UNIX System Administration Handbook - Evi Nemeth [112]
A stacker is a simple tape changer that is used with a standard tape drive. It has a hopper that you load with tapes; it unloads full tapes as they are ejected from the drive and replaces them with blank tapes from the hopper. Most stackers hold about ten tapes.
A jukebox is a hardware device that can automatically change removable media in a limited number of drives, much like an old-style music jukebox that changed records on a single turntable. Jukeboxes are available for several types of media, including DAT, DLT, AIT, and CD. Jukeboxes are often bundled with special backup software that understands how to manipulate the changer. Storage Technologies and Sony are two manufacturers of these products.
Tape libraries are a hardware backup solution for large data sets—terabytes, usually. They are closet-sized mechanisms with multiple tape drives (or CDs) and a robotic arm that retrieves and files media on the library’s many shelves. As you can imagine, they are quite expensive to purchase and maintain, and they have special power, space, and air conditioning requirements. Most purchasers of tape libraries also purchase an operations contract from the manufacturer to optimize and run the device. The libraries have a software component, of course, which is what really runs the device. Storage Technology is a leading manufacturer of tape libraries.
Hard disks
We would be remiss if we did not mention the decreasing cost of hard drives as a reason to consider disk-to-disk backups. Although we suggest that you not duplicate one disk to another within the same physical machine, hard disks can be a good, low-cost solution for storage over a network.
One obvious problem is that hard disk storage space is finite and must eventually be reused. However, disk-to-disk backups are an excellent way to protect against the accidental deletion of files. If you maintain a day-old disk image in a well-known place that’s shared over NFS, users can recover from their own mistakes without involving an administrator.
Summary of media types
Whew! That’s a lot of possibilities. Table 10.1 summarizes the characteristics of the media discussed in the previous sections.
Table 10.1 Backup media compared
a. Uncompressed capacity and speed
b. Maximum burst transfer rate; the manufacturer does not disclose the true average throughput.
W. Curtis Preston has compiled an excellent reference list of backup devices by manufacturer. It’s available from www.backupcentral.com/hardware-drives.html.
What to buy
When you buy a backup system, you pretty much get exactly what you see in Table 10.1. All of the media work pretty well, and among the technologies that are close in price, there generally isn’t a compelling reason to prefer one over another. Buy a system that meets your specifications and your budget.
DAT and Exabyte drives are excellent solutions for small workgroups and for individual machines with a lot of storage. The startup costs are relatively modest, the media are widely available, and several manufacturers are using each standard. Both systems are fast enough to back up a buttload of data in a finite amount of time.
ADR may well deserve to be listed along with DAT and Exabyte, but the newness of the technology and the tenuous existence of its sole supplier make it significantly less desirable in our opinion.
DLT, AIT, and Mammoth-2 are all roughly comparable. There isn’t a clear winner among the three, and even if there were, the situation would no doubt change within a few months as new versions of the formats were deployed. All of these formats work well, and they all address the same market: university and corporate environments that need serious backup hardware (which is to say, pretty much all university and corporate environments).
In the following sections, we use the generic term “tape” to refer to the