Published: 08 Jun 2005
|Veritas' NetBackup 6.0 works better with disk|
This article is the final installment of a three-part series on disk-based data protection. The first two parts covered how to use disk to enhance traditional backup systems. The focus now turns to disk-based data protection options that could replace one or more parts of a backup system. The options include snapshots, replication, continuous data protection (CDP) and data reduction backup (DRB). These technologies will reduce backup and restore times, and help meet requirements such as RTO, RPO, backup window and synchronicity.
RTO--how long it takes to recover a system--can range from zero seconds to several days or even weeks. Each piece of information serves a business function, so the question is how long the business can live without that function. If the business can't live without it for one second, then the RTO is zero.
RPO is determined by how much data a business can afford to lose. If the business can lose three days' worth of a set of data, then the RPO is three days. If the data is real-time transactions essential to the business, the RPO is zero for that application.
There can also be an RPO for a group of machines. If several systems are related to each other, they may need to be recovered to the same point in time. This is the synchronicity requirement; to meet it, all related systems have to be backed up at exactly the same time.
|Pros and cons of alternative backup methods|
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 volume is why snapshots must be backed up to provide recovery from physical failures (see Match snaps to apps). Snapshot functionality resides in a number of places, including advanced file systems, volume managers, enterprise arrays, NAS filers and backup software.
Snapshots can help you to meet aggressive backup requirements. For example, some snapshots can meet 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. The next section discusses how replication is a great way to do that. Finally, creating synchronized snapshots on multiple systems is also fairly easy.
|Adaptec switches from tape to disk|
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.
When used with snapshots, replication allows for tiny backup windows. The snapshot takes just seconds to create, and replication is the quickest way to back up that snapshot to another device. You can also cascade replication to provide multiple copies, such as an onsite and offsite copy. If you want to provide a tape copy of the replicated snapshot, just back up one of the destination devices. But replication software doesn't usually provide recovery features. The RTO, RPO and synchronicity requirements that you'll be able to meet will be based on how you're performing snapshots or backups, and how quickly they'll be able to recover.
|A tale of two divisions|
DRB systems. DRB systems were designed to answer the following questions: If only a few bytes in a file change, why back up the entire file? If the same file resides in two places on the same system, why back it up twice? Why not store a reference to the second file? And why waste server and network resources by backing up the same file across multiple systems?
By backing up a file once, and then backing up only the changed bytes, backup windows are substantially reduced. Tape copies of disk-based backups can usually be created at any time, depending on your requirements. Some DRB products can meet aggressive RTO requirements by restoring only the blocks that have changed since the file was last backed up. The RPO and synchronicity abilities of DRB products are based on how often you back up, but it's common to back up hourly.
The biggest advantage to DRB products is that, from the user adoption perspective, they're the closest to what users know. Their interfaces are similar and they often have database agents like traditional backup software. They're also able to back up faster and more often, and use much less bandwidth. CDP. A CDP system is basically an asynchronous, replication-based backup system. The software runs continuously on the client to be backed up, and each time a file changes, the new bytes are sent to the backup server within seconds or minutes. But unlike replication, a CDP system can roll back to any changes at any time.
CDP products transfer data to the backup server in different ways. Some transfer changed blocks immediately, while others collect changed blocks and send them every few minutes. They also differ in how they do recoveries. Some products are able to restore only the blocks that have changed from a particular point in time, while other programs operate in a more traditional manner by recovering the entire file or file system. Obviously, the first method accommodates more aggressive RTOs and RPOs than the second method. Also, CDP products can meet any type of synchronicity requirement because they can recover one, 10 or 100 systems to any synchronized point in time.
Another difference in CDP products is that some are database-centric and work only with a particular database, such as Microsoft Exchange or SQL Server. Remember that, unlike traditional backup products, file-based CDP products aren't going to provide interfaces for your database applications. These CDP products copy blocks to the backup destination in the same order they're changed on the client. Restarting your database causes it to go into the same mode that it would go into if the server were to crash. It examines the data files, figures out what's inconsistent, rolls backward or forward any necessary transactions or blocks, and then the database is up. If the CDP product puts the blocks back in the exact order in which they were changed, then the database should be able to recover from any point in time. Some products can even present a logical unit number or volume to your database that it can mount and test before you do the recovery.
Of course, your database vendor may have a different opinion about CDP: If you're not using their supported backup method, they may not be helpful if something goes wrong. Discuss the support issue with your database vendor, and include your DBA in the discussion.
|Replicating data every 15 minutes|
You should only consider switching backup products if your current backup product can't meet your requirements (see Pros and cons of alternative backup methods). There are a number of requirements that might prompt you to consider alternatives, such as:
- Remote office data protection
- Backing up large databases
- An application with an RPO of zero
The most common area where backup requirements are difficult to meet is the remote office. Traditional backup schemes can't meet remote office RTO/RPO requirements. There's either too much data or not enough bandwidth to support a reasonable RTO or backup window. Any CDP product can provide backup and recovery of a remote office; most offer two methods. If long RTOs are acceptable, remote sites can back up directly to your central office. In the case of a disaster, just copy the data from the central data center to a disk or tape and send it to the remote site. If this meets RTO requirements, it's the least-expensive option. For tighter RTO requirements, install a backup device at the remote office. The remote office systems can back up to it, and it can then replicate the data to the central site. This provides local recovery and disaster recovery without touching a tape.
CDP products are also superior to traditional backup methods when backing up very large databases. There isn't enough time or horsepower available to transfer several terabytes of data to tape every day. A CDP product could continually back up a database throughout the day, with no noticeable backup window or application impact. Depending on the product, a stringent RTO and short RPO could also be met. Also, some products provide a disk-based copy that can be used in a disaster situation while the real volume is being recovered.
Finally, some database applications require a zero RPO. Most databases can meet such a requirement if they're configured correctly, and if the transaction log is backed up throughout the day. If your database supports that kind of functionality, it's probably best to stick with it. If not, try one of these newer methods.