In setting up monitoring our DataDomain environment, I've found the attached mibs to be quite helpful. Maybe you will as well.
"filesystemcleanstatus"
"filesystempercentused"
"filesystemresourcename"
"filesystemspaceavailable" (Measured in GB's)
"filesystemspacesize" (Measured in GB's)
"filesystemspaceUsed" (Measured in GB's)
"filesystemstatusmessage"