The compression process

The compression process uses zEDC. The average amount of disk space that is saved per file averages approximately 65%, depending on the type of data that is being compressed.

If you cancel a compression that is in progress, the zFS file system will remain partially compressed. In a partially compressed file system, new files may or may not be compressed. You can resume the compression later with another zfsadm compress command.

The compression process is not mandatory. If the compression of a file does not reduce space, the file is left in its uncompressed format.

Restrictions:
  1. Do not enable compression for any file system until you migrate all of your systems to z/OS V2R3. All systems in a sysplex must be at least z/OS V2R3 before any file systems are compressed because compression is not supported prior to z/OS V2R3. Also, do not use compression until you know that no system will be regressed to a prior release. Compressed file systems cannot be mounted on a release prior to V2R3. Therefore, if there is no zFS system in the shared file system environment that is eligible to own a compressed file system, the file system will be inaccessible.

    Decompression is supported if there are pre-V2R3 systems in the sysplex in order to allow the compression to be backed out.

  2. Only files larger than 8 K can be compressed. Directories and other control information inside the zFS file system are not compressed.
  3. You cannot compress or decompress an aggregate that is in a partially encrypted or partially decrypted state. In other words, if an encryption or decryption process was stopped for an aggregate, you cannot compress or decompress that aggregate until after the encryption or decryption is completed.