I use a lot of snmp to monitor our filers. Here are two snmp oid's that I monitor that match what you're asking for:
This indicates the overall status of the appliance.
The algorithm to determine the value uses both
hardware status (e.g. the number of failed fans) and
volume status (e.g. number of volumes that are full).
The algorithm is subject to change in future releases,
but the range of values will not change.
A string describing the global status,
including a description of the condition (if
any) that caused the status to be anything
other than ok(3).
There are lots of other oid's and traps that can be monitored. Have a look at the snmp mibs which you can find on the vol0 volume of the filers in the mib directory.