Category: Wait Types

RESTORE_MSDA_THREAD_BARRIER is recorded during restores from multiple backup devices (striped backups). Threads restoring parts of a single data block from the multiple streams synchronise at a barrier: all must finish their piece before the group advances to the next block. The wait is that rendezvous. Striped restores live at this barrier constantly, in small doses.…

RTDATA_LIST is recorded when a thread waits for access to the data structure holding performance metrics (RunTimeData) for natively-compiled stored procedures, In-Memory OLTP’s compiled-to-machine-code procedures. Both reporting execution statistics and maintaining the structure (cleanup, new entries) pass through it. No native procedures, no meaningful RTDATA_LIST. Is It a Problem? Usually not, with a version-specific exception:…

SESSION_WAIT_STATS_CHILDREN is recorded when a thread waits to synchronise updates to the per-session wait statistics behind sys.dm_exec_session_wait_stats, the DMV that gives you sys.dm_os_wait_stats-style numbers scoped to one session. Parallel queries mean multiple child tasks feeding one session’s tally, hence the synchronisation and the name. Wait statistics about maintaining wait statistics: agreeably meta. Is It a…

SHUTDOWN is recorded when a SHUTDOWN statement is waiting for active connections to exit, specifically when the shutdown request is blocked behind queries being run by connections with sysadmin-level (sa) access. A graceful shutdown wants running work to complete; while it waits, the initiating thread sits here. If SQL Server “will not shut down”, at…

SLEEP_BPOOL_FLUSH is recorded when the checkpoint process deliberately pauses issuing new writes because it detects it is saturating the I/O subsystem (around 20ms average write latency during normal operation, 100ms during shutdown). Checkpoint backs off for a few milliseconds, lets in-flight writes land, then resumes. It is self-throttling by design: checkpoint being a considerate neighbour…

Three sleep waits share the theme of “no free memory right now, nap and retry”, and this page covers them together: Individually benign; collectively a memory-pressure gauge. Are They a Problem? Occasional traces, no. A rising trend, yes, in the informative sense: these waits increasing means threads are routinely finding the free list empty, which…

Three sleep waits measure phases of instance startup, and this page covers them together: They fire once per startup and then go quiet until the next one. Are They a Problem? Not as waits; they are startup phase markers on the benign filter list. Their value is forensic: SLEEP_DBSTARTUP‘s total after a restart tells you…

SNI_CRITICAL_SECTION is recorded when a thread waits for a critical section inside the SQL Server Network Interface (SNI), the layer that encapsulates TDS packets and talks to clients over the configured transports. Processing communication packets involves shared structures, and threads take brief turns through their guards. Connection-handling plumbing, active on every instance that has clients.…

SNI_TASK_COMPLETION is recorded when a thread waits for tasks to finish during a NUMA node state change, specifically around creating a new SNI network listener after a NUMA node comes online. SQL Server distributes connection listeners across NUMA nodes, and topology changes mean listener work. NUMA node state changes are rare events (startup, hot-add scenarios,…

SOS_DISPATCHER_MUTEX is recorded when a thread waits for the critical section inside the generic dispatcher-pool management code, including when a pool is being adjusted (grown or shrunk). Dispatcher pools are the special-purpose thread pools used by the backup subsystem, In-Memory OLTP, and Extended Events; their management operations serialise here. It pairs with DISPATCHER_QUEUE_SEMAPHORE (idle pool…