diff --git a/_posts/2024-03-06-sql-waits.md b/_posts/2024-03-06-sql-waits.md new file mode 100644 index 0000000..3cc8eef --- /dev/null +++ b/_posts/2024-03-06-sql-waits.md @@ -0,0 +1,118 @@ +--- +layout: post +title: "SQL Server Wait Stats: Mapping to Performance Counters" +date: 2024-03-06 +categories: mssql +--- +# SQL Server Wait Stats + +Mapping between performance counter and SQL Server wait type. + +* **Lock waits** - `LCK_M_*` + + Lock waits occur when a query is waiting for a lock to be released by another transaction. Locks are used to ensure the consistency of data. SQL Server has various lock types (e.g., Shared, Exclusive), and the `LCK_M_*` prefix indicates different lock modes such as: + + * **`LCK_M_S`**: Shared lock + * **`LCK_M_X`**: Exclusive lock + * **`LCK_M_U`**: Update lock + + **Examples of scenarios:** + * A read operation is waiting for a write lock to be released (`LCK_M_S`). + * A write operation is waiting for another write lock to be released (`LCK_M_X`). + +* **Log buffer waits** - `LOGBUFFER` + + This wait type occurs when a session is waiting for space to become available in the log buffer to write log records. The log buffer is a memory area where SQL Server stores transaction log records before writing them to disk. + + **Typical cause:** + * Heavy transactional workloads causing the log buffer to fill up faster than it can be written to disk. + +* **Log write waits** - `WRITELOG` + + This wait type occurs when a session is waiting for the log manager to write log records to disk. It indicates the time spent waiting for the transaction log to be flushed to disk. + + **Typical cause:** + * Disk I/O contention or slow disk subsystems impacting log write performance. + +* **Memory grant queue waits** - `RESOURCE_SEMAPHORE` + + This wait type occurs when a query is waiting for sufficient memory to execute. SQL Server uses memory grants for operations such as sorts and hash joins. + + **Typical cause:** + * Insufficient memory available for query execution due to high memory usage by other processes or large queries. + +* **Network IO waits** - `NET_WAITFOR_PACKET` + + This wait type occurs when SQL Server is waiting for a network packet to be sent or received. It can be related to network latency or throughput issues. + + **Typical cause:** + * Network latency or slow network connections impacting data transmission. + +* **Non-Page latch waits** - `LATCH_*` + + These waits occur for internal memory structures and synchronization objects not related to disk I/O. Latches are lightweight synchronization objects used to protect internal data structures. + + **Typical cause:** + * Contention on internal memory structures, such as data buffers or internal caches. + +* **Page IO latch waits** - `PAGEIOLATCH_*` + + These waits occur when SQL Server is waiting for a data page to be read from disk into memory. This can indicate disk I/O bottlenecks. + + **Examples:** + * **`PAGEIOLATCH_SH`**: Waiting for a page to be read into memory for a shared latch. + * **`PAGEIOLATCH_EX`**: Waiting for a page to be read into memory for an exclusive latch. + +* **Page latch waits** - `PAGELATCH_*` + + These waits occur for latch operations on in-memory pages. Unlike page I/O latches, these do not involve disk reads. + + **Examples:** + * **`PAGELATCH_SH`**: Waiting for a shared latch on an in-memory page. + * **`PAGELATCH_EX`**: Waiting for an exclusive latch on an in-memory page. + +* **Thread-safe memory objects waits** - `SOS_MEMORY_TOPLEVELBLOCKALLOCATOR` + + This wait type occurs when threads are waiting for memory allocation using the SOS (SQL Operating System) memory allocator. + + **Typical cause:** + * High contention on memory allocation for thread-safe memory objects. + +* **Transaction ownership waits** - `XACT_OWNERSHIP` + + This wait type occurs when a session is waiting to acquire ownership of a transaction. This can happen during activities like distributed transactions. + + **Typical cause:** + * Contention on transaction objects or coordination of distributed transactions. + +* **Wait for the worker** - `THREADPOOL` + + This wait type occurs when there are no worker threads available to execute tasks. SQL Server uses a pool of worker threads to handle user requests. + + **Typical cause:** + * All worker threads are busy, typically due to a high number of concurrent requests or long-running queries. + +* **Workspace synchronization waits** - `EXECSYNC` + + This wait type occurs when a session is waiting for synchronization of the execution workspace. This can involve coordination between different parts of the query execution process. + + **Typical cause:** + * Synchronization issues between execution tasks, such as parallel query execution coordination. + +### Recap Table: + +| Performance Counter | SQL Server Wait Type | Description | +|-----------------------------------------|------------------------------------|------------------------------------------------------| +| Lock waits | LCK_M_* | Waiting for a lock to be released by another transaction. | +| Log buffer waits | LOGBUFFER | Waiting for space in the log buffer to write log records. | +| Log write waits | WRITELOG | Waiting for log records to be written to disk. | +| Memory grant queue waits | RESOURCE_SEMAPHORE | Waiting for sufficient memory to execute a query. | +| Network IO waits | NET_WAITFOR_PACKET | Waiting for network packets to be sent or received. | +| Non-Page latch waits | LATCH_* | Waiting for internal memory structures and synchronization objects. | +| Page IO latch waits | PAGEIOLATCH_* | Waiting for data pages to be read from disk into memory. | +| Page latch waits | PAGELATCH_* | Waiting for latch operations on in-memory pages. | +| Thread-safe memory objects waits | SOS_MEMORY_TOPLEVELBLOCKALLOCATOR | Waiting for memory allocation using SOS memory allocator. | +| Transaction ownership waits | XACT_OWNERSHIP | Waiting to acquire ownership of a transaction. | +| Wait for the worker | THREADPOOL | Waiting for worker threads to become available. | +| Workspace synchronization waits | EXECSYNC | Waiting for synchronization of the execution workspace. | +