|
|
| Re: Doumentation of wait events [message #346950 is a reply to message #346933] |
Wed, 10 September 2008 03:22   |
 |
Alessandro Rossi
Messages: 166
Registered: September 2008
Location: Rome
|
Senior Member |
|
|
It was just a case where they said the same thing using different words. Probably the two parts are written by different authors with different approaches to the subject.
The first one uses more general terms like address, pointer and buffer. The second one uses more typical terms for a database like file_id and block.
To be clear:
P1 stands for file_id of the resource the db is reading from. That is the id of a datafile you can find in dba_data_files.
P2 stands for the block_id where the read process begins ( remember that direct path reads are reads of consecutive blocks in a file ). That is a block_id you can find in dba_extents.
P3 stand for number of blocks to read during the event.
Mind that none of them is wrong. It's just a difference in the point of view. A pointer, in a relational database, must be a reference to a primary key. A block_id is the logical address of a particular database block.
The dba of the oldest I/O in the context should be the the first database address ( so the block_id ) read as first ( so oldest ).
Bye Alessandro
[Updated on: Wed, 10 September 2008 03:25] Report message to a moderator |
|
|
|
|
|
| Re: Doumentation of wait events [message #347249 is a reply to message #347079] |
Thu, 11 September 2008 03:31 |
|
Alessandro Rossi
Messages: 166
Registered: September 2008
Location: Rome
|
Senior Member |
|
|
As you can see in the Performance Tuning Guide, | Quote: | when a session is reading buffers from disk directly into the PGA (opposed to the buffer cache in SGA), it waits on this event.
|
It's called direct path read because it is a process that reads data directly on the disk without accessing the buffer cache of the SGA.
Disk access is more expensive than memory access and the most of its cost is caused by latency times (time occurring between a read request and the moment when data starts becoming effectively available to read). For this reason, in order to minimize the number of requests (and then the latency waits on disk), the direct read process is able to read consecutive blocks at each request.
The fact that a direct path read is a process that reads consecutive blocks is just the consequence of an optimized implementation, but it could be done also with a slower process that reads only one block at once.
As you can see, always in the Performance Tuning Guide,| Quote: | This happens in the following situations:
- The sorts are too large to fit in memory and some of the sort data is written out directly to disk. This data is later read back in, using direct reads.
- Parallel slaves are used for scanning data.
- The server process is processing buffers faster than the I/O system can return the buffers. This can indicate an overloaded I/O system.
|
Bye Alessandro
[Updated on: Thu, 11 September 2008 04:29] Report message to a moderator |
|
|
|
| Re: Doumentation of wait events [message #347557 is a reply to message #346933] |
Fri, 12 September 2008 02:23 |
harrysmall3
Messages: 109
Registered: April 2008
Location: Massachusetts
|
Senior Member |
|
|
Thanx Allesandro - Correct, I should've done some reference reading as I would have found out I was thinking about the sequential read waits in regards to index relationship.
Very much appreciated for gathering the text quotes for me and your
explanation!
Best Regards
Harry
|
|
|
|
] 
Blog
Search
Help
Members
Register
Login
Home
