A Bε-tree associated with a file system on a storage volume includes a hierarchy of nodes. Each node includes a buffer portion that can be characterized by a fixed maximum allowable size to store key-value pairs as messages in the buffer. Messages can be initially buffered in the root node of the Bε-tree, and flushed to descendent children from the root node. Messages stored in the buffers can be indexed using a B+-tree data structure. As the B+-tree data structure in a buffer grows (due to receiving flushed messages) and shrinks (due to messages being flushed), disk blocks can be allocated from the storage volume to increase the actual size of the buffer and deallocated from the buffer to reduce the actual size of the buffer.