlos.salguero/32/20_2.png) [@carlos.salguero](https://forums.percona.com/u/carlos.salguero)\ **Post date:** [August 18, 2017, 10:56am UTC](https://forums.percona.com/t/observe-many-copy-threads-blocking-there-when-use-pt-online-schema-change/5819/2 "2017-08-18T10:56:31Z")
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 ---
**Author:** ![lujinke](https://avatars.discourse-cdn.com/v4/letter/l/bb73d2/32.png) [@lujinke](https://forums.percona.com/u/lujinke)\ **Post date:** [August 18, 2017, 8:15pm UTC](https://forums.percona.com/t/observe-many-copy-threads-blocking-there-when-use-pt-online-schema-change/5819/3 "2017-08-18T20:15:22Z")
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! ---
**Author:** ![lujinke](https://avatars.discourse-cdn.com/v4/letter/l/bb73d2/32.png) [@lujinke](https://forums.percona.com/u/lujinke)\ **Post date:** [August 20, 2017, 7:02am UTC](https://forums.percona.com/t/observe-many-copy-threads-blocking-there-when-use-pt-online-schema-change/5819/4 "2017-08-20T07:02:09Z")
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.