|Oracle® Database Backup and Recovery User's Guide
12c Release 1 (12.1)
|PDF · Mobi · ePub|
This chapter describes how to perform setup and configuration tasks. This chapter contains the following topics:
See Also:"RMAN Channels" for a conceptual overview of configured and allocated channels, and Oracle Database Backup and Recovery Reference for
Whether you allocate channels manually or use automatic channel allocation, you can use channel commands and options to control behavior. Table 6-1 summarizes the ways in which you can control channel behavior. Unless noted, all channel parameters are supported in both
CONFIGURE CHANNEL and
ALLOCATE CHANNEL commands.
|Type of Channel Control||Commands|
Limit I/O bandwidth consumption
Limit backup sets and pieces
Channel parallel backup and restore operations
Connection settings for database instances
Specify which instance performs an operation with the
In addition to configuring parameters that apply to all channels of a particular type, you can also configure parameters that apply to one specific channel. Run the
n command (where
n is a positive integer less than 255) to configure a specific channel.
When manually numbering channels, you must specify one or more channel options (for example,
FORMAT) for each channel. When you use that specific numbered channel in a backup, the configured settings for that channel are used instead of the configured generic channel settings.
Configure specific channels by number when it is necessary to control the parameters set for each channel separately. This technique is necessary in the following situations:
When running an Oracle Real Application Clusters (Oracle RAC) database in which individual nodes do not have access to the full set of backups. Each channel must be configured with a node-specific connect string so that all backups are accessible by at least one channel.
When using a media manager that requires different
PARMS settings on each channel.
See Also:Oracle Real Application Clusters Administration and Deployment Guide to learn about RMAN backups in an Oracle RAC environment
In this example, you want to send disk backups to two different disks. Configure disk channels as follows:
CONFIGURE DEFAULT DEVICE TYPE TO disk; # backup goes to disk CONFIGURE DEVICE TYPE disk PARALLELISM 2; # two channels used in parallel CONFIGURE CHANNEL 1 DEVICE TYPE DISK FORMAT '/disk1/%U' # 1st channel to disk1 CONFIGURE CHANNEL 2 DEVICE TYPE DISK FORMAT '/disk2/%U' # 2nd channel to disk2 BACKUP DATABASE; # backup - first channel goes to disk1 and second to disk2
Assume a different case in which you have two tape drives and want each drive to use tapes from a different tape media family. Configure your default output device and default tape channels as shown in Example 6-1 to use parallel database backups.
CONFIGURE DEFAULT DEVICE TYPE TO sbt; # backup goes to sbt CONFIGURE DEVICE TYPE sbt PARALLELISM 2; # two sbt channels allocated by default # Configure channel 1 to pool named first_pool CONFIGURE CHANNEL 1 DEVICE TYPE sbt PARMS 'ENV=(OB_MEDIA_FAMILY=first_pool)'; # configure channel 2 to pool named second_pool CONFIGURE CHANNEL 2 DEVICE TYPE sbt PARMS 'ENV=(OB_MEDIA_FAMILY=second_pool)'; BACKUP DATABASE; # first stream goes to 'first_pool' and second to 'second_pool'
In Example 6-1, the backup data is divided between the two tape devices. Each configured channel backs up approximately half the total data.
PARALLELISM setting is not constrained by the number of specifically configured channels. For example, if you back up to 20 different tape devices, then you can configure 20 different SBT channels, each with a manually assigned number (from 1 to 20) and each with a different set of channel options. In such a situation, you can set
PARALLELISM to any value up to the number of devices, in this instance 20.
RMAN always numbers parallel channels starting with
1 and ending with the
PARALLELISM setting. For example, if the default device is SBT and parallelism is set to 3, then RMAN names the channels as follows:
ORA_SBT_TAPE_1 ORA_SBT_TAPE_2 ORA_SBT_TAPE_3
RMAN always uses the name
ORA_SBT_TAPE_n even if you configure
sbt (not the synonymous
sbt_tape). RMAN always allocates the number of channels specified in
PARALLELISM, using specifically configured channels if you have configured them and generic channels if you have not. If you configure specific channels with numbers higher than the parallelism setting, then this setting prevents RMAN from using them.
See Also:"RMAN Channels" to learn about channels
"Configuring the Environment for RMAN Backups" explains the basics for configuring RMAN to make backups. This section explains more advanced configuration options. This section contains the following topics:
In tape backups, it is possible for a multiplexed backup set to span multiple tapes, which means that blocks from each data file in the backup set are written to multiple tapes. If one tape of a multivolume backup set fails, then you lose the data on all the tapes rather than just one. If a backup is not a multisection backup, then a backup set always includes a whole data file rather than a partial data file. You can use
MAXSETSIZE to specify that each backup set fits on one tape rather than spanning multiple tapes.
CONFIGURE MAXSETSIZE command limits the size of backup sets created on a channel. This
CONFIGURE setting applies to any channel, whether manually allocated or configured, when the
BACKUP command is used to create backup sets. The default value is given in bytes and is rounded down to the lowest kilobyte value.
The value set by the
CONFIGURE MAXSETSIZE command is a default for the given channel. You can override the configured
MAXSETSIZE value by specifying a
MAXSETSIZE option for an individual
Assume that you issue the following commands at the RMAN prompt:
CONFIGURE DEFAULT DEVICE TYPE TO sbt; CONFIGURE CHANNEL DEVICE TYPE sbt PARMS 'ENV=(OB_MEDIA_FAMILY=first_pool)'; CONFIGURE MAXSETSIZE TO 7500K; BACKUP TABLESPACE users; BACKUP TABLESPACE tools MAXSETSIZE 5G;
The results are as follows:
The backup of the
users tablespace uses the configured SBT channel and the configured default
MAXSETSIZE setting of
The backup of the
tools tablespace uses the
MAXSETSIZE setting of
5G specified in the
Backup piece size is an issue when it exceeds the maximum file size permitted by the file system or media management software. You can use the
MAXPIECESIZE parameter of the
CHANNEL command to limit the size of backup pieces.
For example, to limit the backup piece size to 2 gigabytes or less, you can configure the automatic
DISK channel as follows and then run
CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 2G; BACKUP DATABASE;
Note:In version 2.0 of the media management API, media management vendors can specify the maximum size of a backup piece that can be written to their media manager. RMAN respects this limit regardless of the settings that you configure for
See Also:Oracle Database Backup and Recovery Reference to learn about the
CONFIGURE CHANNEL ... MAXPIECESIZEcommand
You can use the
CONFIGURE ... BACKUP COPIES command to specify how many copies of each backup piece are created on the specified device type for the specified type of file. This type of backup is known as a duplexed backup set. The
CONFIGURE settings for duplexing only affect backups of data files, control files, and archived logs into backup sets, and do not affect image copies.
Note:A control file autobackup is never duplexed.
RMAN can duplex backups to either disk or tape, but cannot duplex backups to tape and disk simultaneously. When backing up to tape, ensure that the number of copies does not exceed the number of available tape devices. The following examples show possible duplexing configurations:
# Makes 2 disk copies of each data file and control file backup set # (autobackups excluded) CONFIGURE DATAFILE BACKUP COPIES FOR DEVICE TYPE DISK TO 2; # Makes 3 copies of every archived redo log backup to tape CONFIGURE ARCHIVELOG BACKUP COPIES FOR DEVICE TYPE sbt TO 3;
To return a
COPIES configuration to its default value, run the same
CONFIGURE command with the
CLEAR option, as in the following example:
CONFIGURE DATAFILE BACKUP COPIES FOR DEVICE TYPE sbt CLEAR;
COPIES is set to
1 for each device type.
Note:If you do not want to create a persistent copies configuration, then you can specify copies with the
SET BACKUP COPIEScommands.
"Multiple Copies of RMAN Backups" for an overview of duplexed backups
"Duplexing Backup Sets" to learn how to create duplexed backups
Oracle Database Backup and Recovery Reference for
Oracle Database Backup and Recovery Reference for
Oracle Database Backup and Recovery Reference for
A tablespace is easy to rebuild, so it is more cost-effective to rebuild it than back it up every day.
A tablespace contains temporary or test data that you do not need to back up.
A tablespace does not change often and therefore should be backed up on a different schedule from other backups.
You can run
CONFIGURE EXCLUDE FOR TABLESPACE to exclude the specified tablespace from the
BACKUP DATABASE command. The exclusion condition applies to any data files that you add to this tablespace in the future.
For example, you can exclude testing tablespaces
example from whole database backups as follows:
CONFIGURE EXCLUDE FOR TABLESPACE cwmlite; CONFIGURE EXCLUDE FOR TABLESPACE example;
If you run the following command, then RMAN backs up all tablespaces in the database except
You can still back up the configured tablespaces by explicitly specifying them in a
BACKUP command or by specifying the
NOEXCLUDE option on a
DATABASE command. For example, you can enter one of the following commands:
BACKUP DATABASE NOEXCLUDE; #backs up database, including cwmlite and example BACKUP TABLESPACE cwmlite, example; # backs up only cwmlite and example
You can disable the exclusion feature for
example as follows:
CONFIGURE EXCLUDE FOR TABLESPACE cwmlite CLEAR; CONFIGURE EXCLUDE FOR TABLESPACE example CLEAR;
RMAN includes these tablespaces in future whole database backups.
Oracle Database Backup and Recovery Reference for
RMAN supports precompression processing and binary compression of backup sets.
Better backup compression ratios are achieved by consolidating the free space in each data block, and setting that free space to binary zeroes. This precompression processing stage has the most benefit for data blocks that have been the subject of many deletes and inserts operations. Conversely, it has no effect on data blocks that are still in their initial loaded state.
OPTIMIZE FOR LOAD option controls precompression processing. By specifying the default,
OPTIMIZE FOR LOAD TRUE, you ensure that RMAN optimizes CPU usage and avoids precompression block processing. By specifying
OPTIMIZE FOR LOAD FALSE, RMAN uses additional CPU resources to perform precompression block processing.
Oracle Database Backup and Recovery Reference for
Oracle Database provides two categories of compression algorithms: a default compression algorithm and a group of compression algorithms available with the Oracle Advanced Compression option. The default algorithm is a standard feature of Oracle Database while the Oracle Advanced Compression option is a separately purchased option.
You can configure the default compression algorithm, which does not require the Oracle Advanced Compression option, with the following syntax:
If you have enabled the Oracle Advanced Compression option, you can choose from the following compression levels:
|Compression Level||Performance Benefits and Trade-Offs|
||Best suited for backups over slower networks where the limiting factor is network speed.|
||Recommended for most environments. Good combination of compression ratios and speed.|
||Least effect on backup throughput.|
The compression ratio generally increases from low to high, with a trade-off of potentially consuming more CPU resources.
Because the performance of the various compression levels depends on the nature of the data in the database, network configuration, system resources and the type of computer system and its capabilities, Oracle cannot document universally applicable performance statistics. Which level is best for your environment depends on how balanced your system is regarding bandwidth into the CPU and the actual speed of the CPU. It is highly recommended that you run tests with the different compression levels on the data in your environment. Choosing a compression level based on your environment, network traffic characteristics (workload), and data set is the only way to ensure that the backup set compression level can satisfy your organization's performance requirements and applicable service level agreements.
See Oracle Database Licensing Information for more information about the Oracle Advanced Compression option.
If you are backing up to tape and your tape device performs its own compression, then do not use both RMAN backup set compression and the media manager vendor's compression. See the discussion of tuning RMAN's tape backup performance in Chapter 23, "Tuning RMAN Performance".
Restoring a compressed backup is performed inline, and does not require decompression.
For improved security, you can configure backup encryption for RMAN backup sets. Encrypted backups cannot be read if they are obtained by unauthorized users.
The V$RMAN_ENCRYPTION_ALGORITHMS view contains a list of encryption algorithms supported by RMAN. If no encryption algorithm is specified, then the default encryption algorithm is 128-bit Advanced Encryption Standard (AES). RMAN encryption requires the
COMPATIBLE initialization parameter at a target database to be at least 10.2.0.
RMAN offers the following encryption modes:
This is the default mode and uses the Oracle software keystore. A keystore is a password-protected container used to store a Transparent Data Encryption (TDE) key. In previous releases, this container was referred to as a wallet.
This mode uses only password protection. You must provide a password when creating and restoring encrypted backups.
This mode requires either the keystore or a password.
Note:Keystore-based encryption is more secure than password-based encryption because no passwords are involved. Use password-based encryption only when it is absolutely necessary because your backups must be transportable.
Encrypted backups are decrypted automatically during restore and recovery, if the required decryption keys are available. Each backup set gets a separate key. The key is stored in encrypted form in the backup piece. The backup is decrypted with keys obtained by a user-supplied password or the Oracle keystore.
The Oracle Secure Backup media management software is the only supported interface for making encrypted RMAN backups directly to tape. RMAN issues an
ORA-19916 error if you attempt to create encrypted RMAN backups using a media manager other than Oracle Secure Backup.
When you use the
BACKUPSET command with encrypted backup sets, the backup sets are backed up in encrypted form. Because
BACKUPSET copies an already-encrypted backup set to disk or tape, no decryption key is needed during
BACKUPSET. The data is never decrypted during any part of the operation. The
BACKUPSET command can neither encrypt nor decrypt backup sets.
See Also:Oracle Database Advanced Security Guide for details about configuring the Oracle keystore
Transparent encryption can create and restore encrypted backups with no DBA intervention, if the required Oracle key management infrastructure is available. Transparent encryption is best suited for day-to-day backup operations, where backups are restored to the same database from which they were created. Transparent encryption is the default for RMAN encryption.
When you use transparent encryption, you must first configure an Oracle keystore for each database, as described in Oracle Database Advanced Security Guide. Transparent backup encryption supports both the encrypted and autologin forms of the Oracle keystore. When you use the Oracle keystore, the keystore must be opened before you can perform backup encryption. When you use the autologin keystore, encrypted backup operations can be done at any time, because the autologin keystore is always open.
Caution:If you use an autologin keystore, do not back it up along with your encrypted backup data, because users can read the encrypted backups if they obtain both the backups and the autologin keystore. It is safe to back up the Oracle keystore because that form of the keystore cannot be used without the keystore password.
After the Oracle keystore is configured, encrypted backups can be created and restored with no further DBA intervention. If some columns in the database are encrypted with Transparent Data Encryption (TDE), and if those columns are backed up using backup encryption, then those columns are encrypted a second time during the backup. When the backup sets are decrypted during a restore operation, the encrypted columns are returned to their original encrypted form.
Because the Oracle key management infrastructure archives all previous master keys in the Oracle keystore, changing or resetting the current database master key does not affect your ability to restore encrypted backups performed with an older master key. You can reset the database master key at any time. RMAN can restore all encrypted backups that were ever created by this database.
Caution:If you lose your Oracle keystore, then you are unable to restore any transparently encrypted backups.
Password encryption requires that the DBA provide a password when creating and restoring encrypted backups. Restoring a password-encrypted backup requires the same password that was used to create the backup.
Password encryption is useful for backups that are restored at remote locations, but which must remain secure in transit. Password encryption cannot be persistently configured. You do not need to configure an Oracle keystore if password encryption is used exclusively.
Caution:If you forget or lose the password that you used to encrypt a password-encrypted backup, then you are unable to restore the backup.
To use password encryption, use the
ONLY command in your RMAN scripts.
Dual-mode encrypted backups can be restored either transparently or by specifying a password. Dual-mode encrypted backups are useful when you create backups that are normally restored on-site using the Oracle keystore, but which occasionally must be restored offsite, where the Oracle keystore is not available.
When restoring a dual-mode encrypted backup, you can use either the Oracle keystore or a password for decryption.
Caution:If you forget or lose the password that you used to encrypt a dual-mode encrypted backup and you also lose your Oracle keystore, then you are unable to restore the backup.
To create dual-mode encrypted backup sets, specify the
password command in your RMAN scripts.
You can use the
CONFIGURE command to persistently configure transparent encryption of backups. You can use the command to specify the following:
Whether to use transparent encryptions for backups of all database files
Whether to use transparent encryptions for backups of specific tablespaces
Which algorithm to use for encrypting backups
You can also use the
ENCRYPTION command to perform the following actions:
Override the encryption settings specified by the
ENCRYPTION command. For example, you can use
OFF to create an unencrypted backup, even though a database is configured for encrypted backups.
Set a password for backup encryption, persisting until the RMAN client exits. Because of the sensitive nature of passwords, RMAN does not permit configuration of passwords that persist across RMAN sessions.
Using or not using persistent configuration settings controls whether archived redo log backups are encrypted. Backup sets containing archived redo log files are encrypted if any of the following are true:
ON is in effect when the archive log backup is being created.
Encryption is configured for backups of the whole database or at least one tablespace.
This behavior ensures that the redo associated with any encrypted backup of a data file is also encrypted.
Set up the Oracle keystore as explained in Oracle Database Advanced Security Guide.
Issue the following RMAN command:
CONFIGURE ENCRYPTION FOR DATABASE ON;
At this stage, all RMAN backup sets created by this database use transparent encryption by default.
You can explicitly override the persistent encryption configuration for an RMAN session with the following command:
SET ENCRYPTION ON;
The encryption setting remains in effect until you issue the
OFF command during an RMAN session, or change the persistent setting again with the following command:
CONFIGURE ENCRYPTION FOR DATABASE OFF;
You can use the
CONFIGURE command to persistently configure the default algorithm to use for encryption when writing backup sets. Possible values are listed in
V$RMAN_ENCRYPTION_ALGORITHMS. The default algorithm is AES 128-bit.
Start RMAN and connect to a target database and a recovery catalog (if used).
Ensure that the target database is mounted or open.
CONFIGURE ENCRYPTION ALGORITHM command, specifying a valid value from
The following example configures the algorithm to AES 256-bit encryption:
CONFIGURE ENCRYPTION ALGORITHM TO 'AES256';
Assume that you are performing data file tablespace point-in-time recovery (TSPITR) or performing data transfer with RMAN. In this case, you may want to set the names of data files in the auxiliary instance before starting the TSPITR or database duplication. The command is as follows, where
datafileSpec identifies some data file by its original name or data file number, and
filename is the new path for the specified file:
CONFIGURE AUXNAME FOR datafileSpec TO 'filename';
For example, you might configure a new auxiliary name for data file
2 as follows:
CONFIGURE AUXNAME FOR DATAFILE 2 TO '/newdisk/datafiles/df2.df';
As with other settings, the
CONFIGURE command setting persists across RMAN sessions until cleared with
CLEAR, as shown in the following example:
CONFIGURE AUXNAME FOR DATAFILE 2 CLEAR;
If you are performing TSPITR or running the
DUPLICATE command, then by using
CONFIGURE AUXNAME you can preconfigure the file names for use on the auxiliary database without manually specifying the auxiliary file names during the procedure.
When renaming files with the
CONFIGURE AUXNAME is an alternative to
SET NEWNAME command. The difference is that after you set the
AUXNAME the first time, you do not need to reset the file name when you issue another
DUPLICATE command; the
AUXNAME setting remains in effect until you issue
CLEAR. In contrast, you must reissue the
SET NEWNAME command every time you rename files.
See Chapter 21, "Performing RMAN Tablespace Point-in-Time Recovery (TSPITR)," for more details on using
AUXNAME for TSPITR, and Chapter 25, "Duplicating a Database," for more details on using
AUXNAME in performing database duplication.
When RMAN needs a read-consistent version of the control file, it creates a temporary snapshot control file. RMAN needs a snapshot control file when resynchronizing with the recovery catalog or when making a backup of the current control file.
The default location for the snapshot control file is platform-specific and depends on the Oracle home of each target database. For example, the default file name on some Linux platforms is
$ORACLE_HOME/dbs/snapcf_@.f. If a fast recovery area is configured for a target database, then the default location for the snapshot control file is not the fast recovery area.
You can see the current snapshot location by running the
SHOW command. This example shows a snapshot location that is determined by the default rule:
RMAN> SHOW SNAPSHOT CONTROLFILE NAME; CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/oracle/dbs/snapcf_trgt.f'; # default
This example shows a snapshot control file that has a nondefault file name:
RMAN> SHOW SNAPSHOT CONTROLFILE NAME; CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/oracle/oradata/trgt/snap_trgt.ctl';
' command to change the name and path of the snapshot control file. Subsequent snapshot control files that RMAN creates use the specified name and path.
For example, start RMAN, connect to the target database, and then enter:
CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/oracle/oradata/trgt/snap_trgt.ctl';
You can also set the snapshot control file name to a raw device.
To reset the snapshot control file location to the default, run the
Oracle Real Application Clusters Administration and Deployment Guide for details about handling snapshot control files in Oracle RAC configurations
RMAN cannot connect to a target database through a shared server dispatcher. RMAN requires a dedicated server process. If your target database is configured for a shared server, then you must modify your Oracle Net configuration to provide dedicated server processes for RMAN connections.
To ensure that RMAN does not connect to a dispatcher when a target database is configured for a shared server, the net service name used by RMAN must include
(SERVER=DEDICATED) in the
CONNECT_DATA attribute of the connect string.
Oracle Net configuration varies greatly from system to system. The following procedure illustrates only one method. This scenario assumes that the following service name in
tnsnames.ora file connects to a target database using the shared server architecture, where
inst1 is a value of the
SERVICE_NAMES initialization parameter:
inst1_shs = (DESCRIPTION= (ADDRESS=(PROTOCOL=tcp)(HOST=inst1_host)(port=1521)) (CONNECT_DATA=(SERVICE_NAME=inst1)(SERVER=shared)) )
Create a net service name in the
tnsnames.ora file that connects to the nonshared SID. For example, enter:
inst1_ded = (DESCRIPTION= (ADDRESS=(PROTOCOL=tcp)(HOST=inst1_host)(port=1521)) (CONNECT_DATA=(SERVICE_NAME=inst1)(SERVER=dedicated)) )
Start SQL*Plus and then connect using both the shared server and dedicated server service names to confirm the mode of each session.
SQL> SELECT SERVER 2 FROM V$SESSION 3 WHERE SID = (SELECT DISTINCT SID FROM V$MYSTAT); SERVER --------- DEDICATED 1 row selected.
To connect to a shared server session, connect with
SYSDBA privilege to
inst1_shs and then execute the following
SELECT statement (sample output included):
SQL> SELECT SERVER 2 FROM V$SESSION 3 WHERE SID = (SELECT DISTINCT SID FROM V$MYSTAT); SERVER --------- SHARED 1 row selected.
Start RMAN and connect to the target database using the dedicated service name. Optionally, connect to a recovery catalog.
See Also:Your platform-specific Oracle documentation and your Oracle Database Net Services Reference for a complete description of Oracle Net connect string syntax
A data block lost write occurs when an I/O subsystem acknowledges the completion of the block write, but the write did not occur in the persistent storage. On a subsequent block read, the I/O subsystem returns the stale version of the data block, which might be used to update other blocks of the database, thereby corrupting it.
You can set the
DB_LOST_WRITE_PROTECT initialization parameter to
FULL so that a database records buffer cache block reads in the redo log. The default setting is
NONE. When the parameter is set to
TYPICAL, the instance logs buffer cache reads for read/write tablespaces in the redo log, but not read-only tablespaces. When set to
FULL, the instance also records reads for read-only tablespaces. The performance overhead for
TYPICAL mode is approximately 5 to 10% and potentially higher for
Lost write detection is most effective when used with Data Guard. In this case, you set
DB_LOST_WRITE_PROTECT in both primary and standby databases. When a standby database applies redo during managed recovery, it reads the corresponding blocks and compares the SCNs with the SCNs in the redo log. If the block SCN on the primary database is lower than on the standby database, then it detects a lost write on the primary database and throws an external error (
ORA-752). If the SCN is higher, it detects a lost write on the standby database and throws an internal error (
ORA-600 ). In either case, the standby database writes the reason for the failure in the alert log and trace file.
To repair a lost write on a primary database, you must initiate failover to the standby database. To repair a lost write on a standby database, you must re-create the entire standby database or restore a backup of only the affected files.
Enabling lost write detection is also useful when you are not using Data Guard. In this case, you can encounter a lost write in two ways: during normal database operation or during media recovery. In the first case, there is no direct way to detect the error. Indirect symptoms such as inconsistent tables cannot be unambiguously traced to the lost write. If you retained a backup made before the suspected lost write, however, then you can restore this backup to an alternative location and recover it. To diagnose the problem, recover the database or tablespace to the SCN of the stale block read, which then generates the lost write error (
If a lost write error is encountered during media recovery, the only response is to open the database with the
RESETLOGS option. The database is in a consistent state, but all data after the
RESETLOGS SCN is lost. If you recover a backup made after database creation, you have no guarantee that other stale blocks have not corrupted the database. This possibility exists because the restored backup may have been made after an earlier lost write. To guarantee that no lost writes have corrupted the database, you must perform media recovery from database creation, which is not a practical strategy for most database environments.