Performance costs of JFS data compression

Because data compression is an extension of fragment support, the performance associated with fragments also applies to data compression.

Compressed file systems also affect performance in the following ways:

• It can require a great deal of time to compress and extract data so that the usability of a compressed file system might be limited for some user environments.
• Most UNIX regular files are written only once, but some are updated in place. For the latter, data compression has the additional performance cost of having to allocate 4096 bytes of disk space when a logical block is first modified, and then reallocate disk space after the logical block is written to the disk. This additional allocation activity is not necessary for regular files in a uncompressed file system.
• Data compression increases the number of processor cycles. For the software compressor, the number of cycles for compression is approximately 50 cycles per byte, and for decompression 10 cycles per byte.