When defining tape device capacities, you will often see two types of values for capacity and transfer rate. The first is referred to as native rate and the second as rate using compression. The compression rate used will often be 2:1. What do these compression rates mean? Are there differences between hardware and software compression? What should I use?

The first thing to consider is that the tape device will only be able to use one recording density - it cannot record one piece of data at one density and another at a higher density. Secondly, hardware compression uses a software compression algorithm, running in the firmware of the tape device. This means that all the rules applicable to software compression, are valid for hardware compression. Let's list some of these rules:

We can conclude from the above that we should not use hardware compression and software compression at the same time.

Software compression will be done by the backup software. In most cases it is more advanced than hardware compression, but also slower. Some backup applications could for example distinguish between compressed and uncompressed files, and not compress them again. Hardware compression will not do this.

The advantage of software compression should become more apparent when you are using a client/server environment, where all backup data is moved over the network. If the backup client is able to compress files before sending them over the network, network traffic will decrease. The disadvantage is that client processing will become higher, and you could run out of processing power.

So what should you use now? Unfortunately, there is no one answer. It is very much dependent on the environment and the capabilities of the software. In general, the following rules should be observed:

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