DistOS 2015W Session 7

Ceph

• Key advantage is that it is a general purpose distributed file system.
• System is composed of three units:

*Client, *Cluster of Object Storage device (OSDs): It is basically stores data and metadata and clients communicate directly with it to perform IO operations. *MetaData Server (MDS): It is used to manage the file and directories.Client basically interacts with it to perform metadata operations like open, rename. It manages the capabilities of a client.

• system has three key features:

    * decoupled data and metadata: 
    * Dynamic Distributed Metadata Management: It distribute the metadata among multiple metadata servers using dynamic subtree partitioning to increase the performance and avoid metadata access hot spots.
    * Object based storage: Using cluster of OSDs to form a Reliable Autonomic Distributed Object-Store(RADOS) for ceph failure detection and recovery.   

• CRUSH (Controlled, Replicated, Under Scalable, Hashing) is the hashing algorithm used to calculate the location of object instead of looking for them. The CRUSH paper on Ceph’s website can be downloaded from here http://ceph.com/papers/weil-crush-sc06.pdf.
• RADOS (Reliable Autonomic Distributed Object-Store) is the object store for Ceph.

Chubby

It is basically a coarse grained lock service that serves multiple clients with small number of servers (chubby cell).

system consists

• Chubby Cell: mainly consists of 5 servers known as replicas. Consensus protocol is used to elect the master from replicas.
• Client: Find the master between the replicas. Consensus protocol is used to propagate the write request to the majority of servers. Read request is handled by master only.

communication between client and server is via RPCs.

• Is a consensus algorithm among a set of servers to agree on who is the master that is in charge of the metadata.
• Can be considered a distributed file system for small size files only “256 KB” with very low scalability “5 servers”.
• Is defined in the paper as “A lock service used within a loosely-coupled distributed system consisting of moderately large number of small machines connected by a high speed network”.