If a latch is held in one of the preceding modes, all In the wait type indicates the requested mode, not the blocking mode. Not a factor of the mode specified in the wait type but rather a factor of the mode in which the latch is held. It is important to note that the available blocking information is Single task, whereas KP and SH latches can be held by multiple tasks simultaneously. The common factor with these modes is that a latch can be held in only one of these modes at a time and by only a Information is known when a latch is held in UP, EX, or DT modes. Latch waits because latches track information for only certain types of owners so as to remain lightweight. This information is not available for all Although KP use is common, it only conflicts with DT, which is rarely used, so waits of either are quite rare.īlocking task information for latch waits is provided under certain circumstances. Waits for the NL, KP, and DT modes are rarely, if ever, seen. Of the six modes, three are much more common the others. (Lock modes are defined in the Books Online topic sys.dm_os_wait_stats.) The actual wait type is formed by appending one of the modes to the group name (for example, LATCH_EX). In addition to these groups, different wait types exist for the latch mode being requested. The TRANMARKLATCH group can be treated as any other nonpage latch even though it has the special status of having its own wait types. The main groups are the PAGELATCH and PAGEIOLATCH, and TRANMARKLATCH and LATCH wait base wait types, often referred to as page and nonpage latches, respectively. ![]() Latch waits are divided into two main groups: page latch waits and nonpage latch waits. Certain types of latches are a common source of blocking and, unfortunately, latchīlocking is often hard to investigate and resolve (because of the scarcity of diagnostic information available for them).įortunately, SQL Server 2005 provides much more information than was available in previous releases. More than 120 distinct usages of latches. Although originally used mainly for synchronization of physical access toĭatabase pages, their use in SQL Server as a general synchronization primitive has become widespread. Latches are short-term synchronization objects. ![]() ![]() Next, we’ll cover some of the more common wait types. SQL Server 2005 Practical Troubleshooting: The Database Engineīeyond the common tools covered previously, much of the details of determining the cause of blocking and resolving it are
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