// mid-level
BufferHead *split(BufferHead *bh, loff_t off);
void merge_left(BufferHead *left, BufferHead *right);
+ bool can_merge_bh(BufferHead *left, BufferHead *right);
void try_merge_bh(BufferHead *bh);
bool is_cached(loff_t off, loff_t len) const;