Plesk UI is unresponsive: Zend_Db_Adapter_Exception: SQLSTATE[HY000] [2002] No such file or directory

Symptoms

• Plesk UI is unresponsive.

• The following or similar errors are being written to /var/log/plesk/error.log:

  # tailf /var/log/plesk/error.log
  [07-Mar-2018 19:01:57 Asia/Tokyo] PHP Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY000] [2002] No such file or directory' in /usr/local/psa/admin/externals/Zend/Db/Adapter/Pdo/Abstract.php:129
  ...

• The root partition os running out of a free memory:

  # df -h
  Filesystem Size Used Avail Use% Mounted on
  /dev/mapper/centos-root 50G 50G 9M 100% /
  devtmpfs 7.7G 0 7.7G 0% /dev
  tmpfs 7.7G 0 7.7G 0% /dev/shm
  tmpfs 7.7G 121M 7.6G 2% /run

Cause

Connection to the database is repeatedly dropping, causing Plesk unavailability before spare space is freed by automatic deletion of a temporary file.

Resolution

Clearing some free space or expand the partition.