Details
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Bug
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Status: Closed (View Workflow)
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Major
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Resolution: Fixed
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10.3, 10.4, 10.5, 10.6
Description
Occasionally, the test innodb.alter_copy would fail, reporting DB_MISSING_HISTORY in CHECK TABLE. It first caught my attention during the development of MDEV-25180. That change introduced delays to the purge tasks, in the form of purge_sys.stop_SYS(). If we delay purge more during DDL operations, then the test would almost always fail. Here is the first occurrence of this kind in a main branch:
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10.6 66165ae2210ac3230850fa60086564db |
CURRENT_TEST: innodb.alter_copy
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mysqltest: At line 84: query 'CHECK TABLE t' failed: <Unknown> (2013): Lost connection to server during query
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…
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2021-05-21 22:21:54 3 [ERROR] [FATAL] InnoDB: Unknown error Required history data has been deleted
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I was able to repeat this, and this actually affects older versions as well. In 10.2, the transaction isolation level is supposed to be effectively hard-wired as READ UNCOMMITTED when innodb_read_only is set or the setting innodb_force_recovery has the value 4, 5, or 6. But, we actually would use some other isolation level in trx_undo_prev_version_build():
if (trx_undo_get_undo_rec( |
roll_ptr, heap, rec_trx_id, index->table->name,
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&undo_rec)) {
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if (v_status & TRX_UNDO_PREV_IN_PURGE) { |
/* We are fetching the record being purged */ |
undo_rec = trx_undo_get_undo_rec_low(roll_ptr, heap);
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} else { |
/* The undo record may already have been purged, |
during purge or semi-consistent read. */
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return(false); |
}
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}
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The fix is to avoid creating a purge view if purge is not going to be executed, so that the READ UNCOMMITTED mode will be used. There are two possible fixes: Either make CHECK TABLE explicitly use the READ UNCOMMITTED isolation level, or do not create the purge sys view (the below is for MariaDB 10.2):
diff --git a/storage/innobase/trx/trx0sys.cc b/storage/innobase/trx/trx0sys.cc
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index 9138e9475bf..19dfa79d0ff 100644
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--- a/storage/innobase/trx/trx0sys.cc
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+++ b/storage/innobase/trx/trx0sys.cc
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@@ -535,7 +535,9 @@ trx_sys_init_at_db_start()
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trx_sys_mutex_exit();
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- trx_sys->mvcc->clone_oldest_view(&purge_sys->view);
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+ if (!high_level_read_only) {
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+ trx_sys->mvcc->clone_oldest_view(&purge_sys->view);
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+ }
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}
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/*****************************************************************//** |
We could also explicitly use READ UNCOMMITTED isolation level in CHECK TABLE also in this case (see MDEV-15418 and MDEV-18952).
MDEV-15418 claims the following:
Since MariaDB 10.2 (along with upstream MySQL 5.7), InnoDB already does this [READ UNCOMMITTED] for innodb_read_only=1
That hard-wiring occurs at a rather low level, for example, lock_clust_rec_cons_read_sees() is checking srv_read_only_mode.
The fix with the least impact and risk would seem to be to only touch CHECK TABLE, starting with 10.3, where MDEV-15418 first touched this code:
diff --git a/storage/innobase/dict/dict0dict.cc b/storage/innobase/dict/dict0dict.cc
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index 10b878c0e49..1d0e2af3cd6 100644
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--- a/storage/innobase/dict/dict0dict.cc
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+++ b/storage/innobase/dict/dict0dict.cc
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@@ -5425,7 +5425,7 @@ dict_set_corrupted(
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/* If this is read only mode, do not update SYS_INDEXES, just
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mark it as corrupted in memory */
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- if (srv_read_only_mode) {
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+ if (high_level_read_only) {
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index->type |= DICT_CORRUPT;
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goto func_exit;
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}
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diff --git a/storage/innobase/handler/ha_innodb.cc b/storage/innobase/handler/ha_innodb.cc
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index 4e63edc65c2..65039919793 100644
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--- a/storage/innobase/handler/ha_innodb.cc
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+++ b/storage/innobase/handler/ha_innodb.cc
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@@ -14796,10 +14796,9 @@ ha_innobase::check(
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/* We must run the index record counts at an isolation level
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>= READ COMMITTED, because a dirty read can see a wrong number
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- of records in some index; to play safe, we use always
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- REPEATABLE READ here (except when undo logs are unavailable) */
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- m_prebuilt->trx->isolation_level = srv_force_recovery
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- >= SRV_FORCE_NO_UNDO_LOG_SCAN
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+ of records in some index; to play safe, we normally use
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+ REPEATABLE READ here */
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+ m_prebuilt->trx->isolation_level = high_level_read_only
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? TRX_ISO_READ_UNCOMMITTED
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: TRX_ISO_REPEATABLE_READ;
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Attachments
Issue Links
- relates to
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- Closed
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- Closed
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MDEV-25180 Atomic ALTER TABLE
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- Closed
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