Oracle Wait event free buffer waits

Free buffer waits

Website Explanation:

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

Explain:

This wait event indicates that the server process was unable to find the available buffer and created the free buffer by writing out the dirty buffer.

Dirty buffer refers to the blocks whose contents are modified, and when DBWR writes blocks to the disk, the dirty buffers can be freed for reuse.

causes:    

DBWR May is keeping up with writing dirty buffers in the following situations:

• The I/O system is slow.

• There is resources it's waiting for, such as latches.

• The buffer cache is so small this DBWR spends most of it time cleaning out buffers for server processes.

• The buffer cache is so big it 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 whether the session waits for DBWR to see if there is any delay DBWR.

Explain:

When a session reads chunks of data from disk into memory, it needs to find free memory space to hold the blocks.
This wait occurs when there is no free space in the memory;
When a session is doing a consistent read, it needs to construct a pre-image (image) of the data block at some point ,
At this point, you need to request memory to hold these newly constructed blocks of data, but you cannot find such a block of available memory in memory.
When there is a more severe free buffer waits wait event in the database, the possible causes are:

(1) database buffer cache is too small,

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

This article is from the "Linux Oracle MariaDB" blog, so be sure to keep this source http://wangergui.blog.51cto.com/8504247/1912951

Oracle Wait event free buffer waits