How to determine if hardware/software compression is performing correctly in Backup Exec for NetWare Servers

Article:TECH3745  |  Created: 2006-01-12  |  Updated: 2006-01-12  |  Article URL http://www.symantec.com/docs/TECH3745
Article Type
Technical Solution

Environment

Issue



How to determine if hardware/software compression is performing correctly in Backup Exec for NetWare Servers

Solution



The storage capacity of tape media has often been a subject that causes confusion - in particular, whether hardware/software compression is actually being used, and how much data can be written to a tape.

A simple explanation of how compression works is as follows (note, in this explanation a 20/40GB tape is used, but the information applies to any stated tape capacity):

Regardless of the compression mode used, a 20/40GB tape has only 20GB of physical space on the tape and therefore it can hold a maximum of ~20GB of uncompressed data. The absolute maximum capacity of the tape is ~40GB of compressed data. This assumes a 2:1 compression ratio (i.e. a 2MB file can be compressed to 1MB) - however, typical compression ratios are ~1.2 to ~1.7:1 and therefore the maximum compressed capacity of the tape can be lower than ~40GB.

Scenario 1:
The file system is UNCOMPRESSED, and hardware/software compression is not enabled in Backup Exec (tm)
  • A maximum of 20GB of uncompressed data from the file system will be able to fit in the 20GB of available tape space
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Scenario 2:
The file system is UNCOMPRESSED, and hardware/software compression is enabled in Backup Exec
  • ~40GB of uncompressed data from the file system will be compressed to fit in the 20GB of available tape space
·
Scenario 3:
The file system is COMPRESSED, and hardware/software compression is not enabled in Backup Exec
  • A maximum of 20GB of compressed data from the file system will be able to fit in the 20GB of available tape space
·
Scenario 4:
The file system is COMPRESSED, and Hardware/Software Compression is enabled in Backup Exec
  • The selected compression algorithm will attempt to compress the file again. As files can only be compressed once, the files that have already been compressed by the NetWare file system compression algorithm will be backed up as is, but will have an extra header/footer for identification (note that this may actually result in a larger file size on tape than the original)
  • Files that have not already been compressed by the file system will be compressed and written to tape
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Legacy ID



188539


Article URL http://www.symantec.com/docs/TECH3745


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