Purpose

A common comment that is passed to Support is that 'compression isn't working'.

This document describes what hardware and software compression is, and differences between the two. It also describes some common terminology used when talking about compression, and covers how to check and configure both types of compression (where applicable) from the ARCserve GUI Manager.

Scope

Information in this document relates to:

BrightStor ARCserve Backup v9.x, r11.0, r11.1, and r11.5 for Windows

Introduction

A common comment that is passed to Support is that 'compression' isn't working. In most cases compression is in fact working, but for one reason or other is not compressing as much as expected.

Let's start off by looking at the two different types of compression that are commonly used with ARCserve:

Hardware Compression

This is performed by the drive at the drive level, hence the name 'hardware compression'. It has an advantage in that since the compression is done by the drive, there is no system overhead.

Hardware compression is only configurable in so far as it is either on or off, there is no way to tune or configure the rate of data compression either on the Hardware, or using ARCserve.

Compression on most modern drives is soft set, meaning that hardware compression can be enabled or disabled by sending a SCSI command to the drive.

If your drive supports hardware compression settable in this way, an option for compression will be shown in ARCserve device manager, allowing you to enable or disable hardware compression. We will look at this in more detail later on in this document. Older drive technologies may set compression using jumper blocks or DIP switches on the drive unit.

If your drive supports soft-settable compression ARCserve will by default attempt to enable hardware compression for the device each time the tape engine is started.

*NOTE:
The BrightStor log will not update the value of the compression ratio for multiplexing jobs even if the tape drive can provide it.

Software Compression

Software compression is something which can be configured in the ARCserve backup job options.

Software compression involves compressing the data either before it is sent from a remote system if backing up using a remote agent, or before being sent to the tape drive if backing up a local host server system. There is some system overhead involved in this, how much will depend entirely on your environment, but the overhead is normally negligible.

Software compression can be useful if you are performing remote backups and have limited bandwidth between the host server and remote agent and want to try to reduce the backup window. It can in some cases be more efficient than hardware compression.

Software compression and session password protection is only supported for file system backups and cannot be used for database agent sessions like SQL, Notes and Exchange with the exception of the database agent for exchange document-level backup.

We will look at how we can configure both hardware and software compression later in the document.

Can I use Hardware and Software Compression together?

No.

If you attempt to use hardware and software compression together, software compression will be disabled and a message similar to the one below written to your activity log (brightstor.log):

W3032 Compression is disabled due to Hardware Compression

This does not indicate a bug in ARCserve. You cannot compress an already compressed file.

If both hardware and software compression were allowed to be enabled at the same time the resulting file in many cases would actually be bigger than the original file.

My tapes are reporting as full before their stated capacity

This is a very common 'complaint', and in all but a handful of cases this comes down to misinterpretation of what the real capacity of the tape media is.

Tape drive manufacturers refer to capacity of their drives and associated media in two ways.

1. Native
   
   This is the real capacity of the media, and you should receive very near the stated native capacity of any tape drive/media.
   
   
2. Compressed
   
   This is the normally the capacity which the device is marketed as being able to hold. However, this depends on how much compression you get on the data being backed up.
   

An example of this would be someone stating that they are only getting 45Gb of data backed up to a DLT Type IV cartridge using a DLT8000 drive.

The native capacity of a DLT Type IV cartridge when used in a DLT 8000 drive is 40Gb, and compressed capacity (using 2:1 ratio) is 80Gb.

In this example compression is working, as if it wasn't the backup would have prompted for a second tape after 40Gb of data had been backed up to tape. As 45Gb had been backed up to tape compression is working, but perhaps not as much as you might have expected it to.

The tables below show some of the more common media types and their native and compressed capacities:

NB Figures for AIT drives are based on manufacturer's 2.61:1 compression.

How much compression will I get?

This is unfortunately a 'how long is a piece of string' type of question. There is no hard and fast formula for even approximating this.

Hardware manufacturers often market tape drive capacity based their compressed capacity on a 2:1 ratio, although more recently some manufacturers have begun quoting an even higher ratio of 2.6:1.

Whether or not you achieve this in your own environment is influenced by many factors, almost all of which are outside of ARCserve's control.

These include, but are not limited to:

• Compression already enabled on the file system being backed up
• Backing up already compressed files (eg .zip, .arj, .iso, .rar, etc)
• Backing up natively compressed file formats (eg .jpg, .gif, .mpg etc)

As long as the drive backs up over the native capacity of the drive/media then compression is working.

On drives which support it, Compression ratio and media capacity remaining will be written the ARCserve activity log (brightstor.log). If you don't see these fields in the activity log, your drive does not support this feature.

Enabling/Disabling Hardware compression

If compression is settable under software control, you can enable or disable compression using the ARCserve device manager.

In the ARCserve Manager GUI, select device manager, and highlight the drive that you want to enable or disable compression on:

If compression is settable from the GUI you should see a 'Compression' option as in the screenshot above. You can click on this to toggle the current compression mode either on or off.

Note: You can only change compression with a blank tape present in the drive.

A pop-up dialog box will appear asking whether you want to ENABLE or DISABLE compression:

If you select OK, a status dialog will appear afterwards indicating whether the operation was successful or not:

If compression could not be enabled or disabled then check the tape drive manual to determine first of all whether the device supports hardware compression, and secondly, if it is soft settable, or settable only by jumpers or DIP switches on the drive itself.

You can see the current Compression mode status by clicking on the details tab:

Enabling Software compression in your Backup job

Having first of all made sure that hardware compression is disabled using the instructions above, create a new job or modify an existing one using the ARCserve Manager GUI.

When the job is being created/ modified, click on Global Options from the toolbar:

This will bring up the global options dialog window. From this, ensure the 'Backup Media' Tab is selected (it should be by default):

Click the tickbox for 'Compress Files Before Backup Using Software Compression'.

A pop-up message will appear as follows:

Press OK to acknowledge this.

Submit/Re-submit you job to queue, checking the run time is correct.

Your backups should be performed using software compression.

Note: The screenshots in the section above are specific to r11.5, the description of Software Compression in the GUI differs slightly in earlier releases, and the pop up message does not appear in earlier releases.

I'm trying to use Software compression but hardware compression is re-enabled every time I start the tape engine.

This is correct and is the default behaviour. The tape engine will attempt to enable hardware compression for all tape drives each time the tape engine is started if your drive supports it, since this option would have the least overall effect on resources and backup throughput.

If you want to change this behaviour so that the tape engine does not try to enable hardware compression each time it starts then you can make this modification. You should make this modification if you intend to use software compression over hardware compression on a regular basis.

Open up regedit or regedt32 on the machine you are trying to merge the tape from.

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. CA cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

Browse to:

    HKEY_LOCAL_MACHINE \ SOFTWARE \ ComputerAssociates \ BrightStor ARCserve Backup \ Base \ TapeEngine \ Config 

Add the following key and value:

    Value Name         FormatWithoutHardwareCompression     
    Data Type          REG_DWORD     
    Data               1 

You will need to stop and start the tape engine for this setting to take effect.