Shulou(Shulou.com)06/01 Report--

Free buffer waits

The official website explains:

This wait event indicates that a server process was unable to find a free buffer and has posted the database writer to make free buffers by writing out dirty buffers. A dirty buffer is a buffer whose contents have been modified. Dirty buffers are freed for reuse when DBWR has written the blocks to disk.

Explanation:

This wait event indicates that the server process cannot find the available buffer and creates an idle buffer by writing out a dirty buffer.

Dirty buffer refers to those blocks whose contents have been modified. When DBWR writes the blocks to disk, it can release dirty buffers for reuse.

Causes:

DBWR may not be keeping up with writing dirty buffers in the following situations:

The I/O system is slow.

There are resources it is waiting for, such as latches.

The buffer cache is so small that DBWR spends most of its time cleaning out buffers for server processes.

The buffer cache is so big that one DBWR process is not enough to free enough buffers in the cache to satisfy requests.

Actions:

If this event occurs frequently, then examine the session waits for DBWR to see whether there is anything delaying DBWR.

If this event occurs frequently, check if the session waits for DBWR to see if there is any delayed DBWR.

Explanation:

When a session reads blocks of data from disk into memory, it needs to find free memory space to store those blocks

This wait occurs when there is no free space in memory.

When a session is doing a consistent read, it is necessary to construct a pre-image of the data block at some point (p_w_picpath)

At this point, you need to apply for memory to store these newly constructed data blocks, but such available memory blocks cannot be found in memory.

When a serious free buffer waits wait event occurs in the database, the possible reasons are:

(1) database buffer cache is too small

(2) insufficient free space, such as too much dirty data in memory, and DBWR cannot write these dirty data to disk in time to free up space.

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