How does ARCserve control compression?

The software can turn hardware compression on and off for the drive. When hardware compression is enabled the software compression can not be enabled.

Hardware compression is enabled in the Device Manager window and software compression is set as an option in the backup window.

ARCserve manages compression on a per tape basis. Every time a tape is loaded it checks to see if the tape was used with or without hardware compression, and it will automaticly maintain that status. So it makes no difference how the tape drive has hardware compression set ARCserve will change it on the fly as needed.

Load a tape and check in ARCserve Device Manager if it shows compression is enabled or disabled. If it shows compression disabled to change it to enabled do a Quick Erase Plus. Then enable compression by clicking on the Compression icon and do a Format. This will change the status of the tape. From now on that tape will be used with hardware compression. If after the erase the compression icon is still greyed out check to make sure the tape drive supports hardware compression.

If a tape was used with compression disabled ARCserve will always disable compression at the drive for that tape. Once a tape is loaded with compression enabled then ARCserve will automaticly enable compression at the drive.

While on the subject here are a few more notes on compression in general.

Tape Capacity: Tape capacity is usually given in non-compressed and compressed amounts. So a 35/70 would be 35gb non-compressed and 70gb compressed. However this can be deceptive since the first amount is fixed and the second is theoretical. In non-compressed mode the drive can write no more than 35gb to the tape. Anything over 35gb is by definition compressed, it may not be good compression but it is compressed. The second amount is set more by marketing that technical reasons, and by no means can be expected in every case.

Data Type: More than anything else the type of data effects the amount of compression. Zip, jpg, and gif, are examples of files that are already compressed and usually compressing them again will increase the size caused by the overhead. Exe and dll are examples of binary files that will not compress very much. Txt and log are examples of text files and have a very high rate of compression. Beyond that file size also will effect compression since each file has the overhead associated with it. So 100gb of 50k txt files will take up more space on tape than a 100gb single txt file.

Write Errors: There are two types of write errors soft and hard. Hard or UDE Unrecoverable Data Errors will cause the backup to cancel. Soft errors will not, and usually are not even reported. See faq478-4005 for details.

Empty Space: When the data flow does not match the speed data is being written to tape that can create what is called a shoe shine effect. The drive will stop, start, stop, start as data becomes available. But each time it backs up a little similar to a buffing rag over a shoe. This can cause premature failure of the tape and drive. To prevent this the drive can stream. This is were the drive will not stop and instead just pad the tape with empty space until the data is available.