If you are trying to get the size of your Synology Volume via SNMP < you will need to query the correct Index on the OID .1.3.6.1.2.1.25.2.3.1.3.

Synology does not have standard indexes , so you will need to start by downloading run and install SNMP walk on a machine that can communicate to the device over network

Enter the IP of the device in SNMP Walk ( Leave rest options to default ) and get the output to text File

Open the txt file and do a Search for volume and per below you should fine the list of OID’s

The last number will be the index , for example Index will be 41 fo the volume

.1.3.6.1.2.1.25.2.3.1.3.1 = STRING: “Physical memory”
.1.3.6.1.2.1.25.2.3.1.3.3 = STRING: “Virtual memory”
.1.3.6.1.2.1.25.2.3.1.3.6 = STRING: “Memory buffers”
.1.3.6.1.2.1.25.2.3.1.3.7 = STRING: “Cached memory”
.1.3.6.1.2.1.25.2.3.1.3.8 = STRING: “Shared memory”
.1.3.6.1.2.1.25.2.3.1.3.10 = STRING: “Swap space”
.1.3.6.1.2.1.25.2.3.1.3.31 = STRING: “/”
.1.3.6.1.2.1.25.2.3.1.3.36 = STRING: “/tmp”
.1.3.6.1.2.1.25.2.3.1.3.37 = STRING: “/run”
.1.3.6.1.2.1.25.2.3.1.3.38 = STRING: “/dev/shm”
.1.3.6.1.2.1.25.2.3.1.3.41 = STRING: “/volume1”