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Our backup choices are much greater and more complex compared to the days when you simply had to choose a backup application and tape library. Disk has solved the reliability and performance issues that most storage managers have experienced with traditional backup systems, but disk-to-disk-to-tape (D2D2T) has complicated backup and restore processes. Users must now choose from three backup architectures and four types of disk-based backup targets. What follows are the pros and cons of each approach.
The traditional backup architecture
Before explaining how the different D2D backup options work, it's important to understand the backup systems they'll work with and why you might want to augment those systems with disk. In a traditional backup architecture (see "Traditional backup architecture", next page), software resides on the backup client (the server to be backed up) that allows the backup server to transfer that client's data to tape, disk or virtual tape. The data may be transferred across the network (LAN-based), from the client directly to tape/disk across the SAN (LAN-free) or directly from primary storage to secondary storage across the SAN (server-free). In each case, the data is converted into a different format that's understood by the backup software running the backup. This format could be tar, cpio, dump, NTBackup or a custom format understood only by that particular backup package.
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- If you only buy enough disks to hold a few nights' backups (i.e., disk caching), you'll speed up backups but won't speed up restores.
- If you want to speed up backups and restores, you should buy enough disk to hold all onsite backups.
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The biggest advantage of a traditional backup architecture is that it's well understood and has a solid, mature code base. The biggest disadvantage is how it uses tape. It's difficult for a traditional backup system to use the streaming nature of modern tape drives efficiently. To properly stream tape drives, some backup software products (like EMC Corp.'s Legato NetWorker and Veritas Software Corp.'s [now Symantec's] NetBackup) send multiple backup jobs simultaneously to the same tape drive, a technique called multiplexing or interleaving. This helps backups, but has a negative impact on the restore of a single backup; the backup software has to read the entire tape and disregard data it doesn't need. Other backup products, such as IBM Corp.'s Tivoli Storage Manager (TSM), solve the streaming issue with disk staging where backups are first sent to disk before they're sent to tape.
With the advent of lower-priced ATA-based disk arrays, however, everyone can take advantage of disk staging or disk-based backups without switching from a traditional backup architecture. Simply augment your tape library with a combination of disk and tape.
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