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

**URL:** <https://forums.percona.com/t/observe-many-copy-threads-blocking-there-when-use-pt-online-schema-change/5820>\
**Category:** Percona Toolkit\
**Created:** [August 18, 2017, 10:26am UTC](https://forums.percona.com/t/observe-many-copy-threads-blocking-there-when-use-pt-online-schema-change/5820 "2017-08-18T10:26:57Z")\
**Posts on this page:** 1\
**Page:** 1

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**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, 10:26am UTC](https://forums.percona.com/t/observe-many-copy-threads-blocking-there-when-use-pt-online-schema-change/5820/1 "2017-08-18T10:26:57Z")

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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!