MDEV-12289 Keep 128 persistent rollback segments for compatibility and performance
InnoDB divides the allocation of undo logs into rollback segments. The DB_ROLL_PTR system column of clustered indexes can address up to 128 rollback segments (TRX_SYS_N_RSEGS). Originally, InnoDB only created one rollback segment. In MySQL 5.5 or in the InnoDB Plugin for MySQL 5.1, all 128 rollback segments were created. MySQL 5.7 hard-codes the rollback segment IDs 1..32 for temporary undo logs. On upgrade, unless a slow shutdown (innodb_fast_shutdown=0) was performed on the old server instance, these rollback segments could be in use by transactions that are in XA PREPARE state or transactions that were left behind by a server kill followed by a normal shutdown immediately after restart. Persistent tables cannot refer to temporary undo logs or vice versa. Therefore, we should keep two distinct sets of rollback segments: one for persistent tables and another for temporary tables. In this way, all 128 rollback segments will be available for both types of tables, which could improve performance. Also, MariaDB 10.2 will remain more compatible than MySQL 5.7 with data files from earlier versions of MySQL or MariaDB. trx_sys_t::temp_rsegs[TRX_SYS_N_RSEGS]: A new array of temporary rollback segments. The trx_sys_t::rseg_array[TRX_SYS_N_RSEGS] will be solely for persistent undo logs. srv_tmp_undo_logs. Remove. Use the constant TRX_SYS_N_RSEGS. srv_available_undo_logs: Change the type to ulong. trx_rseg_get_on_id(): Remove. Instead, let the callers refer to trx_sys directly. trx_rseg_create(), trx_sysf_rseg_find_free(): Remove unneeded parameters. These functions only deal with persistent undo logs. trx_temp_rseg_create(): New function, to create all temporary rollback segments at server startup. trx_rseg_t::is_persistent(): Determine if the rollback segment is for persistent tables. trx_sys_is_noredo_rseg_slot(): Remove. The callers must know based on context (such as table handle) whether the DB_ROLL_PTR is referring to a persistent undo log. trx_sys_create_rsegs(): Remove all parameters, which were always passed as global variables. Instead, modify the global variables directly. enum trx_rseg_type_t: Remove. trx_t::get_temp_rseg(): A method to ensure that a temporary rollback segment has been assigned for the transaction. trx_t::assign_temp_rseg(): Replaces trx_assign_rseg(). trx_purge_free_segment(), trx_purge_truncate_rseg_history(): Remove the redundant variable noredo=false. Temporary undo logs are discarded immediately at transaction commit or rollback, not lazily by purge. trx_purge_mark_undo_for_truncate(): Remove references to the temporary rollback segments. trx_purge_mark_undo_for_truncate(): Remove a check for temporary rollback segments. Only the dedicated persistent undo log tablespaces can be truncated. trx_undo_get_undo_rec_low(), trx_undo_get_undo_rec(): Add the parameter is_temp. trx_rseg_mem_restore(): Split from trx_rseg_mem_create(). Initialize the undo log and the rollback segment from the file data structures. trx_sysf_get_n_rseg_slots(): Renamed from trx_sysf_used_slots_for_redo_rseg(). Count the persistent rollback segment headers that have been initialized. trx_sys_close(): Also free trx_sys->temp_rsegs[]. get_next_redo_rseg(): Merged to trx_assign_rseg_low(). trx_assign_rseg_low(): Remove the parameters and access the global variables directly. Revert to simple round-robin, now that the whole trx_sys->rseg_array[] is for persistent undo log again. get_next_noredo_rseg(): Moved to trx_t::assign_temp_rseg(). srv_undo_tablespaces_init(): Remove some parameters and use the global variables directly. Clarify some error messages. Adjust the test innodb.log_file. Apparently, before these changes, InnoDB somehow ignored missing dedicated undo tablespace files that are pointed by the TRX_SYS header page, possibly losing part of essential transaction system state.
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