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Shadow copies influence server performance

Shadow copies can save you time, but is there a cost in terms of server performance? There can be. Here is some advice on how you can monitor your server.

The shadow copy feature for shared folders in Windows Server 2003 is a tremendous time saver for administrators....

Using shadow copies of user folders, for instance, lets users recover from their own mistakes. If they deleted a file but didn't mean to, they can easily recover it without asking you.

That's the up side. There is a potential down side: using shadow copy imposes a performance penalty since data is written twice. Generally this isn't an issue. Microsoft uses a copy-on-write mechanism to make an image of the volume and the demands on server bandwidth and storage space are minimal. In fact, even very large user files such as individual users storing Outlook personal file folders (.pst) on the server won't produce any noticeable performance impact unless the server is heavily loaded.

If the server is heavily loaded there may be a noticeable performance penalty because of the additional disk head seeks as the second copy of the data is written to the disk. In this case, Microsoft recommends dedicating a disk for shadow copy storage. That keeps the shadow copy writes contiguous and reduces the amount of seek time. Normally, the shadow copy snapshot is stored on the same NTFS volume as the original.

Just how much of a performance penalty you will pay for user shadow copy is difficult to determine in advance since it varies depending on the workload, the pattern of work and storage activity. However, Windows Server 2003 has several tools that let you monitor shadow copy's performance. Collectively called Volperf (which is short for Volume Shadow Copy Service Performance), these tools can be activated from the Performance Logs and Alerts menu under the system monitor.

The procedure is:

  • From System Monitor > Double-click Performance Logs and Alerts > right-click Counter Logs > New Settings.
  • Choose a name for the log, such as "Shadow Copy Performance," and click OK.
  • Go to General > Add Counters and select the appropriate counters: Go to Log Files > Log File Type drop-down box and select the output format.
  • Choose a CSV file type if you want to be able to manipulate the data in Excel. The data can also be sent to a SQL database format. Make sure the End File Names checkbox is checked. Use the year, month and day format yyyymmdd.
  • Go to the Schedule > Start Log At, enter the start time for logging.
  • Click the Stop Log At checkbox and enter the time at which logging should stop.
  • Click Apply > OK. The log files will be created in \Perflogs by default.

The Volperf tools track the number of shadow copies open, the amount of disk space being used and the growth of disk use, among other things. When combined with the network performance tools, this gives a picture of the load Shadow Copy Services is placing on the server.

Microsoft offers a complete description of the process at:

Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in storage and other computer issues.

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