Hopefully the Zimbra Devs will eventually give us a switch in the GUI to enable to disable at least basic compression and even better would be varying levels of compression.
My solution was to remove two of the 146gb 15k SAS drives from my RAID 10 and add a pair of 1tb 7200rpm Nearpoint SAS (enterprise sata basically) to the server for use as the HSM. This enables Zimbra to move all data older than XX days to the slower/cheaper HSM storage. I also added a new backup folder there and changed my backup location accordingly.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now