Vielleicht helfen Dir die folgenden OIDs:
hr storage index
1.3.6.1.2.1.25.2.3.1.1
A unique value for each logical storage area contained by the host.
hr storage descr
1.3.6.1.2.1.25.2.3.1.3
A description of the type and instance of the storage described by this entry.
hr storage allocation units
1.3.6.1.2.1.25.2.3.1.4
The size, in bytes, of the data objects allocated from this pool. If this entry is monitoring sectors, blocks, buffers, or packets, for example, this number will commonly be greater than one. Otherwise this number will typically be one.
hr storage size
1.3.6.1.2.1.25.2.3.1.5
The size of the storage represented by this entry, in units of hrStorageAllocationUnits. This object is writable to allow remote configuration of the size of the storage area in those cases where such an operation makes sense and is possible on the underlying system. For example, the amount of main memory allocated to a buffer pool might be modified or the amount of disk space allocated to virtual memory might be modified.
hr storage used
1.3.6.1.2.1.25.2.3.1.6
The amount of the storage represented by this entry that is allocated, in units of hrStorageAllocationUnits.
Diese sind Bestandteil der HOST-RESOURCES-MIB. Mit walk musst Du nur den passenden Index für dein Volume raussuchen. Für die prozentuale Angabe gibt's meines Wissens nach keine OID.
Das MIB-Guide von Synology hast Du auch schon beachtet?
https://global.download.synology.com/download/Document/MIBGuide/Synology_DiskStation_MIB_Guide.pdf