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

1. SMON: system monitoring process

1. It is mainly the data that has been submitted, but the data file has not yet been written to the power outage. When it is restarted, smon will restore it.

a. Perform a roll forward to write data that has been written to the redo log file but not yet to the data file (using the scn number to identify the submission record)

b. Open the database

c. Roll back uncommitted transactions

two。 You can also maintain disk space.

a. Reclaim or merge connected free areas in data files

b. Release temporary section

II. PMON: process monitoring process

1. When a process crashes, PMON will be responsible for the following cleanup

a. Roll back the user's current transaction

b. Release all table-and row-level locks added by the user

c. Release all other resources of the user, etc.

3. CKPT: checkpoint and checkpoint process

1. Improve system efficiency and database consistency, and introduce checkpoint events. CKPT is when DBWR writes the data in the cache to the data file

In theory, it is not necessary.

Full recovery can be guaranteed through redo log and scn, and it was introduced to improve efficiency, because all the changes up to the checkpoint have been written to the number

According to the file, when restoring

The redo log before the checkpoint is no longer needed, so the instance recovery is accelerated. CKPT will write the checkpoint to the header of all relevant data files, and the checkpoint number, redo log sequence number, archive log name and minimum and maximum scn number will be written into the control file. Because ckpt will generate a large number of IO operations, checkpoints can not be generated frequently, preferably more than 20 minutes.

4. ARCn: archive log process

1. When one set of redo log files is full, another group is written, and when the last group is full, the first group is written, which is called log switching.

two。 If no archiving is opened, recovery will not be possible after overwriting.

3. If an archive is opened, the arch process is used to write to the archive log file, and oracle ensures that the archive operation is completed in a set of redo logs

This group of logs will not be reused before

5. DBWn: database writing process

1.11g you can only have 10 processes before 0909, and 36 after 11g, and set the number through db_writer_processes.

If the number is not specified at startup

Then the parameter is determined by the number of cpu.

two。 DBWR writes data from the high-speed buffer to the data file when the following occurs

a. When the number of dirty buffers exceeds the set limit

b. The set time interval has arrived.

c. When a process needs a high-speed buffer, but there is no free high-speed buffer space

d. When a check (checkpoint) occurs

e. A table is deleted or truncated truncate,drop

f. A tablespace is set to read-only

g. When the tablespace is backed up online

h. When the tablespace is set to offline

6. LGWR: redo log writing process

1. Write the record order of the redo log buffer to the redo log file. Note here that it is written sequentially, so the IO is relatively small.

2. How commit statements work. Quick commit technology is used here.

a. The server process writes the submitted record together with the resulting scn number to the redo log buffer.

B.LGWR writes the records in the redo log buffer to the redo log file so that recovery is guaranteed.

C.oracle notifies the user (process) that submission has been completed

e. The server process modifies the state of the relevant data in the database cache and releases resources and unlocks (at this point the data may not be written to

To the data file, they are marked as dirty data, and then written to the data file by DBWR)

3. LGWR writes to the redo log file in the following cases

a. Transaction commit

b. The record in the redo log buffer exceeds 1 of the buffer capacity.

Before c.DBWR writes the blocks in the database cache to the data file

d. Every 3 seconds

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.