An Explanation of Compression and Restoring Tape Backups
If software is used to compress a tape, then the speed of the restoration machine is left to decompress the data coming off the tape, and faster systems can be used to increase that process. If the compression was done with the hardware of the tape drive, then the backup system offloads the work to the tape drive when the backup is created.
In the case of hardware compression, you are subject to the speed of the tape drive to uncompress the data. With higher levels of compression, the drive has to do more work, and the restore takes longer as buffers in the drives aren't large enough to keep the tape spinning. Once the tape stops the drive will sit until the data it has read is sent to the host for restoration. Once the host then requests more data, the drive will have to backup the tape to read where it is and then read the next compressed bit of data.
All of the above factors make reading highly compressed tapes much slower than the normal read rate of the drive.
To learn more about compression, read our Discussion of Compression article.
To learn more factors that affect tape restore times, read our How Long Does it Take to Restore a Tape?
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