How do these snapshot isolation level configurations interact on a SQL Server 2005 instance? I started troubleshooting a tempdb issue we are having on SQL Server 2005 Enterprise Edition. A developer is getting an error that tempdb is out of space. Technically, the error is:


  Transaction aborted when accessing versioned row in table
  'dbo.inserted' in database 'dbname'. Requested versioned row was not
  found. Your tempdb is probably out of space. Please refer to BOL on
  how to configure tempdb for versioning


I looked at the database configuration in sys.databases and found the following settings:

snapshot_isolation_state: 0

snapshot_isolation_state_desc: OFF

is_read_committed_snapshot_on: 1


I looked up what this meant in BOL and the salient information is as follows:


  snapshot_isolation_state
  
  State of snapshot-isolation transactions being allowed, as set by the ALLOW_SNAPSHOT_ISOLATION option: 
  
  0 = Snapshot isolation state is OFF (default). Snapshot isolation is disallowed. 
  
  1 = Snapshot isolation state ON. Snapshot isolation is allowed.
  
  2 = Snapshot isolation state is in transition to OFF state. All transactions have
  their modifications versioned. Cannot start new transactions using
  snapshot isolation. The database remains in the transition to OFF
  state until all transactions that were active when ALTER DATABASE was
  run can be completed. 
  
  3 = Snapshot isolation state is in transition to
  ON state. New transactions have their modifications versioned.
  Transactions cannot use snapshot isolation until the snapshot
  isolation state becomes 1 (ON). The database remains in the transition
  to ON state until all update transactions that were active when ALTER
  DATABASE was run can be completed.
  
  snapshot_isolation_state_desc
  
  Description of state of
  snapshot-isolation transactions being allowed, as set by the
  ALLOW_SNAPSHOT_ISOLATION option: 
  
  
  OFF  
  ON
  IN_TRANSITION_TO_ON 
  IN_TRANSITION_TO_OFF
  
  
  is_read_committed_snapshot_on
  
  1 = READ_COMMITTED_SNAPSHOT option is ON. Read operations under the
  read-committed isolation level are based on snapshot scans and do not
  acquire locks.
  
  0 = READ_COMMITTED_SNAPSHOT option is OFF (default). Read operations
  under the read-committed isolation level use share locks.


If I'm understanding this correctly, snapshot_isolation_state is OFF and disallowed. However, the 1 on is_read_committed_snapshot_on indicates that the database is using snapshot isolation. How is this possible if it's disallowed?
As @AlexKuznetsov noted, SNAPSHOT and READ_COMMITTED_SNAPSHOT are two different isolation levels. As such, snapshot_isolation_state indicates the former, while is_read_committed_snapshot_on the latter. Here are a few comments that I found that summarize the differences between the two:

READ COMMITTED SNAPSHOT does optimistic reads and pessimistic writes.
In contrast, SNAPSHOT does optimistic reads and optimistic writes.
~Bill Paetzke here

and ...

[READ_COMMITTED_SNAPSHOT] differs from the SNAPSHOT isolation level in
that instead of providing a reader with the last committed version of
the row that was available when the transaction started
(SNAPSHOT ISOLATION), a reader gets the last committed version of the
row that was available when the statement started
(READ_COMMITTED_SNAPSHOT).
~Uri Dimant (emphasis mine), here