Hardware Compression Versus Software Compression

Backup client machines can compress data during backup, before the data is moved over the network or written to tape. You can implement software compression by selecting compression directives in the Clients resource or adding compressasm to a custom backup command. The compressasm feature typically achieves a 2:1 compression ratio. In addition to the performance advantages of moving less data across the network, software compression works better than some types of hardware compression in cases where a tape has a bad spot.

To handle EOT (end of tape) errors caused by bad spots on a tape, Backup maintains a fixed size, write-behind buffer. When Backup requests the next tape, it flushes the write-behind buffer to the new tape. (EOT will not be handled if the size of the unflushed data is greater than the Backup buffer). The write-behind buffer has a finite size to handle noncompressing tape drives. This write-behind buffer also works with tape drives that compress data as it is written from the drive's buffer to tape, but not with drives that compress data as it is copied into the drive's buffer. The drive's buffer represents a ratio of 1 1/2 to 3 times as much data as it holds, byte for byte, and possibly much more (some drives claim compression ratios of 10:1). The write-behind buffer must be very large to handle a best-case 10:1 compression ratio possible with some drives. Real memory and swap space consumption make this prohibitive.

Use the following tips to decide which compression method is better for your environment:

• Use compressasm to minimize network bandwidth, if you have available CPU power.

• Use compressasm or compressing drives to get more data on a tape.

• Trying to compress data that has already been compressed provides no additional compression. You can use compressing drives and compressasm, but do not expect additional compression on the tape. Selecting both options might even expand the data.

• Do not use compressasm if you have a compressing drive and no networked clients.