[20171128]rman Input or output Memory Buffers.txt

[20171128]rman Input or output Memory Buffers.txt

--//Input Memory Buffers如何測試,不清楚.不過找到一本電子書.摘要如下:
Oracle RMAN 11g Backup and Recovery.pdf

作者:Robert G. Freeman Matthew Hart
頁數:689
出版社:Mc graw hill
出版號: ISBN: 978-0-07-162861-7
        MHID: 0-07-162861-4

RMAN in Memory P80

    RMAN builds buffers in memory through which it streams data blocks for potential backup. This
memory utilization counts against the total size of the PGA and, sometimes, the SGA. There are
two kinds of memory buffers. Input buffers are the buffers that are filled with data blocks read
from files that are being backed up. Output buffers are the buffers that are filled when the
memory-to-memory write occurs to determine whether a particular block needs to be backed up.
When the output buffer is filled, it is written to the backup location. The memory buffers differ
depending on whether you are backing up to or restoring from disk or tape. Figure 2-3 illustrates
input and output buffer allocation. It illustrates a backup of two datafiles being multiplexed into
a single backup set.
        
Input Memory Buffers
    When you are backing up the database, the size and number of input memory buffers depend on
the exact backup command being executed. Primarily, they depend on the number of files being
multiplexed into a single backup. Multiplexing refers to the number of files that will have their
blocks backed up to the same backup piece. To keep the memory allocation within reason, the
following rules are applied to the memory buffer sizes based on the number of files being backed
up together:

■ If the number of files going into the backup set is four or less, then RMAN allocates four
   buffers per file at 1MB per buffer. The total will be 16MB or less.
■ If the number of files going into the backup set is greater than four, but no greater than
   eight, then each file gets four buffers, each of 512KB. This ensures that the total remains
   at 16MB or less.
■ If the number of files being multiplexed is greater than eight, then RMAN allocates four
   buffers of size 128KB. This ensures that each file being backed up will account for 512KB
   of buffer memory.

    Bear in mind that these memory amounts are on a per-channel basis. So, if you allocate two
channels to back up a database with 32 datafiles, for instance, then RMAN will load-balance the
files between the two channels and may not end up with 16 files per channel. If some files are
significantly larger than others, you may end up with only 8 files going into one backup set and
24 files going into the other. If this were the case, then the buffers for the first channel with 8 files
would allocate 16MB of memory for input buffers (four buffers multiplied by 512KB each, multiplied
by 8 files), and the second channel would allocate 12MB of memory buffers (512KB per file
multiplied by 24 files).
    You can use the following query to monitor the size of buffers on a per-file basis while the
backup is running:

SELECT set_count, device_type, type, filename, buffer_size, buffer_count, open_time, close_time
  FROM v$backup_async_io
 ORDER BY set_count, type, open_time, close_time;

--//注意:原來有這個問題!下次備份注意看buffer_size!

Output Buffers When Backing Up to Disk P81
    In addition to input buffers, RMAN allocates output buffers, depending on what the output source
is. If you are backing up to disk, then RMAN allocates output buffers that must fill up with data blocks
from the input buffers before being flushed to the backup piece on your file system. Per channel,
there will be four output buffers, each of which is 1MB. So, the memory footprint per channel will
always be 4MB.

Output Memory Buffers When Backing Up to Tape P81
    Memory allocation is different when backing up to tape, to account for the slower I/O rates
that we expect from tape devices. When you are backing up to or restoring from tape, RMAN
allocates four buffers per channel process, each of which is 256KB, so that the total memory
footprint per channel is 1MB.

--//這也和我前面的測試一樣,我使用as copy備份,單個資料檔案每個通道4個輸出快取,而每個快取1M,一個通道是4M.