3.6 Data Protection

Data-protection software, from coordinating tape-based backup to real-time disk replication, defends corporate data assets. Through a range of mechanisms that deliver onsite and offsite data copies at various time intervals and across multiple media formats, corporations mitigate the chance of data loss while striving to provide continuous availability. For most companies, the costs of data loss far outweigh the costs of data protection. As such, they invest millions of dollars per year on data protection techniques such as those described in this section.

The primary decisions for data protection compare cost to availability. In this case, availability refers to both the time taken for the backup operation and the time needed to restore the data set in the event of a disaster. Figure 3-12 outlines the position of several data-protection schemes based on their cost and availability measures.

Figure 3-12. Cost-availability trade-off.

For most organizations, the redundancy built into storage architectures spills into the data-protection implementations as well. For example, many companies implement more than one data-protection technique, such as combining tape-based offsite backup with onsite snapshot copies to disk. This gives them near instant recovery onsite as well as the insurance of an offsite recovery if needed.

3.6.1 Tape-Based Backup

Tape archiving is often relegated to the low end of the totem pole of backup technologies. While falling disk prices have made disk-based backup solutions more attractive to a larger segment, tape still fills a valuable need. Density alone makes tape an attractive choice for large backup operations ”tape library capacities measure in hundreds of terabytes compared to disk array units in tens of terabytes. Additionally, the flexibility of removable tape media cartridges for both onsite and offsite storage adds another level of protection. Today's IP storage networking options may soon impact transport of tape cartridges. Why bother with moving them between buildings or sites if the data can be easily sent over an IP network? Even with that capability, it may still be more effective to have a tape library for archiving due to device and media capacity.

Storage networks enable sharing of tape libraries among multiple servers, including NAS filers, as shown in Figure 3-13. This type of configuration also requires software to manage access to the tape library, preventing multiple servers from simultaneous access. Backup packages like Veritas NetBackup, Legato Networker, CA Brightstor, and IBM Tivoli for Storage provide the features for managing tape libraries in shared environments, including schedule modules to perform backups at the appropriate intervals, such as daily, weekly, or monthly.

Figure 3-13. Consolidated tape backup across servers and NAS.

In SAN configurations with shared tape libraries, the backup software resides on a backup application server that manages the control information with the individual servers. With clearance established, the backup server grants access to the appropriate server and allows a direct link to the tape library. This process, often referred to as LAN-free backup, facilitates faster backup operations than do more traditional data paths for shared tape libraries through the LAN.

Tape libraries also serve as a useful complement to NAS. Even though NAS delivers file-based storage through its front end, the back end of a NAS system often has a block-based Fibre Channel connection that can be networked to a tape library for block-based backup of the NAS unit. This type of solution provides economical archiving of NAS data and may also operate simultaneously with NAS replication across multiple filers.

Server-free backup (Figure 3-14) is another technique used in conjunction with tape backup, although it can also be used with disk backup. In server-free implementations the data path is completely offloaded to the storage devices, freeing servers to focus on application processing. Server-free backup solutions take advantage of the same LAN-free designs implemented with SANs but also use data-mover agents that reside in the storage fabric to facilitate disk-to-tape or disk-to-disk data paths.

Figure 3-14. Server-free backup through third-party copy.

The data-mover agents employ a third-party copy command called extended copy. This can run off of a fabric-based storage device such as a router or appliance. The backup application server initiates a command to the data mover, and the data mover then assumes control of the copy operation until complete. The ability to conduct a high-speed, image-level backup free of server intervention builds a scalable backup infrastructure. Key benefits include more backup operations within a fixed window, simpler administration, reduced server load, and faster restores .

3.6.2 Disk Backup and Recovery

Disk-based backup ranks high on the simplicity scale but equally high on the required capacity scale. Specifically, a single disk-based backup will double the required disk storage capacity. This may be on top of disk capacity required for RAID features. In the extreme, an organization may find itself with only 25 percent usable capacity of total terabytes owned. This can be a significant cost when compared to tape archiving.

Aside from capacity considerations, disk solutions eclipse tape for achieving short backup and restore windows . For applications requiring guaranteed availability, disk-based solutions are the only mechanism for secure, rapid recovery.

Because of these cost, availability, and performance dynamics, permutations of disk copy, such as point-in-time and snapshot copy as well as replication and mirroring, come into play.

Many disk-backup solutions leverage internal RAID functions built into subsystems. This allows for multilayered redundancy to protect against disk failure, mirror failure, unit failure, and in the case of remote facilities, infrastructure failure.

3.6.3 Point-in-Time and Snapshot Copy

Point-in-time and snapshot copy solutions address the capacity requirement concerns of disk backup while providing short restore times. Point-in-time copies, as the name implies, isolate time-stamped copies of data at regular intervals and provide the ability to revert to those images in the event of application or data failure.

The first point-in-time copy includes the entire data set, doubling the storage capacity requirement. Thereafter, only changes to the data set are copied at regular intervals. Administrators can choose to initiate and maintain multiple point-in-time copies so recovery can roll back to the appropriate data set. These intervals can measure from minutes to hours to days.

With an updated copy available at all times, the recovery process with a point-in-time copy occurs almost immediately. Instead of attempting to restore the database, users are simply redirected to the last stable copy.

Snapshot copies operate in a similar fashion to point-in-time, but with the slight difference that the snapshot volume merely tracks incremental changes with pointers back to the original data set. Like point-in-time, snapshots can be created at regular intervals to provide rollback capabilities in the event of failure. However, snapshots do not replicate the original data set. Therefore, they don't require the upfront capacity increase, although additional capacity is required to maintain the snapshot information. This will vary based on the amount of data changed at each interval and the interval or snapshot frequency. The additional capacity required for a snapshot copy correlates to the amount of original content that must be saved before incremental changes are made.

Like point-in-time, snapshot copies provide quick recovery in the event of disaster, since no restore is required. But snapshot copies alone cannot protect against all disasters, since the copies still point to the parts of the original data set, and a physical storage failure could invalidate several logical snapshot copies. For a complete disk-based backup solution, snapshot would require the addition of replication or mirroring to protect against physical storage failure or site disaster.

3.6.4 Hierarchical Storage Management (HSM)

Serving a data-protection function and also a space optimization function, hierarchical storage management (HSM) helps storage administrators balance the cost per megabyte with the retrieval time of varying storage media. The conventional business driver behind HSM was the relative high cost of disk compared to the relative low cost of tape, coupled with varying types of data and service levels within the organization (see Figure 3-15).

Figure 3-15. Comparisons of media technologies: disk, optical, tape.

For example, a mission-critical e-commerce database must be instantly accessible at all times and must have mirrored or replicated copies available in the event of disaster and disk backup for fast restore. On the other end of the spectrum, large CAD files for products that have been discontinued probably don't merit the same availability, yet designers are reluctant to discard the data ”they want it available if needed. This second example fits perfectly with HSM principles: to take less critical data that doesn't require immediate access times and move it to more economical storage, such as tape. In the event that the design team resurrects a particular product, the CAD files could be transferred back to disk during the production cycle to facilitate faster access times.

With disk costs dropping precipitously, HSM can be applied to disk-only environments as well. For example, mission-critical information might reside on high-end Fibre Channel disk drives, while less critical information could be stored on much less expensive serial Advanced Technology Attachment (ATA) drives .

HSM software manages this entire process, allowing administrators to set guidelines and storage categories that determine when and where files and volumes are stored. Automated HSM policies identify files that haven't been accessed recently, maintain a pointer at the original location, and then move the actual data to less expensive media. This migration process happens transparently to users, requiring only a quick restore when the file is accessed, if ever.

The concepts of HSM have been a part of enterprise storage strategies for many years . Today, with new SAN architectures and the explosive growth of data, applying HSM principles through specific HSM packages or other storage management software can lead to significant cost savings in storage capacity purchases.