Simply you cannot do incremental backup for a linux VM Using VMWare Consolidated Backup. The following text is taken from the following article: http://www.vmware.com/pdf/esx3_backup_wp.pdf

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Consolidated Backup brings many innovations to protecting virtual machines, but it may not be suitable in all situations.

• In VMware Infrastructure 3, Consolidated Backup does not support file-level backup of guest operating systems other than Windows. Only image backups are supported. There is no incremental backup capability for systems other than Windows, either.

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