8.2 Tape Backup

Tape backup is a universal requirement, and a universal problem, for all data networks, regardless of the specific topology employed. Data security via backup is not only desirable but also sometimes mandated by law (for example, in finance and banking operations). For server/storage configurations based on parallel SCSI, tape backup strategies may require a dedicated tape backup subsystem for each server (an approach that optimizes performance but increases costs) or a centralized tape backup configuration that requires a data path across a LAN (an approach that reduces cost but may hinder performance). As applications generate ever-growing storage requirements, the time required to back up data increases. You cannot accommodate this backup window via the production LAN without increasing the number of switch links to accommodate block storage as well as user bandwidth requirements. And, given the amount of data supported by an individual server, provisioning tape subsystems for every large server is cost-prohibitive.

Traditional LAN-based tape backup is based on backup of files. As shown in Figure 8-2, a tape subsystem is attached to a backup server, which in turns sits on the LAN. Each server on the LAN may have gigabytes of direct-attached storage that needs to be secured through backup. The backup server instructs each server to initiate a backup, with the data sent over the LAN from server to backup server. This type of backup involves multiple conversions. Upon launching a backup, the target server must read blocks of SCSI data from disk, assemble the blocks into files, and packetize the files for transfer over the LAN. At the backup server, the inbound packets must be rebuilt into files, and the files, in turn, are disassembled into blocks to be written to tape. Therefore, the original data blocks that reside on the target storage undergo four steps of conversion before reappearing at the destination as blocks: blocksfilepacketsfileblocks. Both the server and the backup server must devote considerable CPU cycles to SCSI as well as network protocol overhead.

Figure 8-2. Traditional LAN-based tape backup

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On shared media LANs, this backup operation places a significant load on the LAN and may disrupt user messaging traffic. Most enterprise LANs, however, use switched Ethernet infrastructures instead of shared media. Bottlenecks may occur on switch-to-switch links, but not on individual Ethernet switches.

SAN-attached storage and tape offers efficiencies for tape backup by eliminating the block-to-file conversion overhead. Instead, blocks are read from disk and written directly to tape across the SAN. For Fibre Channel, the block transport is over a separate topology. For IP SANs, the block tape backup may be over a separate Gigabit Ethernet switched network, or over a VLAN within a larger Gigabit Ethernet complex. In both cases, by leveraging block-based SCSI transfer, you allow more data to be transported in less time and with less overhead.

Placing servers, storage, and tape on a peer-to-peer network also enables new backup applications such as server-free tape backup. As long as the server is the exclusive owner of storage, the server must always be in the data path for any type of access, including tape backup. On a SAN, however, storage is directly accessible by a tape subsystem. As shown in Figure 8-3, the server can be taken out of the backup data path, with blocks of data read from disk and written directly to tape. Server-free backup requires an extended copy (third-party copy) agent, which typically resides in the tape subsystem, in a bridge, or in a SAN switch.

Figure 8-3. Server-free tape backup using extended copy

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Whereas the SAN plumbing and utilities such as extended copy facilitate efficient backup of storage data, the application software that initiates and manages backup processes varies in capabilities from vendor to vendor. Although every storage administrator recognizes the necessity of data backup, it is often difficult to verify that a backup operation was completed and that the tapes can actually be used for data restore. In addition, regular backup operations may repeatedly copy data that is unchanged over time, and that adds to the volume and duration of the backup process. Vendors of backup software may provide utilities for verification, point-in-time (snapshot) backup for active databases, changed-block-only backup, or other value-added backup services. As the volume of storage data grows, the task of securely backing up data in a reasonable time frame becomes increasingly difficult. Backup application software and tape vendors are trying to meet this challenge. Storage vendors are offering data replication, an alternative but complementary solution.



Designing Storage Area Networks(c) A Practical Reference for Implementing Fibre Channel and IP SANs
Designing Storage Area Networks: A Practical Reference for Implementing Fibre Channel and IP SANs (2nd Edition)
ISBN: 0321136500
EAN: 2147483647
Year: 2003
Pages: 171
Authors: Tom Clark

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