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// Tales from software development

The limits of WebDAV

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On my last assignment, each member of the project team had a top of the range dual processor workstation from a leading PC manufacturer on their desk. Unfortunately, a significant number of these machines suffered disk drive failures after about a year of use. In anticipation of a disk failure on my workstation, I attached a 500gb external USB drive and created a scheduled task to backup my data at the end of each day.

The disk drive on my workstation never did fail but I got used to the additional security provided by my backup regime. So, having started a new assignment, I decided to do something similar with a small twist – instead of storing the backup on an attached external drive, I’d look at moving the file across a WAN link to a remote server.

The obvious way to do this is via FTP. However, the IT department at my new assignment have been overly zealous in locking down internet access and have blocked port 21 making FTP unusable. I started looking at alternatives based on HTTP and port 80 and soon realised that WebDAV was worth investigating as it’s implemented as an optional feature in IIS and Windows Explorer can act as a WebDAV client.

After enabling WebDAV on an IIS server that I run at home and configuring access permissions on the virtual directory that I’d be using, I tested access from the customer site using Windows Explorer and confirmed that it worked as expected.

The final step was to add a command to my backup batch file to copy the backup file created to my WebDAV server. I ran the batch file to test it that it all worked. And it did! Or at least, it seemed to, at first…

The batch file executed and, after a few minutes wait while the 15mb backup file was copied to the WebDAV server, it exited to the Windows command prompt. But when I checked the contents of the WebDAV folder the copy of the backup file there was only 1.5mb in size instead of 15mb.

After a few minutes a notification balloon was displayed with a the message, “Windows –  Delayed write failed.”

 WebDAV error

A quick web search revealed that this is a known issue with the Windows WebDAV client implementation:

http://support.microsoft.com/kb/818408

Microsoft’s WebDAV client implementation doesn’t handle large files very well. In fairness, WebDAV is really intended for web site content management and this would typically involve uploading and downloading files of a few kilobytes, not 15mb backup files.

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Written by Sea Monkey

November 3, 2008 at 7:00 pm

Posted in Environments

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