mysql-php hints PDO failed

There is a MySQL server, dozens of new databases are added each day (30 or 40), up to now, there are nearly 3,000 databases. The number of accesses to a single database is small, but may be due to excessive amounts, and recently a frequent occurrence of PDO failed (discuz DB module) error. See the MySQL error log and find no useful hints.

To ask, what are some of the things that should be considered for a problem like this?

Listed below are some of the my.cnf content

key_buffer_size = 32Mmax_allowed_packet = 2Mtable_open_cache = 256sort_buffer_size = 6Mnet_buffer_length = 8Kread_buffer_size = 2Mread_rnd_buffer_size = 2Mmyisam_sort_buffer_size = 4Mmyisam_max_sort_file_size = 32Mmyisam_max_extra_sort_file_size = 32Mquery_cache_size = 128Mthread_cache_size = 128max_connections=256tmp_table_size = 100Mjoin_buffer_size = 8M

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There is a MySQL server, dozens of new databases are added each day (30 or 40), up to now, there are nearly 3,000 databases. The number of accesses to a single database is small, but may be due to excessive amounts, and recently a frequent occurrence of PDO failed (discuz DB module) error. See the MySQL error log and find no useful hints.

To ask, what are some of the things that should be considered for a problem like this?

Listed below are some of the my.cnf content

key_buffer_size = 32Mmax_allowed_packet = 2Mtable_open_cache = 256sort_buffer_size = 6Mnet_buffer_length = 8Kread_buffer_size = 2Mread_rnd_buffer_size = 2Mmyisam_sort_buffer_size = 4Mmyisam_max_sort_file_size = 32Mmyisam_max_extra_sort_file_size = 32Mquery_cache_size = 128Mthread_cache_size = 128max_connections=256tmp_table_size = 100Mjoin_buffer_size = 8M

Database index changed to InnoDB?
But 3,000 databases, a variety of size only a few megabytes and 10 trillion, it seems that the configuration is too low

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