Oracle 10 and 11
Buffer Busy Waits usually happen on Oracle 10 and 11 mainly because of insert contention into tables or Indexes. There are a few other rare cases of contention on old style RBS segments, file headers blocks and freelists.
Before Oracle 10 and 11 there was one other major reason which was readers waiting for readers, ie one user does a phyiscal IO of a block into memory and a second user want to read that block. The second user waits until the IO is finished by the first user. Starting in 10g this wait has been given the name "read by other session". Before Oracle 10g this was also a "buffer busy wait".
The easiest way to analyse the bottleneck and find a solution is to use ASH (active session History) available in Oracle 10g with the diagnostics pack license or using Simulated ASH for free or using a product like DB Optimizer.
Data block class, which can be found in ASH, is the most important piece of information in analysing buffer busy waits. If we know the block class we can determine what kind of bottleneck:
How do we find the block class? With a quick query on the ASH data like:
If we find that CLASS=datablock, then we will want more information to diagnose, such as the object type "OTYPE" , object name and what kind of tablespace the object is stored in. The following query provides that information:
and the output looks like
Oracle 7, 8 and 9
Before Oracle 10, buffer busy waits also happened because IO blocking another user wanting to do the same IO. On Oracle 9, the main reasons for buffer busy waits are
1) IO read contention (only Oracle 9i and below)
On version 8 and 9, the p3 value has a different meaning. Instead of meaning the block type (which is the best thing to know) it means the kind of buffer busy wait. There are only two values that matter to us, values in
100 range = read waits (basically just an IO wait)
200 range = write contetion (same as in 10g)
Writers blocking other writers for example while doing inserts either because of no free lists on the table or because everyone is inserting into the same index block.
If you have set up ASH style collection with S-ASH or have a product like DB Optimizer you can run a query like:
And see what kind of buffer busy waits there are and what the objects are:
Often the Current_obj# is -1 so we can't figure out what the object is . There is an alternative method
Because querying DBA_EXTENTS is a slow operation, I made a copy of DBA_EXTENTS which will be faster to query.
This second option of getting the object from P1 and P2 (file and block) should probably be done only with the users consent, because we would have to create a copy of the dba_extent table which might take a long time if it's big.
No ASH ?
If you don't have ASH data you will have to do some guess work.
Block Class (block type)
The first step in finding out the source of buffer busy waits is looking at
File with contention
You can also get an idea of what file contains the object with the buffer busy waits by looking at:
Object with contention
Starting in version 9i there is the table
Why do buffer busy waits happen?
To put it most succinctly, buffer busy waits happen because two users want to change a block at the same time. Two users can change the same block, or even same row "at the same time" ie without committing, but that's different from the actual operation of modifying the block. The modification on the block in RAM, or computer memory, can only be done by one process at at time in order to avoid memory corruptions. Different users can modify different blocks at the same time but only one user or process can modify a the same block at a time.
In order to really understand what's going on we have to take a look at how Oracle manages memory and block access and modifications.
Here is the layout of
Above is a diagram shows some of the essential parts of Oracle in regards to performance tuning.
In the machine memory are
The SGA is composed of (among other things)
What's important for understanding buffer busy waits is how the buffer cache is managed. Here is view of the buffer cache with more components:
In order to access a block, a user (shadow process) has to get a latch (cache buffer chains latch) which protects buckets or linked lists of buffer headers. Once the header desired if found the latch is released. The buffer headers point to the actual data block in memory. Before modifying a block in memory a user has to lock the buffer header. The buffer header is locked any time a modification is made whether it is reading a block into memory or modifying a block that is already in memory. Usually the header is locked only for a brief amount of time but when there is a lot of concurrent access the buffer header can become a bottleneck.
BBW when readling data - read by other sessionA buffer busy can happen on oracle 7,8 and 9 when one user is reading a block into memory and a second user wants to read that block. Instead of the second user trying to read that block into memory as well, they just wait for the first user to finish. Starting in Oracle 10, this kind of wait was renames "read by other session"
BBW on insert
If multiple concurrent users are inserting into a table that doesn't have free lists or is not in an ASSM tablespace then all users will end up inserting into the same block, the first one on the free list and this block will become the hot block
by adding free lists or moving the table to an ASSM tablespace we will alleviate the bottleneck.
Multiple free lists:
The other option is ASSM or Automatic Segment Space Management which is set at the tablespace level.
In this case free block information is kept in Level 1 BMB (or bitmapped blocks). These Level 1 BMBs are chosen by a hash on the users process ID thus distributing the inserts across the table.
The inserts would look something like this (somewhat exaggerated drawing)
the ASSM BMB blocks take up more space in the table , about 1 extra block for every 16 data blocks and there is overhead first looking in the header/level 3 BMB block then going to the Level 2 then level 1 and finally to the datablock but all in all ASSM is worth reduced costs of management verses free lists.
Identifying and creating ASSM tablespaces
Which tablespaces are ASSM or not?
creating an ASSM tablespace:
BBW on index (because of insert)
If users are inserting data that has a rising key value, especially a monotonically rising value, then all the new inserts will have to update the leading edge leaf block of the index and with high concurrent inserts this can cause buffer busy waits.
Hash partition the index
IF block class > 18 it's an old style RBS segment
IF the block is of class > 18, the there will be no object name, so we have to look it up ourselves to be sure:
move to new AUM or Automatic Undo Mangement
BBW on a file header
The ASH data has two different fields that indicate the file # and block # when the wait is a buffer busy wait.
For a buffer busy wait
File # = p1 *and* File # = current_file#
Block # = P2 *and* Block # = current_block#
if p1 != current_file# or p2 != current_block# then use p1 and p2. They are more reliable.
Notice P1 != BLOCKN (BLOCKN is CURRENT_BLOCK#) and P2 != FN (FN is CURRENT_FILE#)
The real file # is P1 =202 and block # is P2 which is 2
In my database I only had 10 files, so what is this file# 202?!
If you are getting buffer busy waits on the file header block for a tempfile (datafile in a temporary tablespace) then try increasing the "next extent" size in the temporary tablespace.
This wait can happen when lots of extents are being allocated in the temporary tablespace.
What Would ADDM do?
Interstingly enough the ADDM page doesn't show the new load that has recently come on the system but the analysis is there. I clicked on the next to bottom line in the page, "Read and write contention on database blocks was consuming significant database time.
Here are the outputs for the different scenarios.
inserts into a table contention
inserts into a table with contention on index
The database below is maxed out. There is more demand for CPU than there is CPU. There is only a small amount of Buffer Busy Waits, but even with that small about we can drill down into the BBW and get all the details. By clicking on "buffer busy wait" under details we can see what SQL was running int the BBWs, what sessions were running into BBWs and we can get all the info we want in order to be able to analyze the BBW under the "Analysis" tab:
Wait Event Docs >