However, while replication systems are an almost mandatory portion of large DR plans, they can pose some interesting issues when you run utilities on your production systems.
One of the most common culprits is defragmentation software. Windows systems especially benefit from these tools, and they're also becoming a common utility in production data centers.
Unfortunately, replication tools don't always play nicely with these utilities. When the defrag kicks off and starts reading, writing, and moving data all over the disk, replication tools might see this data change and replicate it to the DR systems, causing massive traffic spikes.
The good news is that this is typically restricted to the world of hardware-level replication, so those using host-based products generally don't see the massive push of data across the WAN. That's due to the fact that most defrag systems work at the disk level—not the file-system level—and nonhardware-based replication tools therefore ignore them.
If you use a hardware-based replication tool, you'll have to take this traffic pattern into account when using defrag utilities. Unfortunately, you can't simply shut off replication during the defrag, which would leave you with horribly corrupted files at the DR site.
However, these hardware-based systems typically require a great deal of bandwidth to work properly, so you should have enough pipe to handle the load of a defrag, but you'll experience a pretty bad performance hit when the utility begins. Since defrags generally take place in the middle of the night, proper timing can keep end users from experiencing the hit, but you should still be aware of its occurrence.
Another usual suspect is antivirus software. Again, this is a larger issue in the Windows world than elsewhere, but it still affects all platforms in some way.
Production antivirus systems typically don't cause major issues, and nearly all replication tools can work with antivirus tools without incident. But issues arise on the DR systems, where constant scanning and rescanning of files (replicated either block by block or byte by byte) creates an enormous load on the antivirus systems.
Because of this load, replication streams can slow to a crawl and could force remirroring or other uncomfortable consequences. The fix for this issue is relatively easy: Shut off antivirus scanning in whole or in part on your DR systems—and only the DR systems.
If you're scanning everything at the production site, you don't need to scan that data again at the DR site. You can easily flip on the antivirus services during a fail-over procedure to allow for scanning to occur when users are directly accessing the files.
If corporate regulations don't allow you to completely shut off antivirus systems (or the thought of doing so gives you a case of the screaming heebie-jeebies), most software allows you to exclude directories and files. Simply ignore the directories and files you're replicating, and scan everything else. It may sound risky to shut off antivirus tools, but remember that excluding this data, which you're scanning at the main facility, while scanning everything else shouldn't leave your organization vulnerable to virus attacks.
Of course, these are only two examples of utilities that can interfere with DR planning and maintenance. There are a myriad of tools and utilities that can impact how your DR plan functions, especially when replication is part of the picture.
None of these headaches warrants eliminating DR tools from your toolkit. But each utility requires an awareness of its functionality and how it can impact your overall plan.
Mike Talon is an IT consultant and freelance journalist who has worked for both traditional businesses and dot-com startups.