|  設定PDB快照的最大數量

使用MAX_PDB_SNAPSHOTS引數設定PDB快照最大快照數，首先登入到需要設定的PDB中，通過檢視CDB_PROPERTIES查詢當前MAX_PDB_SNAPSHOT值，可以使用alter pluggabledatabase/alter database更改MAX_PDB_SNAPSHOTS值。  

如下為設定PDB snapshot的例子

下面語句將當前pdb的快照數設定為7

想要刪除所有的快照，我們只需要執行下面一條命令即可。  

| 配置自動生成PDB快照

預設情況下都是手動生成快照，使用SNAPSHOT MODE EVERY子句自動產生PDB快照。

更改自動建立模式為每24小時自動生成PDB快照

下面是每2小時生成快照的例子

首先登入到CDB根目錄。以下語句從名為cdb1_pdb1的現有PDB建立cdb1_pdb3，並將其配置為每2小時自動獲取快照：

| 手工建立PDB快照

使用ALTER PLUGGABLE DATABASE/CREATE PLUGGABLE DATABASE手動生成快照。使用ALTER PLUGGABLE DATABASE SNAPSHOT語句需要滿足的要求。

• CDB必須是LOCAL UNDO模式

• 使用者必須有許可權建立和刪除PDB

下面查詢出當前PDB的快照  

如果你不指定快照名，系統自動生成唯一名稱的快照，下面即為不指定名字建立快照

| 刪除PDB快照

將MAX_PDB_SNAPSHOTS設定為0，即刪除所有PDB快照，同時也關閉此功能。同樣也可以使用ALTER PLUGGABLE DATABASE刪除當前的快照。

| 從snapshot建立PDB

| 基於SCN或時間點生產快照庫

在閱讀官方文件時，看到生成PDB可以基於SCN，我心中不禁暗喜，當再仔細查閱一番則不然，簡直萬念俱寂、黯然銷魂。提供的基於SCN生成的快照庫只能是基於當前生成快照時間點的SCN，似乎有些雞肋。

與recover database until scn指定scn方式開啟資料庫不同，pdb snapshot Carousel指定scn生成的PDB只能是建立snapshot時的固定scn，並且生成快照與snapshot生成pdb的過程中佔據了大量的物理空間與IO資源。

那麼問題來了,說了這麼多關於PDB snapshot Carousel的優勢與好處,是否就說明已經找到了最佳的解決方案呢?

Hold on.

凡事有利也有弊,同樣PDB snapshot Carousel也不能倖免。

快照數量受限

雖然能夠做到Carousel輪轉,但畢竟數量有限,較為死板。在使用者的實際測試環境中條件複雜多變,會限制測試環境的搭建.

佔據大量物理空間與IO資源

對於當前使用者動輒幾十T的資料量，在生成snapshot與snapshot建立pdb的過程中，消耗的時間和佔據的物理資源是巨大的。

拘泥固定時間點

對於PDBsnapshot Carouse而言,只能基於固定時間，即快照點建立，無法根據測試需要靈活指定時間點。面對不斷變化的測試需求，如此死板的模式明顯是不可能滿足要求的。

版本受限

最後是客觀條件的限制,利用新特性需要滿足是在18C的環境下且啟用了PDB功能,但目前很多使用者依然是10G\11G\12C的環境,因此也僅僅只能遠觀而無法上手,對於實際工作很難產生推動作用。

說到這,是否還有更好的解決辦法呢?

答案是必須有!

16 Administering a PDB Snapshot Carousel

您可以為指定的PDB配置一個PDB快照庫，手動或自動建立快照，並設定快照的最大數量。

This section contains the following topics:

• About PDB Snapshot Carousel
  A  PDB snapshot  is a point-in-time copy of a PDB. You can create snapshots manually using the  SNAPSHOT  clause of  CREATE PLUGGABLE DATABASE  (or  ALTER PLUGGABLE DATABASE ), or automatically using the  EVERY interval  clause. A PDB snapshot carousel is a library of snapshots.
• Setting the Maximum Number of Snapshots in a PDB Snapshot Carousel
  You can set the maximum number of PDB snapshots for a PDB.
• Configuring Automatic PDB Snapshots
  Configure a PDB for automatic snapshots by using the  SNAPSHOT MODE EVERY  clause when creating or altering a PDB.
• Creating PDB Snapshots Manually
  To create a PDB snapshot manually, specify the  SNAPSHOT snapshot_name  clause in  ALTER PLUGGABLE DATABASE  or  CREATE PLUGGABLE DATABASE .
• Dropping a PDB Snapshot
  You can drop a PDB snapshot by running an  ALTER PLUGGABLE DATABASE  statement with the  DROP SNAPSHOT  clause.

Parent topic:   Administering a Multitenant Environment

16.1 About PDB Snapshot Carousel

PDB快照是PDB的時間點副本。您可以使用create PLUGGABLE DATABASE(或ALTER PLUGGABLE DATABASE)的SNAPSHOT子句手動建立快照，也可以使用each interval子句自動建立快照

. A PDB snapshot carousel 快照庫.

This section contains the following topics:

• Purpose of PDB Snapshot Carousel
  A PDB snapshot carousel is a useful way of maintaining a library of recent PDB copies for point-in-time recovery and cloning.
• How PDB Snapshot Carousel Works
  The carousel for a specific PDB is a circular library of copies for this PDB.
• User Interface for PDB Snapshot Carousel
  The  SNAPSHOT MODE  clause controls creation of snapshots, and determines whether creation is manual, automatic, or disabled.

Parent topic:   Administering a PDB Snapshot Carousel

16.1.1 Purpose of PDB Snapshot Carousel

PDB快照庫是維護最近的PDB副本庫以便進行時間點恢復和克隆的一種有用方法。
克隆用於開發和測試的PDBs

在一個典型的開發用例中，您克隆一個產品PDB來進行測試。當CDB處於ARCHIVELOG模式和本地撤銷模式時，源生產PDB可以以讀/寫模式開啟，並在操作過程中充分發揮功能。這種技術被稱為熱克隆。當SCN完成時，熱克隆在事務上與源PDB一致of the  ALTER PLUGGABLE DATABASE ... OPEN  statement.

例如，當名為pdb1_prod的生產PDB處於開啟狀態並正在使用時，您將建立一個名為pdb1_test_master的可重新整理克隆。然後配置pdb1_test_master來每天建立自動快照。當您需要新的PDBs進行測試時，請建立任何快照的完整克隆，然後使用  CREATE PLUGGABLE DATABASE ... SNAPSHOT COPY .

下圖顯示了從4月5日拍攝的PDB快照建立的克隆pdb1_test_full1。圖中顯示了從pdb1_test_full1建立的三個PDBs快照副本。

Figure 16-1 Automatic Snapshots of a Refreshable Clone PDB

Description of "Figure 16-1 Automatic Snapshots of a Refreshable Clone PDB"

使用PDB快照庫進行時間點恢復

典型的策略是每天在同一時間對PDB進行快照。另一種策略是在資料載入之前手動進行快照。在這兩種情況下，PDB快照庫都允許您使用任何可用快照恢復PDB。

例如，名為pdb1_prod的銷售歷史記錄PDB每天在12:01生成一個自動快照。在4/9週一下午的daily data load中，您意外載入了錯誤的資料，損壞了PDB。您可以基於Monday 4/9快照建立一個新的生產PDB，刪除損壞的PDB，然後重試資料載入。

Figure 16-2 Restore a Production PDB Using a Snapshot

Description of "Figure 16-2 Restore a Production PDB Using a Snapshot"

See Also:

• " About Cloning a PDB "

• Oracle Database SQL Language Reference  for  CREATE PLUGGABLE DATABASE  syntax and semantics

Parent topic:   About PDB Snapshot Carousel

16.1.2 How PDB Snapshot Carousel Works

The carousel 對於特定的PDB，是該PDB的一個迴圈複製庫。

資料庫根據需要或自動地在庫中建立連續的副本。當達到快照限制時，資料庫將重寫最舊的快照。

This section includes the following topics:

• Contents of a PDB Snapshot
  The data files of the PDB snapshot reside in an archive file on the file system.
• Contents of a PDB Snapshot Carousel
  The PDB snapshot carousel is the set of all existing snapshots for a PDB.

Parent topic:   About PDB Snapshot Carousel

16.1.2.1 Contents of a PDB Snapshot

PDB快照的資料檔案駐留在檔案系統上的歸檔檔案中。
不包括存檔的重做日誌檔案。通常，歸檔檔案儲存在與PDB的資料檔案相同的目錄中。對於系統生成的快照名稱，snap_以惟一識別符號為字首，該識別符號包含快照SCN。該歸檔檔案的副檔名是.pdb。

資料庫使用隱式的using snapshot子句獲取快照。在使用建立可插入資料庫快照獲取快照時，不能指定快照複製子句。

Note:

使用USING snapshot子句建立的PDB快照和使用snapshot copy子句建立的快照複製PDB是兩種不同型別的快照，它們不應該混淆。USING SNAPSHOT子句建立一個不需要物化的完整PDB。快照複製子句建立了一個稀疏的PDB，如果您想刪除它所基於的PDB快照，就必須將其物化。

雖然PDB快照carousel中的快照始終是完整的PDB，但是您可以在carousel中獲取快照的快照副本。在這種情況下，快照副本是一個稀疏克隆。

See Also:

" How a Snapshot Copy PDB Differs from a Standard Clone "

Parent topic:   How PDB Snapshot Carousel Works

16.1.2.2 Contents of a PDB Snapshot Carousel

PDB快照庫是一個PDB的所有現有快照的集合。
max_pdb_snapshot屬性指定庫中允許的最大快照數量。當前設定在CDB_PROPERTIES檢視中是可見的。

下圖顯示了cdb1_pdb1的庫。在本例中，資料庫每24小時自動獲取一個快照，維護一組8個快照。建立了前8個快照之後，每個新快照都會替換舊快照。例如，星期二4/10快照替換星期一4/2快照;星期三4/11快照取代星期二4/3快照;等等。

Figure 16-3 PDB Snapshot Carousel

Description of "Figure 16-3 PDB Snapshot Carousel"

Parent topic:   How PDB Snapshot Carousel Works

16.1.3 User Interface for PDB Snapshot Carousel

The  SNAPSHOT MODE  clause controls creation of snapshots, and determines whether creation is manual, automatic, or disabled.

CREATE PLUGGABLE DATABASE Statement

要為PDB設定快照模式，請在快照模式中使用以下值之一 clause of  ALTER PLUGGABLE DATABASE  or  CREATE PLUGGABLE DATABASE :

• MANUAL

  This clause, which is the default, enables the creation of manual snapshots of the PDB. To create a snapshot on demand, specify the  SNAPSHOT snapshot_name  clause in an  ALTER PLUGGABLE DATABASE  or  CREATE PLUGGABLE DATABASE  statement.

• EVERY snapshot_interval [MINUTES|HOURS]

  This clause enables the automatic creation of snapshots after an interval of time. The following restrictions apply to the interval specified:

  • The minutes value must be less than  3000 .

  • The hours value must be less than  2000 .

  The database assigns each automatic snapshot a system-generated name. Note that manual snapshots are also supported for the PDB when  EVERY  is specified.

• NONE

  This clause disables snapshot creation for the PDB.

See Also:

• " About Point-in-Time Cloning of PDB Snapshots "

• Oracle Database SQL Language Reference  for the syntax and semantics of the  SNAPSHOT  clause

MAX_PDB_SNAPSHOTS Database Property

要設定PDB的最大快照數量，請在ALTER PLUGGABLE DATABASE或CREATE PLUGGABLE DATABASE中指定max_pdb_snapshot屬性。屬性的預設值是8，這也是最大值。當建立了允許的最大快照數量時，資料庫將清除最舊的快照。CDB_PROPERTIES檢視顯示了max_pdb_snapshot的設定。

See Also:

Oracle Database SQL Language Reference  for the syntax of the  ALTER PLUGGABLE DATABASE statement

DBA_PDB_SNAPSHOTS and DBA_PDBS

The following data dictionary views provide snapshot information:

• The  DBA_PDB_SNAPSHOTS  view metadata about the snapshots, including name, creation SCN, creation time, and file name.

• The  DBA_PDBS  view has a  SNAPSHOT_MODE  and  SNAPSHOT_INTERVAL  column.

See Also:

Oracle Database Reference to learn about  DBA_PDB_SNAPSHOTS  and  DBA_PDBS

Parent topic:   About PDB Snapshot Carousel

16.2 Setting the Maximum Number of Snapshots in a PDB Snapshot Carousel

You can set the maximum number of PDB snapshots for a PDB.

max_pdb_snapshot資料庫屬性設定PDB快照旋轉木馬中每個PDB的最大快照數量。預設最大值是8。不能將屬性設定為大於8的數字。

Prerequisites

The PDB must be open in read/write mode.

To set the maximum number of PDB snapshots for a PDB:

1. In SQL*Plus, ensure that the current container is the PDB for which you want to set the limit.

2. Optionally, query  CDB_PROPERTIES  for the current setting of the  SET MAX_PDB_SNAPSHOTS  property.

3. Run an  ALTER PLUGGABLE DATABASE  or  ALTER DATABASE  statement with the  SET MAX_PDB_SNAPSHOTS  clause.

Example 16-1 Setting the Maximum Number of PDB Snapshots for a PDB

The following query shows the maximum in the carousel for  cdb1_pdb1  (sample output included):

SET LINESIZE 150COL CON_ID FORMAT 99999COL PROPERTY_NAME FORMAT a17COL PDB_NAME FORMAT a9COL VALUE FORMAT a3COL DESCRIPTION FORMAT a43SELECT r.CON_ID, p.PDB_NAME, PROPERTY_NAME,PROPERTY_VALUE AS value, DESCRIPTIONFROM   CDB_PROPERTIES r, CDB_PDBS pWHERE  r.CON_ID = p.CON_IDAND    PROPERTY_NAME LIKE 'MAX_PDB%' ORDER BY PROPERTY_NAME;CON_ID PDB_NAME  PROPERTY_NAME     VAL DESCRIPTION------ --------- ----------------- --- -------------------------------------------3 CDB1_PDB1 MAX_PDB_SNAPSHOTS 8   maximum number of snapshots for a given PDB

The following SQL statement sets the maximum number of PDB snapshots for the current PDB to 7:

ALTER PLUGGABLE DATABASE SET MAX_PDB_SNAPSHOTS=7;

Example 16-2 Dropping All Snapshots in a PDB Snapshot Carousel

To drop all snapshots in a PDB snapshot carousel, set the  MAX_PDB_SNAPSHOTS  database property to   (zero), as shown in the following statement:

ALTER PLUGGABLE DATABASE SET MAX_PDB_SNAPSHOTS=0;

This technique is faster than executing  ALTER PLUGGABLE DATABASE ... DROP SNAPSHOTsnapshot_name  for every snapshot.

See Also:

" About Container Access in a CDB "

Parent topic:   Administering a PDB Snapshot Carousel

16.3 Configuring Automatic PDB Snapshots

在建立或更改PDB時，通過使用快照模式EVERY子句為自動快照配置PDB。
預設情況下，PDB是為手動快照配置的。

Prerequisites

Note the following prerequisites for the  ALTER PLUGGABLE DATABASE SNAPSHOT  statement:

• The CDB must be in local undo mode.

• The administrator must have the privileges to create a PDB and drop a PDB.

To configure automatic snapshots when altering a PDB:

1. In SQL*Plus, log in as an administrator to the PDB whose snapshot mode you intend to configure.

2. Optionally, query  DBA_PDBS  to determine the current snapshot mode.

3. Run  ALTER PLUGGABLE DATABASE  with the  SNAPSHOT MODE EVERY interval clause, specifying either  MINUTES  or  HOURS .

To configure automatic snapshots when creating a PDB:

1. In SQL*Plus, log in as an administrator to the CDB root or application root.

2. Optionally, query  DBA_PDBS  to determine the current snapshot mode.

3. Run  CREATE PLUGGABLE DATABASE  with the  SNAPSHOT MODE EVERY interval clause, specifying either  MINUTES  or  HOURS .

Example 16-3  為現有PDB每天配置一個自動快照

本例假設您已登入到要更改其快照模式的PDB。查詢資料字典，確認PDB目前處於手動模式(包括示例輸出):

SELECT SNAPSHOT_MODE "S_MODE", SNAPSHOT_INTERVAL/60 "SNAP_INT_HRS" FROM DBA_PDBS;S_MODE SNAP_INT_HRS------ ------------MANUAL

Change the snapshot mode to every 24 hours:

ALTER PLUGGABLE DATABASE SNAPSHOT MODE EVERY 24 HOURS;

Confirm the change to automatic mode:

SELECT SNAPSHOT_MODE "S_MODE", SNAPSHOT_INTERVAL/60 "SNAP_INT_HRS" FROM DBA_PDBS;S_MODE SNAP_INT_HRS------ ------------AUTO             24

Example 16-4 Creating a PDB That Takes Snapshots Every Two Hours

This example assumes that you are logged in to the CDB root. The following statement creates  cdb1_pdb3  from an existing PDB named  cdb1_pdb1 , and configures it to take snapshots automatically every 2 hours:

CREATE PLUGGABLE DATABASE cdb1_pdb3 FROM cdb1_pdb1FILE_NAME_CONVERT=('cdb1_pdb1','cdb1_pdb3')SNAPSHOT MODE EVERY 120 MINUTES;

See Also:

• " Creating a Point-in-Time Clone of a PDB Snapshot: Example "

• " Configuring a CDB to Use Local Undo Mode "

Parent topic:   Administering a PDB Snapshot Carousel

16.4 Creating PDB Snapshots Manually

To create a PDB snapshot manually, specify the  SNAPSHOT snapshot_name  clause in  ALTER PLUGGABLE DATABASE  or  CREATE PLUGGABLE DATABASE .

Prerequisites

Note the following prerequisites for the  ALTER PLUGGABLE DATABASE SNAPSHOT  statement:

• The CDB must be in local undo mode.

• The administrator must have the privileges to create a PDB and drop a PDB.

To create a PDB snapshot:

1. In SQL*Plus, log in as an administrator to the PDB whose snapshot you intend to create.

2. Optionally, query  DBA_PDBS  to confirm that the snapshot mode is not set to  NONE .

3. Run an  ALTER PLUGGABLE DATABASE  statement with the  SNAPSHOT  clause.

Example 16-5 Creating a Snapshot with a User-Specified Name

以下SQL語句建立兩個cdb1_pdb1的PDB快照，一個在週三資料載入之前，一個在週三資料載入之後:

ALTER PLUGGABLE DATABASE SNAPSHOT cdb1_pdb1_b4wedload;-- data loadALTER PLUGGABLE DATABASE SNAPSHOT cdb1_pdb1_afwedload;

The following query of  DBA_PDB_SNAPSHOTS  shows the locations of two snapshots of the PDB named  cdb1_pdb1  (sample output included):

SET LINESIZE 150COL CON_NAME FORMAT a10COL SNAPSHOT_NAME FORMAT a20COL SNAP_SCN FORMAT 9999999COL FULL_SNAPSHOT_PATH FORMAT a45SELECT CON_ID, CON_NAME, SNAPSHOT_NAME,SNAPSHOT_SCN AS snap_scn, FULL_SNAPSHOT_PATHFROM   DBA_PDB_SNAPSHOTSORDER BY SNAP_SCN;CON_ID CON_NAME   SNAPSHOT_NAME        SNAP_SCN FULL_SNAPSHOT_PATH------- ---------- -------------------- -------- ---------------------------------------------3 CDB1_PDB1  CDB1_PDB1_B4WEDLOAD   2962078 /disk1/oracle/dbs/snap_3489077498_2962078.pdb3 CDB1_PDB1  CDB1_PDB1_AFWEDLOAD   2962938 /disk1/oracle/dbs/snap_3489077498_2962938.pdb

If you do not specify a PDB snapshot name, then the database generates a unique name.

Example 16-6 建立具有系統指定名稱的快照

下面的SQL語句建立一個快照，但沒有指定名稱:

ALTER PLUGGABLE DATABASE SNAPSHOT;

The following sample query shows that the database assigned the snapshot a name prefixed with  SNAP_ :

SET LINESIZE 150COL CON_NAME FORMAT a10COL SNAPSHOT_NAME FORMAT a25COL SNAP_SCN FORMAT 9999999COL FULL_SNAPSHOT_PATH FORMAT a45SELECT CON_ID, CON_NAME, SNAPSHOT_NAME,SNAPSHOT_SCN AS snap_scn, FULL_SNAPSHOT_PATHFROM   DBA_PDB_SNAPSHOTSORDER BY SNAP_SCN;CON_ID CON_NAME   SNAPSHOT_NAME             SNAP_SCN FULL_SNAPSHOT_PATH------- ---------- ------------------------- -------- ---------------------------------------------3 CDB1_PDB1  CDB1_PDB1_B4WEDLOAD        2962078 /disk1/oracle/dbs/snap_3489077498_2962078.pdb3 CDB1_PDB1  CDB1_PDB1_AFWEDLOAD        2962938 /disk1/oracle/dbs/snap_3489077498_2962938.pdb3 CDB1_PDB1  SNAP_3489077498_960130367  2993525 /disk1/oracle/dbs/snap_3489077498_2993525.pdb

See Also:

• " About Container Access in a CDB "

• " Configuring a CDB to Use Local Undo Mode "

Parent topic:   Administering a PDB Snapshot Carousel

16.5 Dropping a PDB Snapshot

You can drop a PDB snapshot by running an  ALTER PLUGGABLE DATABASE  statement with the  DROP SNAPSHOT  clause.

To drop all PDB snapshots based on a PDB, set the  MAX_PDB_SNAPSHOTS  property in the PDB to  (zero).

To drop a PDB snapshot:

1. In SQL*Plus, ensure that the current container is the PDB from which you created the PDB snapshot.

2. Run an  ALTER PLUGGABLE DATABASE  statement with the  DROP SNAPSHOT  clause.

Example 16-7 Dropping a PDB Snapshot

The following SQL statement drops a PDB snapshot named  sales_snap :

ALTER PLUGGABLE DATABASE DROP SNAPSHOT sales_snap;

See Also:

" About Container Access in a CDB "

Parent topic:   Administering a PDB Snapshot Carousel

ORACLE 18C新特性-PDB快照輪播（SNAPSHOT CAROUSEL）

SQL> SELECT r.CON_ID, p.PDB_NAME, PROPERTY_NAME,
  2      PROPERTY_VALUE AS value, DESCRIPTION
  3      FROM   CDB_PROPERTIES r, CDB_PDBS p
  4      WHERE  r.CON_ID = p.CON_ID
  5      AND    PROPERTY_NAME LIKE 'MAX_PDB%'
  6      AND    description like 'maximum%'
  7      ORDER BY PROPERTY_NAME;
    CON_ID PDB_NAME             PROPERTY_NAME        VALUE                DESCRIPTION
---------- -------------------- -------------------- -------------------- ----------------------------------------------------------------------
         3 ORCLPDB1             MAX_PDB_SNAPSHOTS    8                    maximum number of snapshots for a given PDB

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

SQL> alter pluggable database snapshot mode manual;
Pluggable database altered.
SQL> alter pluggable database snapshot;
Pluggable database altered.

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

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

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.

SQL> create pluggable database ORCLPDB3 from ORCLPDB1 file_name_convert=('ORCLPDB1','ORCLPDB3') snapshot mode every 5 minutes;
Pluggable database created.

[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

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

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