Data determines the right disaster recovery


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Backup applications copy primary stored data directly from the application server and move it over TCP/IP networks to a local backup server or remote DR backup server. The server then writes the copied data to disk or tape. RPO is the window between backups or incremental backups. RTO is minimally hours, but usually days to weeks.

While backup is the primary DR application deployed in most IT organizations, it also has the highest failure rate. Failures can be attributed to user error, bandwidth issues, throughput issues, tape issues and even application server availability requirements.

The primary advantage of backup is its familiarity--it's a known quantity, both good and bad. Storage administrators know how to deploy and use backup, and the TCO is relatively low depending on the storage environment.

The two key disadvantages of backup are that its RPO and RTO are usually quite high, and backup is a local process. There are exceptions, however. Several backup programs distribute and centralize backup while providing continuous incremental backups, shrinking the RPO considerably. Unfortunately, recovery time is still a lengthy process. Data consistency and usability--the ability to use the backed up data without modification, reordering or re-creation--may also be a problem. Backup programs require server-based agents and backup costs escalate sharply as the environment scales

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and grows more complex.

Backup products are evolving and improving. Virtual tape, disk-to-disk-to-tape (D2D2T) and massive array of idle disks (MAID) technologies speed backups and recovery times. New types of backup software, such as content-addressable storage (CAS), reduce the amount of data required to back up by sending only changed data and meta tags about data. This significantly reduces recovery times and dramatically increases recovered data usability. Distributed backup eliminates the installation of server agents. These new types of backup have RPOs and RTOs that can be used for critical data.

Replication software replicates data from server to server synchronously and asynchronously. There are incremental and CDP modes. Replicated data travels over TCP/IP networks to a remote server's disk, and then a backup client is needed to move the data to a storage device. RPO for replication is similar to the RPO for storage array remote mirroring, depending on whether it's synchronous or asynchronous. RTO can be a little faster because the DR application servers are already collocated with the DR storage.

Replication software is easy to install and operate. It can run locally and distributed, and because it's server-, storage- and infrastructure-agnostic, there are no hardware lock-ins. Replication software costs are less than those for backup software and much less than storage array-based remote mirroring. Replication has evolved to include application-aware agents, continuous protection and rollback capabilities. One important benefit to replication is data migration. Replication software simplifies the process and replicates only the data that needs to be replicated in a non-disruptive manner.

Replication software can't prevent damaged data from being replicated, and server agents must be maintained and managed. RTO can be significantly increased if there's a single DR server caching the replication from different application operating systems. In the event of a disaster, all data must be recovered and rewritten before the applications can access the data. This is similar to backup. If there's a DR replication server per operating system, the RTO rivals storage array mirroring.

A snapshot provides a point-in-time reference marker to data stored on a storage system. Snapshots are a way to speed RTOs. There are two primary types of snapshots: copy-on-write and split-mirror.

A copy-on-write snapshot stores changes and additions to existing data. Data recovery is rapid in case of a disk write error, corrupted file or program malfunction; however, all of the previous snapshots must be available if complete archiving or recovery is required. A split-mirrored snapshot references all the data on a set of mirrored drives where one is local and the other is local or remote. Each time the snapshot is run, it snaps the entire volume, not just new or updated data.

Snapshot is easy to install and operate. A copy-on-write snapshot provides a short RTO and a relatively slow RPO (data must still be recovered before it can be used). Split-mirror snapshots have a relatively long RPO, but they speed data recovery (RTO), duplication and data archival. One important benefit to split-mirror snapshots is that it's possible to access data offline for tasks such as data mining and offline production data testing. Some snapshot applications provide continuous snapshots and rollback capabilities based on a point in time, which offers faster RTO.

A split-mirror snapshot uses a lot of system resources and will degrade the performance of the platform it's running on while it creates the snapshot. And snapshots can't prevent a rolling disaster of snapping corrupt data.

This was first published in January 2005

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