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Reads through mapped drive 700% slower

Posted: 04/19/2013 16:54:40
by Paul Jones (Basic support level)
Joined: 04/19/2013
Posts: 1

Using the Mapper demo app, I've setup a mapping between the existing C:\temp directory and the Z:\ mounting point. I've populated the C:\temp directory with around 800 exe files of various sizes.

I've compared reads between accessing the files directly from C:\temp to accessing the files from Z:\. Using multiple runs, I've found that reads of file content from Z:\ is about 700% slower that reads from C:\temp.

I used two methods of doing the reads that generally agree in performance. First, I simply xcopy the files from the source location (either C:\temp or Z:\) to a separate drive (e.g. D:\temp). Second, I wrote a few-lines C# program to iterate the files, read all their content into memory, and write the number of bytes read to the console.

Is this the expected behavior and performance that should be expected for this scenario? Are there optimizations that could be used that is not employed by the Mapper demo app?
Posted: 04/20/2013 00:58:54
by Eugene Mayevski (Team)

Thank you for contacting us.

Of course you need to use some caching in a real-life application and Mapper does none.

Next you need to note, that CBFS was designed to create virtual disks from non-disk data (usually network or database-based) where usage scenarios don't involve fast transfer of large amounts of data.

The main slowdown is caused by CBFS nature and the main feature: callbacks to the user mode. Context switches from the user mode to kernel mode, then back to the user mode *and again* back to the kernel mode (to access the real disk) are slow.

If you take VDisk sample, which keeps data in memory, you will see faster operations because half of context switches is not present there. And you can measure the difference (it would be nice to know it). VDisk will also show you how caching in user-mode (in memory) would improve speed.

Sincerely yours
Eugene Mayevski



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