9.5. Automated Backup Hardware So far, this chapter has covered only the tape and optical drives themselves. However, today's environments are demanding more and more automation as databases, filesystems, and servers become larger and more complex. Spending a few thousand dollars on some type of automated volume management system can reduce the need for manual intervention, drastically increasing the integrity of a backup system. It reduces administrator frustration by handling the most common (and most boring) task associated with backupsswapping a volume. There are essentially three types of automated backup hardware. Some people may use these three terms interchangeably. For the purposes of this chapter, these terms are used as they are defined here:
Stacker -
This is how many people enter the automation market. Stackers get their name from the way they were originally designed. Tapes appeared to be "stacked" on top of one another in early models, although many of today's stackers have the tapes sitting side by side. A stacker is traditionally a sequential access device, meaning that when you eject tape 1, it automatically puts in tape 2. If it contains 10 tapes, and you eject tape 10, it puts in tape 1. You cannot tell a true stacker to "put in tape 5." (This capability is referred to as random access.) It is up to you to know which tape is currently in the drive and to calculate the number of ejects required to get to tape 5. Stackers typically have between 4 and 12 slots and 1 or 2 drives. Many products that are advertised as stackers support random access, so the line is slightly blurred. However, in order to be classified as a stacker, a product must support sequential-access operation. This allows an administrator to easily use shell scripts to control the stacker. If you purchase a commercial backup product or use an open-source product that can control a tape library, you have the option of putting the stacker into random-access mode and allowing the backup product to control it. (Control of automated backup hardware is almost always an extra-cost option in commercial software.)
Library -
This category of automated backup hardware is called many things, but the most common terms are " library," "autoloader," and "jukebox." Each of these terms connotes an addressable group of volumes that can be automatically loaded via unique volume addresses. This means that each slot and drive within a library is given a location address. For example, the first slot may be location 0000, and the first drive may be location 1000. When the backup software controlling the library tells it to put the tape from slot 1 into drive 1, it actually is saying "move the volume in location 0000 to location 1000." The primary difference between a library and a stacker is that a library can operate only in random-access mode. Today's libraries are starting to borrow advanced features that used to be found only in silos, such as import/export ports, bar code readers, visual displays, and Ethernet ports for SNMP monitoring. Libraries may range from 12 slots to 1000 or more slots. The largest libraries even offer pass-through ports, which allows one library to pass tapes to another library. (This is usually a standard feature in silos.) Some libraries can expand with extra cabinets that actually become part of the base chassis; the robot track is extended into the next cabinet.
Silo -
Since many libraries now offer features that used to be found only in silos, the distinction between the two has blurred. The main distinction between a silo and a library used to be whether or not it allowed multiple hosts with disparate backup applications to connect to the same silo. However, libraries now offer this feature through something called partitioning. Therefore, many people just use the term silo to refer to a really large tape libraryand it's hard to correct them. Another way to subdivide the different types of automated backup hardware is how they allow you to expand them, if they allow that at all. Automated backup hardware can be divided this way into three different categories:
Nonexpandable -
These types of libraries cannot be expanded beyond their base configuration. While this is most common in smaller units, there are plenty of large, nonexpandable libraries as well.
Connectable -
A connectable tape library can be expanded by buying a second library and connecting the two. Each library has its own distinct robot, but they are allowed to share media with each other using pass-through ports. The big advantage to this method is that you can disconnect the different libraries and use them as separate libraries at a later date.
Expandable -
An expandable library allows you to add additional capacity without adding additional robotics. The original robotics simply expand to handle the additional capacity. The big advantage to these types of libraries is that you can buy exactly the amount of capacity when you need itand only when you need it. |
