Tablespaces tablespace truncate operations, which can occur if  innodb_max_undo_log_size  is set too low on a busy system, for example. If the limit is exceeded, an undo tablespace can still be made inactive, but it is not truncated until after the next checkpoint. The the limit was raised from 64 to 50,000 in MySQL 8.0.22. An undo tablespace truncate operation creates a temporary  undo_ space_number _trunc.log  file in the server log directory. That log directory is defined by  innodb_log_group_home_dir . If a system failure occurs during the truncate operation, the temporary log file permits the startup process to identify undo tablespaces that were being truncated and to continue the operation. The following status variables permit tracking the total number of undo tablespaces, implicit ( InnoDB - created) undo tablespaces, explicit (user-created) undo tablespaces, and the number of active undo tablespaces: For status variable descriptions, see Section 5.1.10, “Server Status Variables” . InnoDB  uses session temporary tablespaces and a global temporary tablespace. Session temporary tablespaces store user-created temporary tables and internal temporary tables created by the optimizer when  InnoDB  is configured as the storage engine for on-disk internal temporary tables. Beginning with MySQL 8.0.16, the storage engine used for on-disk internal temporary tables is  InnoDB . (Previously, the storage engine was determined by the value of internal_tmp_disk_storage_engine .) Session temporary tablespaces are allocated to a session from a pool of temporary tablespaces on the first request to create an on-disk temporary table. A maximum of two tablespaces is allocated to a session, one for user-created temporary tables and the other for internal temporary tables created by the optimizer. The temporary tablespaces allocated to a session are used for all on-disk temporary tables created by the session. When a session disconnects, its temporary tablespaces are truncated and released back to the pool. A pool of 10 temporary tablespaces is created when the server is started. The size of the pool never shrinks and tablespaces are added to the pool automatically as necessary. The pool of temporary tablespaces is removed on normal shutdown or on an aborted initialization. Session temporary tablespace files are five pages in size when created and have an  .ibt  file name extension. A range of 400 thousand space IDs is reserved for session temporary tablespaces. Because the pool of session temporary tablespaces is recreated each time the server is started, space IDs for session temporary tablespaces are not persisted when the server is shut down, and may be reused. The  innodb_temp_tablespaces_dir  variable defines the location where session temporary tablespaces are created. The default location is the  #innodb_temp  directory in the data directory. Startup is refused if the pool of temporary tablespaces cannot be created. 3066 15.6.3.5 Temporary Tablespaces Session Temporary Tablespaces Undo Tablespace Status Variables Undo Tablespace Truncation Recovery mysql>  SHOW STATUS LIKE ’Innodb_undo_tablespaces%’; +----------------------------------+-------+ | Variable_name | Value | +----------------------------------+-------+ | Innodb_undo_tablespaces_total | 2 | | Innodb_undo_tablespaces_implicit | 2 | | Innodb_undo_tablespaces_explicit | 0 | | Innodb_undo_tablespaces_active | 2 | +----------------------------------+-------+