How disk has changed backup


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Setting RPO, RTO requirements
All RPO, RTO and synchronicity requirements must be business-centric. Before deciding what these requirements are, you should first analyze and prioritize the business functions, and assign each computer system the recovery priority of the business function it serves. Next, decide on an RTO and RPO for each system and type of disaster--from the loss of a disk to the loss of a metropolitan area. Some systems will have the same requirements for all types of disasters; others may have tougher requirements for specific types of disasters.

Once you've determined an RTO and RPO for each system and disaster type, the final step is to determine how long it will take to back up the system and how much the backup will impact the production system.

Everything should start with RTO and RPO, although very few people do it that way. Most people go right to the backup window. Instead, you should concentrate on meeting your RTO and RPO requirements, and the backup window will almost always fall right in line. The reverse isn't necessarily true, however. There are many things that will shrink your backup window but not help your recovery objectives. If your requirements are impossible to meet with a traditional backup system, the following technologies are worth considering.

SNAPSHOTS. The most common type of snapshot is a virtual copy of an original volume or file system. The reliance on the original

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volume is why snapshots must be backed up to provide recovery from physical failures (see "Match snaps to apps," p. 46). Snapshot functionality resides in a number of places, including advanced filesystems, volume managers, enterprise arrays, NAS filers and backup software.

First American Trust Federal Savings Bank, Santa Ana, CA, handles up to $2 billion worth of wire transfers each day. The bank was recently asked by the Securities and Exchange Commission (SEC) to restore one year's worth of Microsoft Exchange e-mail data--a significant request.

One division used Network Appliance (NetApp) Inc.'s unified storage solution, SnapManager for Exchange, and Single Mailbox Recovery software, while another division used traditional backup and tape. The results from the two divisions couldn't have been more different. "The SEC request made the need for using nearline storage to easily recover and access e-mail undisputable," says Henry Jenkins, chief technology officer at First American. "Our disk-based solution rose to the occasion, but damaged tapes and botched backups made restoring from tape excruciating for our sister division."

It took the bank only a few days to restore roughly 360GB of e-mail using the combination of hardware and software from NetApp. In contrast, it took several months for one IT bank staffer to restore a smaller volume of e-mail from tape.

First American also uses offsite replication of critical SQL Server databases, Exchange e-mail and flat-file data that's used to perform routine wire services. All of this critical data creates only 200MB of changed data blocks per day, which are then asynchronously replicated to a remote system located at a disaster recovery (DR) site approximately 100 miles away. The DR system has an RPO of four hours in the event of a site failure.

"SnapMirror software saves us time by not having to replay logs and data at the remote site is, on average, less than 15 minutes behind," says Jenkins. "Every year for the past three years, we've done a disaster recovery test and every year it's just a matter of bringing up the warm servers," he adds.

Snapshots can help you to meet aggressive backup requirements. For example, some snapshots can satisfy an RTO of a few seconds by simply changing a pointer. An aggressive RPO can be achieved by creating several snapshots per day and, because snapshots can be created in seconds, you can also meet stringent backup window requirements. For instance, it's possible to create a stable, virtual backup of a multiterabyte database in seconds--reducing the impact on the application to potentially nothing--which leaves hours to perform a backup of that snapshot. Finally, creating synchronized snapshots on multiple systems is also fairly easy.

There's a growing list of APIs that allow different vendors' products to interface with snapshots; the network data management protocol (NDMP) and Microsoft Corp.'s Volume Shadow Copy Service (VSS) are examples. NDMP lets backup products create a snapshot, and catalog and restore from its contents. VSS allows storage vendors with snapshot capability to have the files in those snapshots listed in and restored from the Previous Versions tab in Windows Server 2003. Hopefully, this capability will be added to workstation versions of Windows and more NAS vendors will support VSS.

Another interesting development is the creation of database agents that work with snapshots. The database agent communicates with the database so that the database believes it's being backed up, when all that's really happening is the creation of a snapshot. Recoveries can be incredibly fast when the process is controlled by the database application.

REPLICATION. Replication is the practice of continually copying from a source system to a target system all files or blocks that have changed on the source system. Replication used to be what companies implemented after everything was completely backed up and redundant, which meant that few used replication. However, many people are now using replication as their first line of defense for providing backup and disaster recovery.

Replication by itself is not a good backup strategy; it copies everything, including viruses and file deletions. Therefore, a replication-based backup system must be able to provide a history by either occasionally backing up the replicated destination or through the use of snapshots. It's usually preferable to make a snapshot on the source and replicate that snapshot to the destination. That way, you can prepare database applications for backup, take a snapshot and then have that snapshot replicated.

This was first published in November 2005

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