5.1 Compressing and Decompressing Files

One way to conserve disk space is to compress files. If you set the File Compression attribute for a Traditional volume, NetWare compresses files that have been inactive for a period of time. Compression typically occurs at non-peak hours.

IMPORTANT:You cannot use compression for volumes on removable drives, such as CDs, DVDs, or ISO files.

NetWare maintains the original version of a file during compression. When compression completes, NetWare replaces the original with the compressed version of the file, if no errors occurred. If errors do occur during compression, NetWare leaves the original version intact.

This section discusses the following topics:

5.1.1 Planning for File Compression

To effectively use file compression for your Traditional volumes, you must understand several key concepts:

Only Inactive Files Are Candidates for Compression

Files automatically pass in and out of their compressed state as they are unused, then used. It is not necessary to separate application files from data files for file compression because NetWare compresses files based on the interval of time that a file remains inactive. Most application files are used regularly.

Use the set command to preclude compression of frequently used applications. For instructions, see SET in the NW 6.5 SP8: Utilities Reference.

Decompression Activity Depends on Available Space

Compressed files are uncompressed as they are needed, then remain uncompressed until they are inactive for an extended period. For a file to be uncompressed, there must be enough free space on the volume to accommodate the uncompressed file size.

Immediate Compression Impacts CPU Performance

Compression is usually a low-priority process thread because of compression’s impact on performance. If you flag an item for immediate compression during peak system usage, performance might deteriorate.

Files Remain Compressed during Backup and Restore

Backup applications that use Novell Storage Management Services™ (SMS™) can back up and restore files in their compressed state. Other applications might decompress them.

Compressed Volumes Remain Compressed

The File Compression attribute can be turned on when you create the Traditional volume or at any time afterwards. However, after you enable file compression for a Traditional volume, you cannot turn it off. Instead, you can suspend the compression activity, as needed.

If you want to turn off file compression, you must back up the volume in its uncompressed state, then restore the data to a new volume on which the File Compression attribute is not set.

How to Monitor Compression Activity

Monitor compression activity via the set command’s Compress Screen parameter. For instructions on how to use the set command, see SET in the NW 6.5 SP8: Utilities Reference.

5.1.2 Setting Server-Level File Compression Attributes

For the NetWare server, the compression parameters apply to all files and directories in compression-enabled volumes on the server. They have no affect on the file compression for volumes where the volume’s File Compression attribute is disabled.

The following list identifies set command parameters that affect file compression. For details about each parameter and instructions for setting them, see Configuring Compression for a Server in the NW 6.5 SP8: NSS File System Administration Guide.

  • Compression Daily Check Stop Hour

  • Compression Daily Check Starting Hour

  • Minimum Compression Percentage Gain

  • Enable File Compression

  • Maximum Concurrent Compressions

  • Convert Compressed to Uncompressed Option

  • Decompress Percent Disk Space Free To Allow Commit

  • Decompress Free Space Warning Interval

  • Deleted Files Compression Option

  • Days Untouched Before Compression

5.1.3 Enabling File Compression for a Traditional Volume

You choose to compress files when you create volumes by setting the File Compression attribute.You can also set the File Compression attribute later. However, after you enable file compression for a volume, you cannot turn it off. For instructions on setting file compression for volumes, directories, and files, see Setting Server-Level File Compression Attributes.

The following set command parameters affect how file compression behaves. For information on how to use the set command, see SET in the NW 6.5 SP8: Utilities Reference.

Days Untouched Before Compression

Use the set command’s parameter named Days Untouched Before Compression to set this interval of inactivity. This parameter specifies the number of days that must pass without access to a file before the file can be compressed. The parameter uses the date the file was last accessed to gauge whether a file should be compressed.

Minimum Percentage Compression Gain

To avoid the overhead of uncompressing files that do not compress well, the system calculates the compressed size of a file before actually compressing it. If no disk space can be saved by compression, or if the size difference does not meet the value specified by the set command’s parameter named Minimum Percentage Compression Gain, the file is not compressed.

5.1.4 Disabling File Compression for a Traditional Volume

File compression is enabled and disabled at the volume level. If you do not enable the File Compression attribute when you create a volume, you can subsequently enable it using the set command. However, after it is enabled, file compression cannot be disabled on the volume unless you re-create the volume.

You can temporarily suspend file compression using the set command’s Enable File Compression parameter. For more information, see Section 5.1.5, Suspending File Compression.

5.1.5 Suspending File Compression

Use the set command’s Enable File Compression parameter to temporarily suspend file compression for a volume. For instructions, see SET in the NW 6.5 SP8: Utilities Reference.

While file compression is suspended, files that would have been compressed are queued and compressed when compression is re-enabled.

You can also use the monitor utility to change file compression parameters. For a general description, see MONITOR in the NW 6.5 SP8: Utilities Reference.