What is hot block in Oracle?
What is hot block in Oracle?
A hot block is one that is accessed disproportionately often. One example would be an index block where the index is generated via a sequence; the database will need to write to the same block with the next value over and over again.
How do you fix latch cache buffer chains?
Solving the cache buffers chain latch Where there are hot blocks, you can relieve contention by doing one of the following: Splitting the buffer pool into multiple pools. Altering PCTFREE/PCTUSED to allow fewer rows per block, thus reducing contention on a certain block.
What is cache buffer chain in Oracle?
Description. The cache buffers chains latches are used to protect a buffer list in the buffer cache. These latches are used when searching for, adding, or removing a buffer from the buffer cache. Blocks in the buffer cache are placed on linked lists (cache buffer chains) which hang off a hash table.
What causes buffer busy waits in Oracle?
Description. This wait happens when a session wants to access a database block in the buffer cache but it cannot because the buffer is busy. Another session is modifying the block and the contents of the block are in flux during the modification.
What is DB file sequential read in Oracle?
“The db file sequential read Oracle metric event signifies that the user process is reading buffers into the SGA buffer cache and is waiting for a physical I/O call to return. This call differs from a scattered read, because a sequential read is reading data into contiguous memory space.
What is GC buffer busy acquire?
The ‘gc buffer busy acquire’ wait event occurs when a session is trying to access a block. But that session is waiting for another session to finish opening that block. The other session is on the same instance as the waiting session.
What is latch free wait event Oracle?
The Oracle latch free wait event occurs when a session needs a latch, tries to get the latch, but fails because someone else has it. So, it sleeps with a wait on latch free, wakes up and tries again.
How do you reduce buffer busy waits?
The main way to reduce buffer busy waits is to reduce the total I/O on the system. This can be done by tuning the SQL to access rows with fewer block reads (i.e., by adding indexes). Even if we have a huge db_cache_size, we may still see buffer busy waits, and increasing the buffer size won’t help.
How do I fix free buffer waits?
User Action. Sometimes the easy solution is to increase the buffer cache to allow for more free blocks. This works in many cases, but if the application is generating a sustained amount of dirty blocks then increasing the buffer cache may only help or delay the problem but not solve it.
How do you avoid db file sequential read waits?
There are two things you can do to minimize the db file sequential read waits:
- Optimize the SQL statement that initiated most of the waits by reducing the number of physical and logical reads.
- Reduce the average wait time.
What is the difference between db file sequential and scattered reads?
A db file sequential read is an event that shows a wait for a foreground process while doing a sequential read from the database. A db file scattered read is the same type of event as “db file sequential read”, except that Oracle will read multiple data blocks. Multi-block reads are typically used on full table scans.
What are the top 5 wait events in Oracle?
Oracle Wait Events and Solution
- Buffer Busy Wait:
- Db File Sequential Read :
- Enq: TX – row lock contention :
- Enq: TM – index contention :
- Row Cache Lock Wait :
- Read by Other Session :
What is Gc current block 2way?
Block-Oriented RAC Waits. – “gc current block 2-way” – Indicates that a current block was sent from either the resource master requiring 1 message and 1 transfer. – “gc current block 3-way” – Indicates that a current block was forwarded to a 3rd node from which it was sent, requiring 2 messages and one block transfer.
What is latch hit in Oracle?
The Latch Hit Ratio Oracle metric is the ratio of the total number of latch misses to the number of latch gets for all latches. A low value for this ratio indicates a latching problem, whereas a high value is generally good.
What is Oracle latch?
A latch is a low-level internal lock used by Oracle to protect memory structures. Latches are similar to short duration locks that protect critical bits of code. This wait indicates that the process is waiting for a latch that is currently busy (held by another process).
How do I resolve log file sync wait event?
The Oracle “log file sync” wait event is triggered when a user session issues a commit (or a rollback)….If slow I/O, investigate the following:
- Reduce contention on existing disks.
- Put log files on faster disks.
- Put alternate redo logs on different disks to minimize the effect archive processes (log files switches).
How do you fix buffer busy waits?
What is free buffer?
The free buffer wait is an Oracle metric that has generally been considered to be related to the database writer (DBWR). Generally, the free buffer wait event indicates that dirty blocks are not being cleared fast enough by the DBWR and some server process is reporting that it is unable to find a free buffer.
How can I improve db file sequential read?
How do I fix a scattered read database?
The cure for the db file scattered read waits is the same as the one for the db file sequential read: find and tune the SQL statement that clocked the most time on the event. The goal of tuning is also to minimize the number of logical and physical I/Os. Follow the examples in the db file sequential read event section.
What is a hot block in SQL?
A hot block is one that is accessed disproportionately often. One example would be an index block where the index is generated via a sequence; the database will need to write to the same block with the next value over and over again. This can make parallel operations difficult.
What is the current block size in Oracle 9i?
Starting in Oracle9i Database, Oracle supports multiple block sizes. If the current block size is 16K, you may move the table or recreate the index in a tablespace with an 8K block size. This too will negatively impact full table scans operations.
How to tell if a hot block is in the cache?
If you don’t see those waits, you are in the “miss” stage. A hot block is going to manifest itself as a cache buffers chains latch — it is doubtful you would be performing IO to read in a hot block, it would be definition be in the cache. for some details.
What is the best way to handle hot blocks?
As previously mentioned, hot blocks are an application issue. Find out why the application has to repeatedly access the same block (or blocks) and check if there is a better alternative. As for the workaround, the idea is to spread the hot blocks across multiple cache buffers chains latches.
