Plesk backup fails: No space left on device

Symptoms

• A backup is finished with one of the following warnings/errors:

  Warning: domain "example.com"
  Not all the data was backed up into /var/lib/psa/dumps/clients/john_doe/domains/example.com successfully.
  Total space: 37287124992;
  Available space: 2109317120;
  Unable to write to the output file '/var/lib/psa/dumps/clients/john_doe/domains/example.com/backup_domainmail_1710251030_1710300108.tgz':
  No space left on device at /usr/local/psa/PMM/agents/shared/Storage/Splitter.pm line 92. pigz: abort: write error on <stdout> (Broken pipe) Broken pipe at /usr/local/psa/PMM/agents/shared/Storage/Storage.pm line 310.

  ---

  Warning: mysql "wordpress_0"
  Not all the data was backed up into /var/lib/psa/dumps/domains/example.com/databases/wordpress_0_1 successfully. Total space: 94.00 GB; Available space: 22.00 GB; Mounted on: /. mysqldump: Error: 'Disk full (/var/tmp/#sql_626_2); waiting for someone to free some space...' when trying to dump tablespaces mysqldump: Couldn't execute 'show fields from `hl6UY_commentmeta`': Disk full (/var/tmp/#sql_626_0); waiting for someone to free some space... (1021)

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  STDOUT: Selected filesystem at has only 0 bytes free, but 4096 bytes are needed for operation

  ---

  There is not enough disk space at /var/lib/psa/dumps to backup creation

  ---

  Unable to make database dump. Error: Failed to exec mysqldump: Exit code: 5

• The following errors appear in /usr/local/psa/PMM/logs/backup-2020-05-08-02-33-12-032/backup.log:

  ERR [panel] PHP Warning: file_put_contents(/var/lib/psa/dumps/backup_info_2005080233.xml): failed to open stream: No space left on device

• FTP backup cannot be restored with the following error message in /var/log/plesk/panel.log:

  Error: Unable to import file as dump: pmm-ras failed (Error code = 1): STDOUT: boost::filesystem::create_directories: No space left on device: "/tmp/pmm-repo-icon-content-uPBsvu/clients/example/domains/example.com/databases" STDERR:

  ---

  ERR [panel] DB query failed: SQLSTATE[HY000]: General error: 1 Can't create/write to file '/var/tmp/#sql_674_0.MAI' (Errcode: 28), query was: DESCRIBE `sessions`:
  0: /usr/local/psa/admin/plib/Db/Adapter/Pdo/Mysql.php:53
  Db_Adapter_Pdo_Mysql->query(string 'DESCRIBE `sessions`')

• Restoration of a backup located on an FTP server fails with the following error message:

  external error ;-P
  ERROR: PleskExceptionDatabase: DB query failed: SQLSTATE[HY000]: General error: 1 Can't create/write to file '/var/tmp/#sql_658_3.MAI' (Errcode: 28), query was: DESCRIBE `sessions` (Mysql.php:53)

• WordPress site backup via  ‘Back Up / Restore’  functionality fails with the following error message:

  Failed to back up site
  Task is not responding: id=837, pid=2279579, type=ext-wp-toolkit-create-backup

• Disk space where temporary backup files need to be created is full (or insufficient for the necessary temporary backup files):

  # grep TMP /etc/psa/psa.conf
  DUMP_TMP_D /tmp

  # df -h /tmp
  Filesystem    Size    Used    Avail    Use%    Mounted on
  /dev/dev1     7.1G    7.0G    0.1G     99%     /

Cause

During the execution of the backup creation process, a point is reached where the used free space of the…