Understanding VSS and ShadowProtect

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How do ShadowProtect and VSS interact to ensure complete and accurate point-in-time volume backups when VSS-Aware applications reside on these volumes?


What is VSS and what does it do?

VSS (Volume Shadow Services) is software first built into Windows 2003 Server by Microsoft to resolve the issues associated with backing up open files. Utilizing VSS correctly helps ensure that Administrators do not have to log users off of VSS enabled databases when backups occur. VSS does not backup files as such but rather the sectors on which the files reside thus giving very accurate and small point-in-time, real-time backups.

What applications are VSS enabled today?

Type vssadmin list writers at the command prompt to list all VSS-enabled applications' respective writers on a system. Today, these Microsoft applications and databases include:

• Windows Server 2003 and later
• Exchange 2003 and later
• SQL 2005 including MSDE and later
• Windows Vista and later

How does VSS work?

An external application (such as StorageCraft ShadowProtect) uses the correct and certified API calls to VSS (these calls are referred to as VSS Writers) to freeze the relevant application. This means that during the process of creating the backup, VSS tells the application to commit all transactions to disk and then freezes the database so it can gather the list of sectors that need to be backed up. This freeze process allows the data to be read from the frozen sectors but temporarily blocks writing to these same sectors.

Once the snapshot has been completed, the VSS Writer informs VSS that all data (sectors) have been backed up and that VSS can now release or thaw the database. At this point, if an application needs to write to a protected sector, VSS makes a copy of the sector before allowing the write. When the affected sector needs to be backed up the copied one is the sector that is stored in the backup. This entire process typically takes only a few seconds, depending upon the size of the database and number of changed sectors. Escessively long snapshot times can indicate underlying problems with the application, OS, or hardware of the system. They can also be the byproduct of an extremely busy database-using system such as large Exchange or SQL implementations.

What are the currently certified VSS Writers?

A VSS writer is an application that seamlessly and fully integrates with VSS to instruct VSS to freeze and thaw the relevant VSS application or database (such as Exchange 2003 or SQL 2005). Type vssadmin list providers to list the VSS Providers on a system. Today, only two applications are certified VSS Providers:

        • StorageCraft Volume Snapshot Manager (as used in ShadowProtect)
        • Microsoft VCS

Why is a Certified VSS Provider Important?

Using VSS-Aware applications is NOT recommended. Often an application is referred to as VSS-Aware – this typically means that a work-around has been written to VSS because the application is not actually VSS certified. This can often lead to a corruption of the VSS database or backup image file.

 How does ShadowProtect use VSS?

StorageCraft ShadowProtect is a certified VSS Writer and enables any VSS application or database to be protected in real-time without taking the application or database off-line. StorageCraft ShadowProtect uses an automated 'Set & Forget' scheduling feature to help ensure that organizations amd users can protect their data in real-time up to every 15 minutes throughout the day. This minimizes data loss after a crash or corruption. As StorageCraft leverages the sector based capabilities of VSS, incremental changes or backups are often tiny compared to traditional file-based backups. This substantially reduces or eliminates backup windows in organizations of all sizes.

 Troubleshooting VSS

The VSS Framework was developed by Microsoft and as such StorageCraft support is limited in troubleshooting these issues. VSS largely relies on the environment that it is being used in rather than the software that is using it. If the VSS Framework needs further diagnosis and troubleshooting after all StorageCraft resources have been exhausted, you may need to contact Microsoft for further assistance. The following StorageCraft Knowledge Base Articles may assist in resolving VSS issues with backups or provide additional information on utilizing VSS in your backup and disaster recovery plan.

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