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How can I increase the system transfer rate to my tape drive, autoloader or library?
(KB # F1030358)

A large number of items affect performance while transfering data between a system and a tape drive. Some you may be able to change, and others not.

Some items that can affect the transfer rate to the tape drive are:

  • Network performance. If data is being backed up over a network, low transfer rates may be due to the network bandwidth and amount of traffic on the network. To check if the network is a bottleneck, perform a local backup to see if the transfer rates improve.
  • The transfer size and block size used during the backup. Performance will be increased by increasing the data transfer size. Why does this help? With every command used to send data to the tape drive, a certain amount of additional communications 'overhead' must also take place on the SCSI bus. By increasing the amount of data transfered with each SCSI command, the 'overhead' takes a relatively smaller portion of bus bandwith. Your backup application may allow you to set the blocksize and/or transfer size (sometimes called "buffer size"). Refer to the documentation for your backup application for help on increasing blocksize and transfer size.
  • The number and size of files and directories being backed up.More system overhead is required when reading many small files than when reading fewer large files. Throughput may be decreased if the data being backed up consists of many small files, especially if they are scattered through different directories. Backup throughput may be increased by consolidating data in larger files or fewer directories.
  • Multiple applications running on the system. Limit the number of applications which are running on the system. If your system is multi-tasking and running many different programs, the backup transfer rate may be greatly affected. Several items that can affect transfer rate are screen savers, caching programs, and anti-virus protection pro grams.
  • System resources. Data throughput may be limited by low system memory, processor speed, swap file settings or free space on the hard disk. Verify that the system meets at least the minimum requirements for your backup software.
  • Fragmented files. A fragmented file is broken up into pieces, scattered in various locations across the hard disk. Reading a fragmented file from the hard disk takes more time because the heads must physically move to different locations on the disk. Perform de-fragmentation on the hard disks where the data is located.
  • Compression. Different data types will compress at different ratios, and the transfer rate will vary depending on the compressibility of the data. Another thing to keep in mind is that some files such as JPEG and MPEG are already compressed and will not compress further. Additionally, if files are first compressed using software compression, they will not compress any further using hardware compression. The best option is to use hardware compression instead of software compression, and never use both.
  • Tape wear. If you are using a tape that is well worn or of poor quality, the tape drive may be performing high numbers of rewrites to correct errors. Use tapes that are in good condition.
  • Dirty heads in the tape drive.When the recording heads get dirty, error rates may increase and backups can take longer because the tape drive has to rewrite the data in error. To maintain top performance, you must clean your tape drive regularly.
  • SCSI bus configuration. Improper configuration may lead to failed backups or error recovery operations. Review SCSI bus configuration tips and the documentation with your adapter card to insure proper configuration.
  • Shared bus bandwidth. Don't connect a tape drive and a hard disk on the same bus. If the same bus is being used to read data from the hard disk and to write data to the tape drive, overall performance to both devices will be decreased due to sharing the bus bandwidth. Connect your tape drive to a separate bus for better performance. Additionally, limit the number of tape drives connected to the same bus. If the backup application is configured to write data to multiple tape drives at the same time, performance will be increased if fewer drives are sharing the same bus.


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