作者：liumaclean

liumaclean — Thu, 05 Jan 2012 13:13:56 +0000

请见这篇文章《Does Rman Backup benefit from Large Pool?》 http://www.oracledatabase12g.com/archives/does-rman-backup-benefit-from-large-pool.html

作者：roger

roger — Thu, 05 Jan 2012 01:13:17 +0000

to vmcd：笔误了，已纠正!

作者：VMCD

VMCD — Wed, 04 Jan 2012 16:15:04 +0000

从10g开始，实际上默认就是开启了异步IO，换句话说，对于10g以后，默认rman使用的内存是从
large pool中分配的，

—-？

10g 开始async_io 默认是true ，但是是从PGA分配内存的

作者：roger

roger — Wed, 04 Jan 2012 05:40:03 +0000

总结：从10g开始，实际上默认就是开启了异步IO，换句话说，对于10g以后，默认rman使用的内存是从
PGA中分配的，如果使用dbwr_io_salves 非0方式，将使用sga内存，即large pool，如果large pool没有设置，
那么将从shared pool中分配，10g的官方文档说的十分的详细，如下：

总结：从10g开始，实际上默认就是开启了异步IO，换句话说，对于10g以后，默认rman使用的内存是从

PGA中分配的，如果使用dbwr_io_salves 非0方式，将使用sga内存，即large pool，如果large pool没有设置，

那么将从shared pool中分配，10g的官方文档说的十分的详细，如下：

Step 1: Remove RATE Parameters from Configured and Allocated Channels
 
The RATE parameter on a channel is intended to reduce, rather than increase, backup throughput, so that more disk bandwidth is available for other database operations.
 
If your backup is not streaming to tape, then make sure that the RATE parameter is not set on the ALLOCATE CHANNEL or CONFIGURE CHANNEL commands.
 

Step 2: If You Use Synchronous Disk I/O, Set DBWR_IO_SLAVES
 
If and only if your disk does not support asynchronous I/O, then try setting the DBWR_IO_SLAVES initialization parameter to a nonzero value. Any nonzero value for DBWR_IO_SLAVES causes a fixed number (four) of disk I/O slaves to be used for backup and restore, which simulates asynchronous I/O. If I/O slaves are used, I/O buffers are obtained from the SGA. The large pool is used, if configured. Otherwise, the shared pool is used.
 
Note: By setting DBWR_IO_SLAVES, the database writer processes will use slaves as well. You may need to increase the value of the PROCESSES initialization parameter.
 

Step 3: If You Fail to Allocate Shared Memory, Set LARGE_POOL_SIZE
 
Set this initialization parameter if the database reports an error in the alert.log stating that it does not have enough memory and that it will not start I/O slaves. The message should resemble the following:
 ksfqxcre: failure to allocate shared memory means sync I/O will be used whenever async I/O to file not supported natively
 
When attempting to get shared buffers for I/O slaves, the database does the following:
 •
If LARGE_POOL_SIZE is set, then the database attempts to get memory from the large pool. If this value is not large enough, then an error is recorded in the alert log, the database does not try to get buffers from the shared pool, and asynchronous I/O is not used.
 
•
If LARGE_POOL_SIZE is not set, then the database attempts to get memory from the shared pool.
 
•
If the database cannot get enough memory, then it obtains I/O buffer memory from the PGA and writes a message to the alert.log file indicating that synchronous I/O is used for this backup.
 

The memory from the large pool is used for many features, including the shared server (formerly called multi-threaded server), parallel query, and RMAN I/O slave buffers. Configuring the large pool prevents RMAN from competing with other subsystems for the same memory.
 
Requests for contiguous memory allocations from the shared pool are usually small (under 5 KB) in size. However, it is possible that a request for a large contiguous memory allocation can either fail or require significant memory housekeeping to release the required amount of contiguous memory. Although the shared pool may be unable to satisfy this memory request, the large pool is able to do so. The large pool does not have a least recently used (LRU) list; the database does not attempt to age memory out of the large pool.
 
Use the LARGE_POOL_SIZE initialization parameter to configure the large pool. To see in which pool (shared pool or large pool) the memory for an object resides, query V$SGASTAT.POOL.
 
The formula for setting LARGE_POOL_SIZE is as follows:
 LARGE_POOL_SIZE =  number_of_allocated_channels * 
                   (16 MB + ( 4 *  size_of_tape_buffer ) )

Step 1: Remove RATE Parameters from Configured and Allocated Channels

The RATE parameter on a channel is intended to reduce, rather than increase, backup throughput, so that more disk bandwidth is available for other database operations.

If your backup is not streaming to tape, then make sure that the RATE parameter is not set on the ALLOCATE CHANNEL or CONFIGURE CHANNEL commands.

Step 2: If You Use Synchronous Disk I/O, Set DBWR_IO_SLAVES

If and only if your disk does not support asynchronous I/O, then try setting the DBWR_IO_SLAVES initialization parameter to a nonzero value. Any nonzero value for DBWR_IO_SLAVES causes a fixed number (four) of disk I/O slaves to be used for backup and restore, which simulates asynchronous I/O. If I/O slaves are used, I/O buffers are obtained from the SGA. The large pool is used, if configured. Otherwise, the shared pool is used.

Note: By setting DBWR_IO_SLAVES, the database writer processes will use slaves as well. You may need to increase the value of the PROCESSES initialization parameter.

Step 3: If You Fail to Allocate Shared Memory, Set LARGE_POOL_SIZE

Set this initialization parameter if the database reports an error in the alert.log stating that it does not have enough memory and that it will not start I/O slaves. The message should resemble the following:

ksfqxcre: failure to allocate shared memory means sync I/O will be used whenever async I/O to file not supported natively

When attempting to get shared buffers for I/O slaves, the database does the following:

•

If LARGE_POOL_SIZE is set, then the database attempts to get memory from the large pool. If this value is not large enough, then an error is recorded in the alert log, the database does not try to get buffers from the shared pool, and asynchronous I/O is not used.

•

If LARGE_POOL_SIZE is not set, then the database attempts to get memory from the shared pool.

•

If the database cannot get enough memory, then it obtains I/O buffer memory from the PGA and writes a message to the alert.log file indicating that synchronous I/O is used for this backup.

The memory from the large pool is used for many features, including the shared server (formerly called multi-threaded server), parallel query, and RMAN I/O slave buffers. Configuring the large pool prevents RMAN from competing with other subsystems for the same memory.

Requests for contiguous memory allocations from the shared pool are usually small (under 5 KB) in size. However, it is possible that a request for a large contiguous memory allocation can either fail or require significant memory housekeeping to release the required amount of contiguous memory. Although the shared pool may be unable to satisfy this memory request, the large pool is able to do so. The large pool does not have a least recently used (LRU) list; the database does not attempt to age memory out of the large pool.

Use the LARGE_POOL_SIZE initialization parameter to configure the large pool. To see in which pool (shared pool or large pool) the memory for an object resides, query V$SGASTAT.POOL.

The formula for setting LARGE_POOL_SIZE is as follows:

LARGE_POOL_SIZE = number_of_allocated_channels *

(16 MB + ( 4 * size_of_tape_buffer ) )

从官方文档的描述来看，其实也验证了一种说法，large pool适当设置的大一点，还是有好处的，因为对于MTS，parallel query等情况下都会用到large pool。)

1	从官方文档的描述来看，其实也验证了一种说法，large pool适当设置的大一点，还是有好处的，因为对于MTS，parallel query等情况下都会用到large pool。)

作者：kk

kk — Wed, 04 Jan 2012 05:23:47 +0000

好

《rman备份与large_pool_size的关系》的评论

作者：liumaclean

作者：roger

作者：VMCD

作者：roger

作者：kk