Oracle 18c推出的一个新功能就是PDB快照轮播，最多可以创建8个pdb的快照，而这8个构成的一组快照就被称作快照轮播。当达到第8个快照限制时，新的快照就会覆盖最旧的快照，就像redo log一样。PDB的快照主要作用是当出现了问题可以闪回到快照的时间点，另一个功能是基于快照来查询历史数据。
SQL> SELECT r.CON_ID, p.PDB_NAME, PROPERTY_NAME,
PROPERTY_VALUE AS value, DESCRIPTION
FROM CDB_PROPERTIES r, CDB_PDBS p
WHERE r.CON_ID = p.CON_ID
AND PROPERTY_NAME LIKE 'MAX_PDB%'
AND description like 'maximum%'
ORDER BY PROPERTY_NAME;
CON_ID PDB_NAME PROPERTY_NAME VALUE DESCRIPTION
---------- -------------------- -------------------- -------------------- ----------------------------------------------------------------------
3 ORCLPDB1 MAX_PDB_SNAPSHOTS
maximum number of snapshots for a given PDB
默认情况下是手动创建快照，我们可以把它改成自动创建。
SQL> select snapshot_mode,snapshot_interval/60 from dba_pdbs; SNAPSH SNAPSHOT_INTERVAL/60 ------ -------------------- MANUAL SQL> alter pluggable database snapshot mode every 4 hours; Pluggable database altered. SQL> select snapshot_mode,snapshot_interval/60 from dba_pdbs; SNAPSH SNAPSHOT_INTERVAL/60 ------ -------------------- AUTO 4
要返回手动模式，输入mode manual即可。
SQL> alter pluggable database snapshot mode manual; Pluggable database altered. SQL> alter pluggable database snapshot; Pluggable database altered.
手动创建快照，可以自己指定名字，也可以使用系统自动生成的名字。通过查看DBA_PDB_SNAPSHOTS视图，可以找到快照存放的路径及创建快照的SCN号。
SQL> SELECT CON_ID, CON_NAME, SNAPSHOT_NAME, SNAPSHOT_SCN AS snap_scn, FULL_SNAPSHOT_PATH FROM DBA_PDB_SNAPSHOTS ORDER BY SNAP_SCN;
CON_ID CON_NAME SNAPSHOT_NAME SNAP_SCN FULL_SNAPSHOT_PATH
---------- -------------------- ------------------------- ---------- ---------------------------------------------------------------------------
3 ORCLPDB1 SNAP_2953839490_989014667 2821702 /opt/oracle/oradata/ORCLCDB/ORCLPDB1/snap_2953839490_2821702.pdb
3 ORCLPDB1 PDB_SNAP 2823303 /opt/oracle/oradata/ORCLCDB/ORCLPDB1/snap_2953839490_2823303.pdb
3 ORCLPDB1 SNAP_2953839490_989015970 2823679 /opt/oracle/oradata/ORCLCDB/ORCLPDB1/snap_2953839490_2823679.pdb
[oracle@oracle-18c-vagrant ORCLPDB1]$ ls -lsh
total 1.3G
4.0K drwxr-x---. 8 oracle oinstall 4.0K Oct 8 22:39 ORCLCDB
171M -rw-r--r--. 1 oracle oinstall 171M Oct 8 22:18 snap_2953839490_2821702.pdb
171M -rw-r--r--. 1 oracle oinstall 171M Oct 8 22:38 snap_2953839490_2823303.pdb
171M -rw-r--r--. 1 oracle oinstall 171M Oct 8 22:40 snap_2953839490_2823679.pdb
371M -rw-r-----. 1 oracle oinstall 371M Oct 8 22:39 sysaux01.dbf
271M -rw-r-----. 1 oracle oinstall 271M Oct 8 22:39 system01.dbf
56K -rw-r-----. 1 oracle oinstall 63M Oct 1 23:21 temp01.dbf
101M -rw-r-----. 1 oracle oinstall 101M Oct 8 22:39 undotbs01.dbf
5.1M -rw-r-----. 1 oracle oinstall 5.1M Oct 8 22:39 users01.dbf
如果要删除快照，则有两种方法，第一种就是使用alter pluggable database删除，第二种是修改max_pdb_snapshots参数，当max_pdb_snapshots参数设置成0时，将删除全部快照。
SQL> alter pluggable database drop snapshot SNAP_2953839490_989015970;
Pluggable database altered.
SQL> SELECT CON_ID, CON_NAME, SNAPSHOT_NAME, SNAPSHOT_SCN AS snap_scn, SNAPSHOT_TIME, FULL_SNAPSHOT_PATH FROM DBA_PDB_SNAPSHOTS ORDER BY SNAP_SCN;
CON_ID CON_NAME SNAPSHOT_NAME SNAP_SCN SNAPSHOT_TIME FULL_SNAPSHOT_PATH
---------- -------------------- ------------------------- ---------- ------------- ---------------------------------------------------------------------------
3 ORCLPDB1 SNAP_2953839490_989014667 2821702 1539008272 /opt/oracle/oradata/ORCLCDB/ORCLPDB1/snap_2953839490_2821702.pdb
3 ORCLPDB1 PDB_SNAP 2823303 1539009503 /opt/oracle/oradata/ORCLCDB/ORCLPDB1/snap_2953839490_2823303.pdb
SQL> alter pluggable database set max_pdb_snapshots=0;
Pluggable database altered.
SQL> SELECT CON_ID, CON_NAME, SNAPSHOT_NAME, SNAPSHOT_SCN AS snap_scn, SNAPSHOT_TIME, FULL_SNAPSHOT_PATH FROM DBA_PDB_SNAPSHOTS ORDER BY SNAP_SCN;
no rows selected
演示了上述功能之后，还有一个功能，就是我们可以根据快照创建pdb。
SQL> alter pluggable database set max_pdb_snapshots=8;
Pluggable database altered
SQL> alter pluggable database snapshot pdb_snap;
Pluggable database altered.
SQL> SELECT CON_ID, CON_NAME, SNAPSHOT_NAME, SNAPSHOT_SCN AS snap_scn, SNAPSHOT_TIME, FULL_SNAPSHOT_PATH FROM DBA_PDB_SNAPSHOTS ORDER BY SNAP_SCN;
CON_ID CON_NAME SNAPSHOT_NAME SNAP_SCN SNAPSHOT_TIME FULL_SNAPSHOT_PATH
---------- -------------------- ------------------------- ---------- ------------- ---------------------------------------------------------------------------
3 ORCLPDB1 PDB_SNAP 2825283 1539010328 /opt/oracle/oradata/ORCLCDB/ORCLPDB1/snap_2953839490_2825283.pdb
SQL> !
[oracle@oracle-18c-vagrant ~]$ mkdir -p /opt/oracle/oradata/ORCLCDB/ORCLPDB2
SQL> create pluggable database ORCLPDB2 from ORCLPDB1 using snapshot PDB_SNAP create_file_dest='/opt/oracle/oradata/ORCLCDB/ORCLPDB2';
create pluggable database ORCLPDB2 from ORCLPDB1 using snapshot PDB_SNAP create_file_dest='/opt/oracle/oradata/ORCLCDB/ORCLPDB2'
*
ERROR at line 1:
ORA-65040: operation not allowed from within a pluggable database
SQL> alter session set container=CDB$ROOT;
Session altered.
SQL> create pluggable database ORCLPDB2 from ORCLPDB1 using snapshot PDB_SNAP create_file_dest='/opt/oracle/oradata/ORCLCDB/ORCLPDB2';
Pluggable database created.
SQL> show pdbs;
CON_ID CON_NAME OPEN MODE RESTRICTED
---------- ------------------------------ ---------- ----------
2 PDB$SEED READ ONLY NO
3 ORCLPDB1 READ WRITE NO
5 ORCLPDB2 MOUNTED
SQL> alter pluggable database ORCLPDB2 open;
Pluggable database altered.
这样我们就根据快照创建了一个新的PDB，假设当前PDB出现了数据覆盖，我们就可以使用这个历史快照的PDB来恢复数据。
当然还有一个很好用的功能就是我们在创建pdb的过程中把他设置成15分钟产生一个快照。当然配置这个功能有一个先决条件：CDB必须处于本地undo模式.
SQL> create pluggable database ORCLPDB3 from ORCLPDB1 file_name_convert=('ORCLPDB1','ORCLPDB3') snapshot mode every 5 minutes;
Pluggable database created.
可以看到每隔5分钟自动创建一个快照
[oracle@oracle-18c-vagrant ORCLPDB3]$ ls -lrt snap* -rw-r--r--. 1 oracle oinstall 179134736 Oct 8 23:13 snap_2091710291_2830531.pdb -rw-r--r--. 1 oracle oinstall 179167158 Oct 8 23:18 snap_2091710291_2832545.pdb -rw-r--r--. 1 oracle oinstall 179285979 Oct 8 23:23 snap_2091710291_2833104.pdb -rw-r--r--. 1 oracle oinstall 179284365 Oct 8 23:28 snap_2091710291_2833687.pdb -rw-r--r--. 1 oracle oinstall 179302205 Oct 8 23:33 snap_2091710291_2834965.pdb -rw-r--r--. 1 oracle oinstall 179315484 Oct 8 23:38 snap_2091710291_2836695.pdb -rw-r--r--. 1 oracle oinstall 115724288 Oct 8 23:43 snap_2091710291_2837222.pdb
我们可以做个实验测试一下。在生成快照之前创建一个表插入点数据，这里上一次快照是23:43分。
SQL> alter session set container=ORCLPDB3;
Session altered.
SQL> create table a1 as select * from dba_objects;
Table created.
SQL> select count(1) from a1;
COUNT(1)
----------
72897
[oracle@oracle-18c-vagrant ORCLPDB3]$ ls -lrt snap*
-rw-r--r--. 1 oracle oinstall 179134736 Oct 8 23:13 snap_2091710291_2830531.pdb
-rw-r--r--. 1 oracle oinstall 179167158 Oct 8 23:18 snap_2091710291_2832545.pdb
-rw-r--r--. 1 oracle oinstall 179285979 Oct 8 23:23 snap_2091710291_2833104.pdb
-rw-r--r--. 1 oracle oinstall 179284365 Oct 8 23:28 snap_2091710291_2833687.pdb
-rw-r--r--. 1 oracle oinstall 179302205 Oct 8 23:33 snap_2091710291_2834965.pdb
-rw-r--r--. 1 oracle oinstall 179315484 Oct 8 23:38 snap_2091710291_2836695.pdb
-rw-r--r--. 1 oracle oinstall 179353508 Oct 8 23:43 snap_2091710291_2837222.pdb
-rw-r--r--. 1 oracle oinstall 5767168 Oct 8 23:48 snap_2091710291_2839217.pdb
SQL> SELECT CON_ID, CON_NAME, SNAPSHOT_NAME, SNAPSHOT_SCN AS snap_scn, SNAPSHOT_TIME, FULL_SNAPSHOT_PATH FROM DBA_PDB_SNAPSHOTS ORDER BY SNAP_SCN;
CON_ID CON_NAME SNAPSHOT_NAME SNAP_SCN SNAPSHOT_TIME FULL_SNAPSHOT_PATH
---------- -------------------- ------------------------- ---------- ------------- ---------------------------------------------------------------------------
7 ORCLPDB3 SNAP_2091710291_989017984 2830531 1539011592 /opt/oracle/oradata/ORCLCDB/ORCLPDB3/snap_2091710291_2830531.pdb
7 ORCLPDB3 SNAP_2091710291_989018284 2832545 1539011887 /opt/oracle/oradata/ORCLCDB/ORCLPDB3/snap_2091710291_2832545.pdb
7 ORCLPDB3 SNAP_2091710291_989018584 2833104 1539012188 /opt/oracle/oradata/ORCLCDB/ORCLPDB3/snap_2091710291_2833104.pdb
7 ORCLPDB3 SNAP_2091710291_989018884 2833687 1539012489 /opt/oracle/oradata/ORCLCDB/ORCLPDB3/snap_2091710291_2833687.pdb
7 ORCLPDB3 SNAP_2091710291_989019184 2834965 1539012787 /opt/oracle/oradata/ORCLCDB/ORCLPDB3/snap_2091710291_2834965.pdb
7 ORCLPDB3 SNAP_2091710291_989019484 2836695 1539013087 /opt/oracle/oradata/ORCLCDB/ORCLPDB3/snap_2091710291_2836695.pdb
7 ORCLPDB3 SNAP_2091710291_989019784 2837222 1539013386 /opt/oracle/oradata/ORCLCDB/ORCLPDB3/snap_2091710291_2837222.pdb
7 ORCLPDB3 SNAP_2091710291_989020084 2839217 1539013686 /opt/oracle/oradata/ORCLCDB/ORCLPDB3/snap_2091710291_2839217.pdb
23点48分产生了新的快照，此时我们使用新的快照做一个克隆，登陆到ORCLPDB4数据库后，我们会发现是有数据的。
SQL> alter session set container=CDB$ROOT; Session altered. SQL> create pluggable database ORCLPDB4 from ORCLPDB3 using snapshot SNAP_2091710291_989020084 create_file_dest='/opt/oracle/oradata/ORCLCDB/ORCLPDB4'; Pluggable database created SQL> alter pluggable database ORCLPDB4 open; Pluggable database altered. SQL> alter session set container=ORCLPDB4; Session altered. SQL> select count(1) from a1; COUNT(1) ---------- 72897
这个功能对于测试人员非常有用，例如移动电信现在有BCV环境，专门用于经分抽数及测试，而这个BCV环境是底层同步的一个克隆。使用18c数据库，我们就可以直接创建快照，然后使用快照克隆一个新数据库形成BCV环境给经分抽数及测试使用了。
总结
以上所述是小编给大家介绍的Oracle 18c新特性-PDB快照轮播（Snapshot Carousel），希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对网站的支持！
