[irods/irods#6940] Add documentation for unixfilesystem voting

The process for how storage resources vote has not been documented
outside of the code. This commit includes the voting values for all
the various situations in which a resource or a replica on the resource
may find themselves. These votes are, of course, filtered up to the
parent resource for decision making higher up, but these policies have
been documented for some time.

docs/plugins/composable_resources.md

@@ -305,6 +305,47 @@ The following are some rules around using detached mode:
 4. The resource vault path must be identical on every host that services requests for the resource. This vault path must be in a mount to a common filesystem.
 5. Registrations outside the vault are not allowed.  We can't enforce that the path being registered is shared by all hosts that service requests for the resource.
 
+#### UnixFileSystem Resource Voting
+
+This resource returns vote values based on the type of operation.
+
+For a "create" operation, the following criteria are considered *in order*:
+
+1. If the resource is marked "down", a `SYS_RESC_IS_DOWN` error is returned. The vote value is 0.0.
+2. If the size of the data to be created exceeds the configured minimum required free space on the resource, a `USER_FILE_TOO_LARGE` error is returned. The vote value is 0.0. 
+3. If the client is connected to the server to which the resource is attached, the vote value is 1.0.
+4. Otherwise, the vote value is 0.5.
+
+For a "write" operation, the following criteria are considered *in order*:
+
+1. If the resource is marked "down", a `SYS_RESC_IS_DOWN` error is returned. The vote value is 0.0.
+2. If the replica on the resource has a status value of '3' or '4' (read locked or write locked, respectively), the vote value is 0.0.
+3. If the replica on the resource has a status value of '2' (intermediate) and the client has not supplied the correct replica access token, the vote value is 0.0.
+4. If the client has requested a specific replica number and the replica on the resource has that replica number, the vote value is 1.0. If the client has requested a specific replica number and the replica on the resource does NOT have that replica number, the vote value is 0.25.
+5. If the replica on the resource has a replica status value of '0' (stale), the vote value is 0.25.
+6. If the client is connected to the server to which the resource is attached, the vote value is 1.0.
+7. Otherwise, the vote value is 0.5.
+
+For a "read" operation, the following criteria are considered *in order*:
+
+1. If the resource is marked "down", a `SYS_RESC_IS_DOWN` error is returned. The vote value is 0.0.
+2. If the replica on the resource has a status value of '3' or '4' (read locked or write locked, respectively), the vote value is 0.0.
+3. If the replica on the resource has a status value of '2' (intermediate), the vote value is 0.0.
+4. If the client has requested a specific replica number and the replica on the resource has that replica number, the vote value is 1.0. If the client has requested a specific replica number and the replica on the resource does NOT have that replica number, the vote value is 0.25.
+5. If the replica on the resource has a replica status value of '0' (stale), the vote value is 0.25.
+6. If the client is connected to the server to which the resource is attached, the vote value is 1.0.
+7. Otherwise, the vote value is 0.5.
+
+For an "unlink" operation, the following criteria are considered *in order*:
+
+1. If the resource is marked "down", a `SYS_RESC_IS_DOWN` error is returned. The vote value is 0.0.
+2. If the replica on the resource has a status value of '3' or '4' (read locked or write locked, respectively), the vote value is 0.0.
+3. If the replica on the resource has a status value of '2' (intermediate), the vote value is 0.0.
+4. If the client has requested a specific replica number and the replica on the resource has that replica number, the vote value is 1.0. If the client has requested a specific replica number and the replica on the resource does NOT have that replica number, the vote value is 0.25.
+5. If the replica on the resource has a replica status value of '1' (good), the vote value is 0.25.
+6. If the client is connected to the server to which the resource is attached, the vote value is 1.0.
+7. Otherwise, the vote value is 0.5.
+
 ### Structured File Type (tar, zip, gzip, bzip)
 
 The structured file type storage resource is used to interface with files that have a known format.  By default these are used "under the covers" and are not expected to be used directly by users (or administrators).