In reply to Weeian:
Depends. After the arete pitch you'll have to dig around for a belay, as you'll not get to the top in one pitch with a 30. Belays do exist between the arete and the top, but last week they were all buried under the snow. On the lower pitches 30m would be fine, as there are plenty of anchor options. You might make yourselves unpopular if the route is busy though! Enjoy.