cis-Cyclohexane-1,2-diol is treated with $\mathrm{HIO}{}_4$ (one equivalent). The product is

Cleavage of the ring C1-C2 bond opens the ring, and each former carbinol carbon becomes an aldehyde carbon. The product is the six-carbon dialdehyde $\mathrm{OHC}-(\mathrm{CH}{}_2){}_4-\mathrm{CHO}$. One equivalent of periodate is too mild to oxidise the aldehydes further.