Dell PowerVault 110T LTO2 Manual

Also see for PowerVault 110T LTO2: Compatibility Matrix Handbook

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Dell PowerVault 110T LTO2 Manual Manual pdf 13 page image

Performance Considerations for Tape Drives and Libraries 13SAN ConfigurationsFibre Channel (FC) offers many advantages over SCSI. First, it overcomes the distance limitationsof SCSI (12 meters for LVD SCSI versus 300 meters for a short-wave 2-Gb FC link) and allows forthe transmission of data at higher speeds. As a serial network protocol rather than a bus-basedarchitecture like SCSI, FC has also become the protocol of choice for the implementation of SANs,allowing for the consolidation of data storage resources. In addition, each FC connection is madeup of a transmit link and a receive link, allowing for full-duplex operation. This means that data canbe transmitted in two directions simultaneously. Therefore, during a backup operation across asingle FC connection, data can be read from a source and written to tape without taking turns incommunication, effectively doubling the bandwidth of a connection. See Figure 1-5.Figure 1-5. Fibre Channel Link DiagramWhen setting up tape libraries in a SAN, performance can still be affected by various factors. Thesefactors include FC link speeds, data flow between the source and tape library, and performancelimitations of external storage arrays. With an understanding of the overall setup and managementof the solution, many of these factors can be avoided.Even with the high data bandwidth offered by the FC protocol in SANs, proper considerationsmust be made for tape drives in order to avoid a situation in which the FC link may limitperformance. The data rate of a 2-gigabit (Gb) FC link is 200 MB/sec (that is, 200 MB/sec on thetransmit link and 200 MB/sec on the receive link). Therefore, attempting to operate multiple tapedrives across the same link can potentially exceed the full bandwidth of a link. If the host isoperating with a legacy 1-Gb adapter, backing up data to two drives may be sufficient to revealsignificant performance limitations.Therefore, when using three or more tape drives simultaneously on a 2-Gb link, you may need todistribute the backups across a number of connections, rather than relying on a single link. This iswhere understanding the SAN solution's topology is beneficial. Following the data path during abackup operation as it is read from the source and then written out to tape will help administratorsrecognize any potential bottlenecks. If any bottlenecks are identified, measures may be takendepending on the configuration. For example, if the backup solution requires multiple drives to bein operation at once, splitting the tape hardware across separate fabrics may improve performanceby splitting the connections. See Figure 1-6.HOSTFibreChannelDeviceTransmitReceiveReceiveTransmit2 Gb = 200 MB/sec2 Gb = 200 MB/sec