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|How much tape do you need?|
Beware the backhitch
IBM's senior program manager, Bruce Master, notes that the streaming data rates of these new tape drive technologies have gotten so fast that the backups can run faster than the rate at which they receive data. This creates a new problem during the backup period called backhitch.
A backhitch occurs when the flow of data onto a tape during a backup gets interrupted. When this occurs, the tape drive can't immediately stop. Instead, it must slow down and stop the tape, back the tape up and reposition the tape at the point where the data flow was interrupted. The time to perform a repositioning is short, but frequent backhitching can significantly degrade performance.
To counter backhitching, IBM incorporated a new technology called Digital Speed Matching into its LTO Ultrium 2 tape drives, which lets the drive adjust as closely as possible to the speed and flow of the incoming data. This decreases backhitches and improves throughput performance, says IBM.
Other vendors have encountered similar issues and implemented their own solutions. StorageTek provides an adequate buffer size to mask the backhitch. On their high-end 9840B and 9940B tape drives, they incorporate 32MB and 64MB buffers to mask this backhitch. DLT and SDLT tape drives use a methodology similar to LTO's methodology called adaptive cache buffering which automatically adjusts the DLT or SDLT tape drive's transfer rate to match the host system's data rate.
Reliability, density and scalability
Hewlett-Packard's enterprise class ESL9595 tape library typifies the three requirements of nearly every tape library: reliability, density and scalability. For reliability, the ESL9595 tape drives connect directly into the backpane which is part of the intelligence of the unit. While other tape library manufacturers offer hot-swappable tape drives, because ESL9595's tape drives plug directly into a backpane, no cabling must be disconnected to remove the drive, which reduces the risk associated with exchanging failed tape drives on the fly by isolating the other drive in the library from this change.
How often tape drives fail depends on the tape drive. The ESL9595 supports both LTO and SDLT tape drives. The LTO tape drive has the longest head life of any tape drive on the market--coming in at 60,000 hours--or nearly seven years. This contrasts with the SDLT tape drive which rates at 30,000 hours or about three and a half years. For those looking beyond the LTO and SDLT mediums, Qualstar rates the life of the heads of their AIT tape drives at over 50,000 hours, or nearly six years.
Density is also important as a tape library function. HP helps customers maximize capacity per square meter on their data center floors, without taking up precious floor space. To achieve this, a fully configured ESL9595 may hold 595 SDLT tape drives--or up to 190TB compressed--in over just one square meter of floor space.
Spectra Logic's Spectra 64K holds nearly 168TB of compressed data using 645 AIT tape cartridges in under one square meter of floor space. On the LTO side, StorageTek's single frame L700e scales to 271TB compressed using 678 LTO Gen 2 tape cartridges in a single library on about 1.75 square meters of floor space.
Scalability is the final must for most tape library products today. The pass-through mechanism (PTM) in the HP ESL9595 reflects this third necessity. The ESL9595's configuration allows up to five ESL9595 tape libraries to be joined together into one multiunit tape library system. The PTM technology helps give this library the additional functionality to allow tape cartridges to be shared between any of the libraries connected in its multiunit configuration.
To what degree tape libraries scale vary according to vendor. StorageTek L700e enables two of these units to be configured as one logical unit. Quantum's ATL P7000 equals HP by allowing five of their libraries to function as one logically scaled unit. ADIC ATL/2 enables nine of their modules to be configured as one, while StorageTek 9310 PowderHorn grows to manage 24 units as one logical unit.
This was first published in April 2003