现象
sys.processes spid 和 blocked是同一个
同时我观察到还有PAGEIOLATCH_XX或Latch_XX这样的等待事件
解释
When an SPID is waiting for an I/O page latch, you may notice that the blocked column briefly reports that the SPID is blocking itself. This behavior is a side effect of the way that latches are used for I/O operations on data pages. When a thread issues an I/O request, the SPID that issues the I/O request acquires a latch on the page. All SQL Server 2000 I/O operations are asynchronous. Therefore, the SPID will try to acquire another latch on the same page if the SPID that issued the I/O request must wait for the request to finish. This second latch is blocked by the first latch. Therefore, the blocked column reports that the SPID is blocking itself. When the I/O request finishes, the first latch is released. Then, the second latch request is granted.
For example, the following conditions may occur:
SPID 55 wants to read a data page that does not exist in the buffer pool.
SPID 55 acquires an EX latch on the page. Because the page does not exist yet in memory, the requested latch mode is EX. The EX latch mode forces other SPIDs that may also want to access the page to wait for the I/O request to finish. The EX latch mode also prevents other SPIDs from issuing a duplicate I/O request for the same page.
SPID 55 issues the I/O request to read the page from disk.
Because SPID 55 wants to read the page, SPID 55 must wait for the I/O request to finish. To wait for the I/O request to finish, SPID 55 tries to acquire another latch that has the shared (SH) latch mode on the same page. Because an EX latch has already been acquired, the SH latch request is blocked, and the SPID is suspended. Because the EX latch that blocks the SH latch request was also acquired by SPID 55, the SPID is temporarily reported as blocking itself.
When the I/O request finishes, the EX latch on the page is released.
The release of the EX latch gives the SH latch to SPID 55.
SPID 55 can now read the page.
Between step 4 and step 5, the sysprocesses table indicates that SPID 55 is blocked by itself together with a waittype of PAGEIOLATCH_XX. In this waittype, XX may be SH, UP, or EX. This behavior indicates that SPID 55 issued an I/O request and SPID 55 is waiting for the I/O request to finish.