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Consider the library capacity for tape drives and tape media. A tape library must hold not only today's backup, but backups that might reach back months -- even years. Ultimately, the capacity of a tape library is limited not by the number of drives, but by the tape media selection and the number of tapes housed in the library. For example, IBM's TS3500 tape library touts uncompressed capacities from 52 TB to 5.5 petabytes (PB) when using IBM Ultrium 4 tape cartridges even though one frame can only use up to 12 drives (192 drives total in the library). Similarly, libraries like the PX720 from Quantum Corp. can support from one to 20 tape drives per frame. The library supports SDLT 320, SDLT 600, DLT-S4, LTO-2, LTO-3, and LTO-4 drive types and offers up to 726 media slots (scaling to 3600 slots). Quantum boasts capacities to 581 TB with data transfers of 11.5 TB/hr per single frame.
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Consider the connectivity options. Many tape libraries employ FC or SCSI host bus adapters (HBAs) for connectivity directly to a SAN switch. As an example, Qualstar's XLS includes integrated support for up to four dual-ported HBAs -- eight ports total. Gigabit Ethenet ports may be available for connection across a LAN. A standard Ethernet port may allow LAN connectivity for tape library management applications like Qualstar's X-Link or the BlueScale Environment with SpectraLogic Corp., libraries.
Consider data transfer rates between the library and network. Transfer rates limit your backup window. As data volumes grow, faster transfer rates are often necessary to keep backup windows within manageable levels. For example, the XLS product from Qualstar Corp. is available in a variety of modular configurations touting native data transfer rates from 1.1 TB/h up to 27.6 TB/h. An enterprise with 10 TB to backup would then require a nine hour backup window with the 1.1 TB/h configuration, or less than 20 minutes with the 27.6 TB configuration. Transfer rates are generally influenced by additional I/O modules installed in expanded library configurations.
Evaluate the need for advanced tape tracking systems. It's virtually impossible to track thousands of tapes rotating between off-site and library storage. One solution has been to implement bar codes to match tapes to an index of contents allowing administrators to quickly determine what content resides on a given tape. More recently, radio frequency identification (RFID) has emerged, allowing entire groups of tapes to identify themselves to the tape library. Large libraries should include advanced tape tracking features.
Consider the tape storage environment. When considering a tape library, be sure to evaluate the long-term internal storage conditions such as temperature, humidity and contamination. Some libraries try to optimize tape and drive reliability by introducing filtered positive air pressure to reduce dust and other contaminants. If tapes are stored offsite, remember to periodically inspect the conditions at any remote location as well to ensure suitable environmental conditions as well as security protocols.