This is where dbms' designed for data warehouses might come
into play, something like SybaseIQ. It is adapted for long
term storage and retrieval.
If you understand the finer details of schema design for
data warehousing such as star schemas and snowflake schemas
then you will probably have more luck shoving your SNMP
poll data into an SQL database. But it is still a question
of making do with a suboptimal database that is not designed
for dealing with huge time-series datasets.