Quiescence (n): A 50-cent term often used when discussing how to back up virtual machines (VMs) without installing
backup software into the VM itself. Quiescence is an issue because while in an image-level backup the file system is quiesced prior to the backup, applications and databases are not quiesced, resulting in an inconsistent database restore, possible data loss and a longer total time to restore. Although not a critical issue, VMware has alleviated the problem with VMware Server 2.0, and the integration of Microsoft's Volume Shadow Copy Service (VSS) in VMs running Microsoft OSes.
Backup in the early days of virtualization
Since the dawn of virtualization, one of the greatest benefits it has offered IT is its ability to perform backups without installing backup software in each VM. By backing up the virtual machine from the perspective of its host, the backed-up virtual machine is effectively a single file.
This is in contrast to the tens of thousands of files that make up a typical server. By backing up VMs from the perspective of their host -- sometimes called an "image-level" backup -- the chance that a future restore will be completed quickly is greatly enhanced.
With image-level backups, the file system is traditionally quiesced (or quieted) prior to the backup. This ensures that images written to the file system during the backup period do not corrupt the backup during its processing.
The problem is that this activity historically didn't "quiet" any applications or their databases residing on the host. So if your server was Microsoft Exchange, Active Directory or SQL Server, the result after a restore would be a database that returns in an inconsistent state. Although this wasn't a critical problem, it could have resulted in loss of data and certainly resulted in an extension to the overall time to restore (Exchange Server database utilities, anyone?).
VMware Server 2.0 and VSS
To combat this problem, VMware Server 2.0 has introduced an integration with virtual machine backup utilities and Microsoft's Volume Shadow Copy Service (VSS). This integration between host and residing VMs enables backup applications that are VSS aware to correctly and fully quiesce the file system along with the on-board databases. The end result of this integration is that a restored image-level backup of a virtual machine is likely to have its databases return intact as well.
As of press time, VSS is supported only on host OSes running Windows Server 2003 and 2008. Oddly enough, only the x86 version of Windows Server 2008 is supported and application VSS writers for Server 2008 are not supported. This effectively means that databases on Server 2008 cannot utilize any of these new consistency features.
With the strong push to move virtualization hosts to 64-bit hardware and Server 2008, it is likely this omission will change in the future. Obviously, any guest operating systems must also run the same versions of Windows and other OS support is not available.
Utilizing VSS on VMware Server 2.0
Enabling this capability requires some configuration file creation.
- Create a file named vmvsswriter.cfg in the C:\Program Files\VMware\VMware Server folder.
- Open that file in a text editor and add the line vmwriter.overwriteSnapshots = "TRUE".
This enables the automatic overwrite of any existing snapshots associated with the virtual machine. This is necessary if quiescing is to occur during the backup. Be careful with this setting if you use snapshots with your virtual machines.
- Once this is completed, navigate to the Services Control Panel and look for the VMware VSS Writer Service.
- Modify the Log On properties of this service to assign it a user with administrative privileges on all backed up virtual machines. That user must also have access to modify the host's virtual machine disk file folder.
Restoring a backed up virtual machine is a two-step process. First, restore the image-level backup of the virtual machine using your existing backup software. Then, to restore the virtual machine's state at the time of the backup, revert to its existing snapshot in the VMware interface.
Be aware that if this process seems challenging in a large-scale environment, other third-party add-on tools also include VSS support for backups. Veeam Software and Vizioncore are two that add administrative and workflow features that make the use of VSS-based backups easier across the board.
ABOUT THE AUTHOR: Greg Shields, MCSE, is a co-founder and IT guru with Concentrated Technology with nearly 15 years of IT architecture and enterprise administration experience. He is an IT trainer and speaker on such IT topics as Microsoft administration, systems management and monitoring, and virtualization. His recent book, Windows Server 2008: What's New/What's Changed, is available from SAPIEN Press.