环境：Oracle 10.2.0.5
现象：RMAN分配多个通道，但实际无法使用到并行。
构建测试用例：

create tablespace dbs_d_test;
alter tablespace dbs_d_test add datafile; --这里是11
alter tablespace dbs_d_test add datafile; --这里是12
alter tablespace dbs_d_test add datafile; --这里是13

alter database datafile 11,12,13 resize 1G;

• 1.RMAN多通道但未用到并行
• 2.备份语句改写使用到并行
• 3.备份方式改变提高效率

1.RMAN多通道但未用到并行

使用RMAN备份

run {
allocate channel c1 device type disk;
allocate channel c2 device type disk;
allocate channel c3 device type disk;

backup as copy datafile 11 format '/tmp/incr/copy11.bak';
backup as copy datafile 12 format '/tmp/incr/copy12.bak';
backup as copy datafile 13 format '/tmp/incr/copy13.bak';

release channel c1;
release channel c2;
release channel c3;
}

使用下面SQL查询长操作：

select inst_id, sid, username, opname, target, sofar, totalwork, sofar * 100 / totalwork from gv$session_longops where sofar < totalwork;

发现上面这种备份写法，虽然分配了多个通道，但实际观察并没有使用到并行。3个文件的备份是串行操作的。这点从上面的长操作中可以看到，同时从RMAN输出日志中同样也可以看到：

RMAN> run {
2> allocate channel c1 device type disk;
3> allocate channel c2 device type disk;
4> allocate channel c3 device type disk;
5> 
6> backup as copy datafile 11 format '/tmp/incr/copy11.bak';
7> backup as copy datafile 12 format '/tmp/incr/copy12.bak';
8> backup as copy datafile 13 format '/tmp/incr/copy13.bak';
9> 
10> release channel c1;
11> release channel c2;
12> release channel c3;
13> }

using target database control file instead of recovery catalog
allocated channel: c1
channel c1: sid=128 instance=jy1 devtype=DISK

allocated channel: c2
channel c2: sid=117 instance=jy1 devtype=DISK

allocated channel: c3
channel c3: sid=129 instance=jy1 devtype=DISK

Starting backup at 29-AUG-18
channel c1: starting datafile copy
input datafile fno=00011 name=+ZHAOJINGYU/jy/datafile/dbs_d_test.615.985387387
output filename=/tmp/incr/copy11.bak tag=TAG20180829T002101 recid=13 stamp=985393279
channel c1: datafile copy complete, elapsed time: 00:00:25
Finished backup at 29-AUG-18

Starting backup at 29-AUG-18
channel c1: starting datafile copy
input datafile fno=00012 name=+ZHAOJINGYU/jy/datafile/dbs_d_test.613.985387391
output filename=/tmp/incr/copy12.bak tag=TAG20180829T002127 recid=14 stamp=985393305
channel c1: datafile copy complete, elapsed time: 00:00:25
Finished backup at 29-AUG-18

Starting backup at 29-AUG-18
channel c1: starting datafile copy
input datafile fno=00013 name=+ZHAOJINGYU/jy/datafile/dbs_d_test.611.985387395
output filename=/tmp/incr/copy13.bak tag=TAG20180829T002153 recid=15 stamp=985393330
channel c1: datafile copy complete, elapsed time: 00:00:25
Finished backup at 29-AUG-18

released channel: c1

released channel: c2

released channel: c3

实际是串行操作，都是用的通道c1，这3个数据文件的copy备份消耗3个25s=75s。

2.备份语句改写使用到并行

改进写法，用到了并行：

run {
allocate channel c1 device type disk;
allocate channel c2 device type disk;
allocate channel c3 device type disk;

backup as copy datafile 11,12,13 format '/tmp/incr/copy_%u.bak';

release channel c1;
release channel c2;
release channel c3;
}

从日志看到：

RMAN> run {
2> allocate channel c1 device type disk;
3> allocate channel c2 device type disk;
4> allocate channel c3 device type disk;
5> 
6> backup as copy datafile 11,12,13 format '/tmp/incr/copy_%u.bak';
7> 
8> release channel c1;
9> release channel c2;
10> release channel c3;
11> }

using target database control file instead of recovery catalog
allocated channel: c1
channel c1: sid=129 instance=jy1 devtype=DISK

allocated channel: c2
channel c2: sid=127 instance=jy1 devtype=DISK

allocated channel: c3
channel c3: sid=119 instance=jy1 devtype=DISK

Starting backup at 29-AUG-18
channel c1: starting datafile copy
input datafile fno=00011 name=+ZHAOJINGYU/jy/datafile/dbs_d_test.615.985387387
channel c2: starting datafile copy
input datafile fno=00012 name=+ZHAOJINGYU/jy/datafile/dbs_d_test.613.985387391
channel c3: starting datafile copy
input datafile fno=00013 name=+ZHAOJINGYU/jy/datafile/dbs_d_test.611.985387395
output filename=/tmp/incr/copy_14tbnq76.bak tag=TAG20180829T002302 recid=16 stamp=985393432
channel c1: datafile copy complete, elapsed time: 00:00:55
output filename=/tmp/incr/copy_15tbnq76.bak tag=TAG20180829T002302 recid=17 stamp=985393432
channel c2: datafile copy complete, elapsed time: 00:00:55
output filename=/tmp/incr/copy_16tbnq76.bak tag=TAG20180829T002302 recid=18 stamp=985393435
channel c3: datafile copy complete, elapsed time: 00:00:55
Finished backup at 29-AUG-18

released channel: c1

released channel: c2

released channel: c3

实际是并行操作，分别用的通道c1、c2、c3，这3个数据文件的copy备份消耗1个55s=55s。
那为什么并行没有成倍增加效率？跟上一篇提到的一样，系统的整体I/O能力达到瓶颈了。所以一味的增加并行度并不总是有意义的。

3.备份方式改变提高效率

如果数据文件很大，但实际使用的并不多，则可以考虑使用备份集的方式，减少备份对空间的占用，一般同时也会加快备份的速度：

run {
allocate channel c1 device type disk;
allocate channel c2 device type disk;
allocate channel c3 device type disk;

backup as compressed backupset datafile 11,12,13 format '/tmp/incr/datafile_%u.bak';

release channel c1;
release channel c2;
release channel c3;
}

从日志可以看到：

RMAN> run {
2> allocate channel c1 device type disk;
3> allocate channel c2 device type disk;
4> allocate channel c3 device type disk;
5> 
6> backup as compressed backupset datafile 11,12,13 format '/tmp/incr/datafile_%u.bak';
7> 
8> release channel c1;
9> release channel c2;
10> release channel c3;
11> }

using target database control file instead of recovery catalog
allocated channel: c1
channel c1: sid=128 instance=jy1 devtype=DISK

allocated channel: c2
channel c2: sid=134 instance=jy1 devtype=DISK

allocated channel: c3
channel c3: sid=116 instance=jy1 devtype=DISK

Starting backup at 29-AUG-18
channel c1: starting compressed full datafile backupset
channel c1: specifying datafile(s) in backupset
input datafile fno=00011 name=+ZHAOJINGYU/jy/datafile/dbs_d_test.615.985387387
channel c1: starting piece 1 at 29-AUG-18
channel c2: starting compressed full datafile backupset
channel c2: specifying datafile(s) in backupset
input datafile fno=00012 name=+ZHAOJINGYU/jy/datafile/dbs_d_test.613.985387391
channel c2: starting piece 1 at 29-AUG-18
channel c3: starting compressed full datafile backupset
channel c3: specifying datafile(s) in backupset
input datafile fno=00013 name=+ZHAOJINGYU/jy/datafile/dbs_d_test.611.985387395
channel c3: starting piece 1 at 29-AUG-18
channel c1: finished piece 1 at 29-AUG-18
piece handle=/tmp/incr/datafile_17tbnqi9.bak tag=TAG20180829T002857 comment=NONE
channel c1: backup set complete, elapsed time: 00:00:02
channel c3: finished piece 1 at 29-AUG-18
piece handle=/tmp/incr/datafile_19tbnqia.bak tag=TAG20180829T002857 comment=NONE
channel c3: backup set complete, elapsed time: 00:00:01
channel c2: finished piece 1 at 29-AUG-18
piece handle=/tmp/incr/datafile_18tbnqi9.bak tag=TAG20180829T002857 comment=NONE
channel c2: backup set complete, elapsed time: 00:00:05
Finished backup at 29-AUG-18

released channel: c1

released channel: c2

released channel: c3

由于我这里这几个文件根本没有业务数据，所以效率提升尤为明显，只需要5s钟就完成了备份。