A. Arpaci-Dusseau Department of Computer ScienceCS739: Distributed Systems University of Wisconsin, MadisonTaming aggressive replication in the Pangaea wide-area filesystem - OSDI’021 Introduction• What environment is Pangaea targeting? What workload? Is this well motivated?• What consistency model do they provide? Why do they think this model is okay? Why don’tthey want to provide strong data consistency?2 Pangaea: A structural overview• Where is your data stored in Pangaea? Do you think this is a good idea?• What are the two purposes of replication? How do Gold and Bronze replicas in Pangaeafulfill these different purposes?• What meta-data must be tracked for each type of replica? Why don’t you want to make allcopies Gold?3 Replica set management• How does a user create a file? (Example: User on server S creates file F in directory D.)• How does a user on node S access an existing file F?• After S has a replica, what must it do?4 Propagating updates• Pangaea chooses to update replicas through flooding; i.e., pushing the new contents to ev-ery neighbor. Since the graph of replicas is strongly connected, all replicas will eventuallyreceive the updates. What optimizations do they implement beyond flooding?• How does Pangaea determine whether or not it is correct to update a replica?• Updating directory entries is particularly interesting. For example, if simultaneously executemv /foo /alice/foo and mv /foo /bob/foo, hard to detect conflict since aliceand bob might be stored on different nodes. How does Pangaea solve this problem?1• If a user is worried about eventual consistency, what can they do?5 Evaluation• How much of the system is implemented?• What do they show in Table 2 and Figure 7?• What is the point of Figure 8?• What is the point of Figure 9?6 Class Discussion Questions• What do you think is this major weakness of this work?• Given its goals and assumptions, do you think Pangaea is designed well? Why?• There is no “central authority” for determining who owns a file or who should host goldreplicas. What complications does this raise in Pangaea?• What do you think are the pros and cons of the allocation policy used in Pangaea?• Could Pangaea be modified to perform replication at the block level instead of the whole filelevel? What complications would this raise?• Are there any disadvantages to hosting a bronze replica? Why do you sometimes want toremove a bronze
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