How VSS Is Used By CDP

h2. Question
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How is MS Shadow Copy (VSS) used by CDP?

h2. Answer
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Most Windows CDP applications (R1Soft is an exception) use MS VSS for shared folders. These applications do not delete the snapshot in step 10 when done and instead must keep the snapshot and perform copy-on-write the entire time until the next backup operation. This is because the method of computing deltas (VSS for shared folders) works by comparing two Volume Shadow Copy snapshots.

R1Soft uses Volume Shadow Copy Service to take cluster level point in time volume snapshots of the running volume/disk.

The process of making a file system snapshot usually takes less than a second and if done correctly is never noticed by users or running applications. MS VSS usually takes around a second but can be delayed by Writers (notified applications) taking too long to become consistent for the backup operation.

Contiguous blocks are needed by VSS and really any other snapshot technology. A user space daemon (the VSS System Service Provider) creates a contiguous file in the file system in the hidden SYSVOL folder and gives the block range to the VSS device driver in the Windows kernel. The Windows kernel then writes cached blocks needed to maintain the snapshot directly to disk, bypassing the file system.