Recovering from a data center disaster


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The disk alternative
I have yet to meet an IT manager who wouldn't like to remove or reduce the financial burdens associated with tape media management. More staff, more off-site vendor contracts and more tapes are some of the larger itemized expenses coming from out of your media management budget. Tape management requires staff to collect the most recent backup tapes from the library, possibly entering them into a separate tracking system, and coordinate the pickup and drop-off of tapes needed for recovery and re-initialization.

In addition to these tasks, this group will also determine the recoverability of tapes that were vaulted some time ago. Although most organizations outside of the government aren't prudent in their efforts to ensure recoverability, it's still a best practice.

An increasingly interesting alternative to tape that alleviates many problems is one of the many disk-based DR solutions on the market. With this solution, disk can be used to stage data at a remote site.

That's exactly what one of my clients has done. They already owned space tens of kilometers away from the primary data center, and they decided to mirror their data to this location (see "

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Disk-only approach yields fastest time to recovery"). Every night before the backup, the mirrors were split and the secondary copy mounted on a backup server local to the remote tape library. Then, the backup data was transferred to the library, and the mirrors merged when the backup was complete. This solution put the backup data off-site immediately, and it removed the application server from the process of moving data to tape by splitting the mirrors and mounting them on a different server.

At first glance, this approach may not look like a disk-based solution because the data is being driven to tape. However, the functionality of mirroring data across distances makes this a disk-based DR solution with benefits. In addition to the aforementioned benefits, there's one less copying function necessary for archiving. The mirrored data is already located off-site in a second or third disk array for protection, and the ability to split and mount the mirrors on a separate server implies application-free backup and a higher quality of application availability.

Should the primary data center be destroyed, mirrored and recently synchronized copies of each file system or volume will be resident on Switch B and discovered by leased servers provisioned at the recovery site some time after the disaster. By locating the backup server at the remote site and provisioning application servers, as well as having a plan in place for changing your SAN's zoning and perhaps LUN configurations to include the new host bus adapters (HBAs) in the provisioned servers, you have done much to ensure the expedited recovery of your applications. Furthermore, with your boot disks being mirrored across the extended SAN, you'll be in a better position to recover by connecting a server to Switch B, install the necessary drivers, change your zoning configuration and then boot the server to assume the identity of the destroyed application server at the primary data center. I can't think of a better way to perform a bare-metal restore of an OS.

Because this DR solution still uses tape, the media management issues surrounding tape still exists. But by combining the mirroring functionality of your disk array with the ability to extend your SAN over distances, it takes the off-site tape vendor out of the critical path of a DR exercise by alleviating the need to wait for the vendor to retrieve and deliver the latest full backup tapes. Instead, your recovery team can start connecting servers and discovering volumes much faster than was possible with recovering complete volumes from full backup tapes, leaving tape recoveries for incremental restores only. And guess what? Those tapes are already loaded in the library. Thus, you will only need the full backup tapes being retrieved by your off-site vendor for file systems that were corrupted when its mirrored partner went off the air. A side benefit of having your tape library located at a remote site is you may be able to save budget dollars by refining your off-site pickup schedule because your backup tapes are off-site.

Another disk-based DR solution involves using a disk volume as the target of the backup and recovery application. In this approach, the scalability of capacity and performance is essential. On the front end, you can configure the backup server to recognize the storage nodes or media servers as data movers in the transfer of application server data to disk. As for resource management, you should approach storage provisioning the same way as any other application server. After all, backup and recovery in its simplest form is just another application.

Because the storage nodes and accompanying SAN basically replaces your tape library and perhaps the duties of your tape management staff, you can assume that there are administrative burdens associated with this solution. These burdens mostly mirror those associated with managing your primary and/or secondary disk arrays. The problem is that if this solution is going to make any kind of economical sense, the chosen disk array in your DR solution will probably be less of a robust storage solution than the one chosen for your primary storage solution, thereby suggesting different management tools and more administrative staff. To increase your ROI in a disk-based DR solution, storage management software that provides a management interface into your disk arrays should be chosen prior to implementing your solution.

An enterprise-class tape library at the end of an extended SAN is a tape vendor's best chance at keeping real estate in enterprise data centers for DR purposes. With the right distance between the primary data center and the remote tape libraries and a less-than-aggressive recovery objective, tape libraries still make economical sense for business applications. However, the more servers, applications and file systems that must be recovered with an aggressive recovery objective, the less likely a tape-based solution will satisfy your SLAs.

This was first published in August 2003

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