In my previous post
, I wrote about the Restart Recovery which is automatically done by SQL Server 2005 in the event of SQL Server startup. There is another type of recovery available that is, well known, commonly practiced and a manual process, RESTORE RECOVERY.
Restore Recovery is triggered manually by DBAs during the data loss events, to bring back the SQL Server database to a particular point of working state. The data is recovered from the BACKUP of the database taken and stored away in a media either tape or disk file.
Before looking further inside, let us discuss some basics of Backups.
What is the need of a Backup?
Backup is the backbone for the mission where critical data involved. Even though having high availability system configured with compatible RAID level of Disk Subsystems or fully redundant Storage Area Networks and for Servers that are clustered with failovers with Microsoft Cluster Services and SQL Server 2005 failover clusters, backups of mission-critical databases are so important for many reasons. Say suppose, a developer executed a DELETE FROM query forgetfully missed a WHERE clause in it against a production server, instead of Development Server where he supposed to execute! This is one simple example. A lot of such situations may arrive to test your Database Administrative abilities. You have to rely on your database Backup.
In a Restore Recovery, Backup is so important. But a behaviour of Restore Recovery is based on a property of the Database that is ‘Recovery’. There are three Recovery Models available in SQL Server. They are SIMPLE, FULL, and BULK-LOGGED. When you create a Database, the default value of this option is ‘FULL’. This can be changed with ‘Alter Database’ command with a SET RECOVERY option. For example Alter Database mydb SET RECOVERY SIMPLE.
Simple Recovery Model.
This model provides a very simplest form of backup. This model minimizes the administrative overheads to a DBA. When this RECOVERY option is set in a database, then its transaction log will not be included during the backup and it is not possible to take TRANSACTION LOG Backup. When you take backup, the SQL Server automatically truncates transaction log by dropping the inactive log records and free up the space used by them.
This model of recovery is advisable to
· The databases that are under development process.
· The databases that are mainly used for data ware houses.
· The databases that are used for read-only purposes.
There are no log backups involved in Simple Recovery model; the database can be restored to the end of the most recent backup. So the work done after the last full backup can be lost.
Simple recovery model has following restrictions.
1. Page restore can not be done.
2. File Restore and Piecemeal Restore are available only for read only databases.
3. Point-in-time restore is not available.
Full Recovery Model.
This is the default Recovery Model when you create a database in SQL Server 2005. This model provides a full protection to the data. Thus this is best option to prevent data loss. These recovery models full rely on transaction log backups. To avoid data loss you have to frequently take backup of transaction log along with data backups. If you have a transition log backup after a failure, then you can restore the data to the point of time when the failure occurs.
As all activities including Bulk Copy operations, SELECT INTO, and even Create Index, are logged into the Transaction Log file, and it keeps the log records even after taking the data backup, the Transaction log file may grow high in volume in disk size if you specify auto grow during creating the database. This is one disadvantage of this recovery model, but can be easily handled with DBA’s high attention. As the storage is growing high in this model, the restoration time will be relatively high. For each time a transaction log backup is performed, the inactive log records are truncated and the space used by them is freed up for future usages.
The following scenarios are highly suited to have a database with FULL Recovery Model.
1. If the database contains multiple filegroups or read-only file groups.
2. If having efficient DBAs who can perform point-in-time recovery, Individual page restorations.
3. For high cost tolerance scenarios to tolerate the disk cost due to highly growing transaction logs.
Bulk-logged Recovery model.
This model is very similar to the full recovery model excepts it won’t log the Bulk Copy Operations, SELECT INTO, CREATE INDEX, ALTER INDEX REBUILD, DROP INDEX, WRITETEXT and UPDATETEXT BLOB operations. That means these operations are minimally logged. Because these recovery model can not log these operations as they run very fast. But transaction log records are created for such operations that took place and the page extents which are affected by these operations are also recorded in that log records. Still Log Backup is required to free up the inactive transaction log records. The Bulk-Logged Recovery model does not support Point-In-Time Restoration. This model is very useful where frequent bulk copy operations take place, so that, they are minimally logged and the performance degradations due to bulk copy operations will not be there.
What is LOG Truncation?
If the log records are not eventually not deleted in a frequency of time, Transaction LOG file will grow in high volume (of course depends upon the file size mentioned CREATE DATABASE command) upto even completely full your disk drive. So, inactive transaction records should be deleted in frequency of time. Deleting all inactive Transaction records from Transaction Log file is called as LOG Truncation.
LOG truncates occurs automatically
· for a simple recovery model database after a checkpoint occurs
· for the FULL and Bulk-logged model after taking the Transaction Log backup, if a checkpoint occurs after the previous backup.
In the next part of this series, we will see the Backup Types and Restore Sequences.