13. Compression

There are several compression methods available.

External compression - type 1

This method calls an external compression program to do the compression. The default is set to gzip but can be changed in the defaults.h file at compile time. The compression program is expected to read the file from standard input and write the compressed output to standard output. If your compression program doesn't do this, you will have to modify the taper sources.

Internal compression - type 2

This is a very good compression method - it is very fast and reasonably good at compression. Because of this, it is the default compression method. The only problem is that is uses an extra 2MB of memory which can degrade system performance on heavily loaded machines.

Internal gzip - type 3

This method is basically the gzip compression method, however, the source has been hacked around to accommodate taper. The advantage of using this over the external compression method is that it is a bit quicker because it is internal, however, it is still a very slow compression method. The other disadvantage is that although the compression method is that of gzip, it doesn't produce compressed files that can be read by gzip. You need taper to read the archives.

In summary then:

• Memory use, type 2 > type 3 > type 1.
• Speed, type 2 > type 3 > type 1.
• Compression ratio, type 3 > type 1 > type 2.

You should use compression 2 unless you have a very heavily loaded system or less than 4MB of RAM.

If taper is still too memory hungry for you, try the following:

• Turn off triple buffering
• In defaults.h, reduce the size of DEFAULT_TR_SIZE
• Don't use use compress-type set at 2 because this is a very memory hungry compression (but also the quickest of the lot). Compress-type set at 3 is a good alternative.
• In defaults.h, change the line

  #define COMPRESS2_BUFFER_SIZE 1000000

  to:

  #define COMPRESS2_BUFFER_SIZE 1

  You will not be able to use compress-type 2, but if memory is a problem, you shouldn't be using this anyway.
• In defaults.h, add the following line:

  #define MAXNAMLEN 125

  This assumes that the maximum filename length you have is 125 characters. If this is not the case, change it to match your system.