This ensures that we will never include any partial information that corresponds to the shrunk region of a data file. If growth took place before the partial information is collected, then the partial information should have included the allocated pages in the grown region.
If the growth took place after the partial information is collected, then the partial information does not include changes in the grown region. In the following sections, we will see that such changes will be restored by the log roll-forward. A logical rename of the file does not affect the backup or restore, since the file's logical name is not referenced anywhere in the Writer Metadata Document or in the Backup Component Document.
See the Writer Metadata document: An Example for more detail. A physical database file rename does not take effect until the database restarts.
Therefore, the database configuration information or the file path information in the partial information buffer is still based on the old physical paths, which are the only valid paths to those database files on the snapshot. During a differential restore, the backup metadata that the requestor gives back to the SQL writer has the backup type information. Therefore, no special treatment from the SQL writer is needed.
SQL Server will figure out that it's a differential restore by itself. SQL Server handles such a differential restore in the same way as against a native differential restore that is not performed through VSS. During this phase SQL Server will resize all files to the appropriate size based on the differential backup's file metadata. If the file is grown, SQL Server zeros out the grown portion. If a new file has to be created it was created after the base was taken , SQL Server zeros out the new file.
It also closes all the file handles so that the backup application can overwrite the files with the restored data from the backup media. The client should restore the files based on the partial file specification. Such files must have been pre-created by SQL Server during the restore preparation phase. They should have been extended to the right size and zeroed out. The client only needs to lay down the data as per partial specification the partial specification includes all allocated extents.
The partial information that SQL Server has provided does not include any tracking information for such file drops. SQL Server is responsible for detecting the files to be deleted, by comparing the restored files metadata against the existing containers, and actually deleting them. This is done prior to the restore as a preparation step. Such files must have been extended to the right size by SQL Server during the restore preparation phase.
The extended region must have been zeroed out by SQL Server as well. Therefore, the client can safely lay down the data even in the grown region as per partial specification.
If the file was grown after the partial information was taken, the changes in the grown region will be restored by replaying the log that was backed up along with the differential backup. SQL Server is responsible for truncating the file to the required size as per metadata.
This would not affect the restore as the logical name does not appear in the Writer Metadata Document or the Backup Component Document. The logical name change will be restored when the client applies the change to the primary database file, which contains the system catalog information.
If by the time of differential backup, the rename had not taken effect, then the client still restores data to the old location. A database restart post-restore will cause the physical rename to take effect. If by the time of differential backup, the physical file rename had already taken effect, then the partial data, if any, was backed up from the new physical path.
During the post restore events, the SQL writer will perform the normal redo operation and recovery if SetAdditionalRestores is set to False of the database.
SQL Server full-text catalogs are part of the database resources that need to be backed up or restored together with the rest of the database files. A differential backup is timestamp-based for full-text catalog.
In other words, there will not be different bases for different containers. For VSS full-text catalog backup, this means for all full-text catalog containers, the differential backup will be single-timestamp based, unlike the case of native SQL differential backup, in which there is one timestamp base per full-text catalog container.
In VSS, this timestamp is expressed as a component-wide property that is set during the full backup, and used during a subsequent differential backup. This indicates to the backup application that the SQL writer owns the management of the differential base. The base timestamp is set during a full backup. The backup application will retrieve this timestamp from the base full backup, and make the timestamp available for the writer by calling IVssComponent::GetBackupStamp to retrieve the base stamp from the previous base backup.
Backup application's responsibility during differential backup During a differential backup, the backup application is responsible for:. Backup application's responsibilities during a differential restore During a differential restore, the backup application is responsible for:. It is sometimes necessary to take a backup that is intended for a special purpose. For example, you might need to make a copy of a database for testing purposes. This backup should not impact the overall backup and restore procedures for the database.
When a copy-only backup is selected, it is assumed that files on disk will be copied to a backup medium by the requestor regardless of the state of each file's backup history. SQL Server will not update the backup history. This type of backup will not constitute as a base backup for further differential backup operations and also it does not disturb the history of the previous differential backups.
The backup application is only allowed to specify new targets for the physical path, but not the file specification. A requestor may need to restore an SQL database with a new name, especially if the database is to be restored side by side with the original database.
The SQL writer will take the entire content of New Component Name's value and use it as the new name for the restored database. If no option is specified, SQL will restore the database with its original name component name. The SQL writer currently does not support "Rename across Instances" to move a database to a new instance. Data in the snapshot cannot be safely accessed prior to going through the recovery phase to roll back in-flight transactions and placing the database in a consistent state.
Since the snapshot is in a read-only state, it cannot be recovered by the normal process of attaching the database. It is possible to autorecover the snapshots as part of the snapshot creation process. SQL Server the auto recovery should be applied only to app-rollback snapshots but not for backup snapshots.
This process will do the following for each explicitly selected by the requestor SQL Server database in the snapshot set:. Attach the snapshot database to the original SQL Server instance that is, the instance to which the original database is attached.
Shrinking the log files is the default behavior. This can be disabled by setting the value to the following registry key to 1. This may be useful in scenarios where the snapshot may be used to export data from a specific page at a specific point in time from the log to fix a problem in the online database.
In some cases, the snapshot databases may contain some in-flight multi-database transactions. During recovery operation, the SQL writer will attach the database on the snapshots with the Presumed Abort option. This would roll back any multi-database transaction that is not yet committed including any transactions that are in a Prepared to Commit state. This may lead to some inconsistencies between databases in the snapshot set.
For example, consider two databases A and B. There is a distributed transaction between these two databases and this transaction is in Committed state in database A and in Prepared to Commit state in database B. As part of the autorecovery process, this transaction will be committed in database A and rolled back in database B. This may lead to some inconsistencies in the snapshot set.
This implies that member of either box admin or that special group will be able to attach the database. If Master database is restored together with user databases that are using the Simple Recovery model, the user databases can be restored with the same technique as the master database: with the instance shut down, just copy or mount the volume s.
When the SQL instance is started, everything recovers. If user databases are to be recovered and rolled forward together with master database recovery, the instance must not start up and recover the master and user databases together.
If the server instance is the default instance on the machine, the logical path becomes one part — "Server1". This section describes some of the special cases encountered during SQL writer-based backup and restore operations. For noncomponent-based backups, autoclosing of databases is done, when checking for torn conditions, but the autoclosed databases are not explicitly frozen during backup operations.
The expected scenario here is that many closed databases may exist and we want to minimize the cost of the snapshot. Autoclosed databases are typically used in low-end configurations where resources are scarce.
That is, even if these operations take place successfully the log file will record detailed entries. It's an improvement that offers an easy built-in way of establishing these details at SQL Server level. Additionally, SQLWriter service start-up events will also be recorded and will report active logging parameters.
The above entry will be observed for each start of SQL Writer Service it may even be logged twice per service start, it's a minor known cosmetic issue. In order of appearance, we can see the following information is logged:.
OnIdentify is a common VSS operation. It's triggered by vssadmin list writers command. Previously, only active profiler tracing would allow the DBA to detect such an event. With the new logging feature, each event will lead to the above entry. This event leads to a more sizeable set of entries. These entries provide details on the VSS operations that were previously difficult to establish quickly and required advanced tracing to do so.
With SQL Server It can occur when a VSS backup attempts to create a snapshot set of volumes, which includes only a partial set of files of a given database. From SqlWriterLogger. This value is difficult to interpret without the error code references. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services. Privacy policy.
Applies to: SQL Server all supported versions. To configure the service, use the Microsoft Windows Services applet. When running, Database Engine locks and has exclusive access to the data files. The VSS provides a consistent interface that allows coordination between user applications that update data on disk writers and those that back up applications requestors. The VSS captures and copies stable images for backup on running systems, particularly servers, without unduly degrading the performance and stability of the services they provide.
For more information on the VSS, see your Windows documentation. When using VSS to backup a virtual machine that is hosting a Basic Availability Group, if the virtual machine is currently hosting databases that are in a secondary state, starting with SQL Server This is because Basic Availability Groups do not support backing up databases on the secondary replica.
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