This month the SQLskills team is presenting a series of blog posts aimed at helping Accidental/Junior DBAs ‘keep the SQL Server lights on’. It’s a little taster to let you know what we cover in our Immersion Event for The Accidental/Junior DBA, which we present several times each year. You can find all the other posts in this series at http://www.SQLskills.com/help/AccidentalDBA. Enjoy!
In the final post in the Backup section of our Accidental DBA series, I’m going to a look at backups using virtual machine (VM) snapshots, which are popular among VM administrators but may not be the right solution for your SQL Server recovery needs, depending on your RPO and RTO requirements.
VM Snapshot Backup Support
The “Support policy for Microsoft SQL Server products that are running in a hardware virtualization environment” Knowledge Base article includes information about the support of virtualization-aware backups for SQL Server. As long as the backup solution uses the volume shadow-copy service (VSS) to perform volume-based snapshots of the virtual machine hard disks, those backups are supported for recovery with SQL Server in a virtualized environment. Some examples of these tools include:
- Hyper-V backup
There are also many other VM backup tools available; this is just a brief list of the ones that come immediately to my mind. Additionally, many of the traditional backup products like Symantec NetBackup also support VSS-based backups of virtual machine images as well. VSS integration is required to provide an application-consistent backup of the databases contained within the SQL Server instance. This ensures that disk I/O activity is quiesced (frozen) properly prior to the snapshot being created. Any snapshot functionality that does not use VSS may leave SQL Server in an inconsistent state, and this may include standard VM snapshots that can save the VM memory and device state to disk without quiescing through VSS, unless specifically configured to do so.
Limitations of VM Backups
VM snapshot-backup solutions are popular with VM and server administrators because they standardize the backup implementation across the enterprise and remove the need for application-specific backup configurations. While this is generally a benefit, there are other considerations that should be evaluated when it comes to relational database management systems (RDBMS) like SQL Server. I specifically say RDBMS here because these same considerations need to be applied to any RDBMS and not just SQL Server for backups. Below are a few of the considerations that you should keep in mind while evaluating whether to use VM snapshot backups for SQL Server.
One of the features provided by VM backup solutions is the ability to perform a point-in-time restore of the virtual machine image, or even the files contained within the VM. While it is true that you can restore to a point in time, that point is simply the last snapshot backup point for the VM being backed up. Depending on the frequency of the backups that are occurring, this might meet your business recovery requirements, but it doesn’t provide the same capabilities as native SQL Server backups, which provide the ability to restore to any point in time using a combination of the latest full backup of the database, and all of the transaction log backups since that full backup (using the full recovery model, or the bulk_logged recovery model with some restrictions). If you are not currently using the full recovery model for databases and also taking transaction log backups at regular intervals, the point-in-time recovery provided by VM snapshot backups can reduce the risk of data loss over only performing daily full or differential SQL Server backups of the database, depending on the snapshot backup interval configured for the VM.
Single Database Restore?
Depending on the tool being used for VM backups, it may or may not be possible to restore a single database from the VM backup without first having to restore the entire VM image from the backups to obtain access to the database files contained in the image. Some tools do allow guest-OS file indexing of the VM backups which allows for individual files to be restored from the VM backup without having to restore the entire VM image. Other tools also offer the ability to mount a VM backup as a VM that is boot-able to allow for object-level recovery of individual tables through data transfers back to the production VM SQL Server.
Transaction Log Clearing?
For databases that use the full or bulk_logged recovery models, transaction log clearing only occurs when the log is backed up using SQL Server native backup commands. The VSS backup will not cause log clearing to occur inside of SQL Server on it’s own. Some of the VM backup tools offer options to perform transaction log clearing, but you need to be very careful with these options, especially if you have concurrent SQL Server backups being performed for the VM. The way that certain tools clear the transaction log during a VM snapshot backup is to issue a subsequent command to the SQL Server, BACKUP LOG <database_name> TO DISK =’NUL’, which dumps the transaction log records to nowhere, essentially throwing them away completely and breaking the log backup chain for any native SQL Server backups being performed. If you are still using native SQL Server backups, it is recommended that the VM snapshot backups be configured to not truncate the transaction logs for the databases.
While VM snapshot backups can provide a simplified method of backing up SQL Server VMs, there are some trade-offs that exist when using them that should be considered to ensure that you are going to be able to meet your business RPO and RTO requirements. The ability to restore a VM, database files, or even an individual object inside of a database to the point-in-time of the last snapshot backup can help minimize the data loss associated with a crash or accidental deletion of data inside of a database.
However, contrary to popular belief, it is not possible to restore to an intermediate point-in-time that exists between the VM snapshot backups, so if that level of recovery is needed or expected, you will still need to perform SQL Server native backups of the databases. Paul explained how to get near zero data loss using SQL Server backups in his post The Accidental DBA (Day 8 of 30): Backups: Planning a Recovery Strategy.
This level of recovery currently can only be accomplished through the use of SQL Server native backups.