Jan Stender

BabuDB: Fast and Efficient File System Metadata Storage

Todays distributed file system architectures scale well to large amounts of data. Their performance, however, is often limited by their metadata server. In this paper, we reconsider the database backend of the metadata server and propose a design that simplifies implementation and enhances performance.In particular, we argue that the concept of log-structured merge (LSM) trees is a better foundation for the storage layer of a metadata server than the traditionally used B-trees. We present BabuDB, a database that relies on LSM-tree-like index structures, and describe how it stores file system metadata.We show that our solution offers better scalability and performance than equivalent ext4 and Berkeley DB-based metadata server implementations. Our experiments include real-world metadata traces from a Linux kernel build and an IMAP mail server. Results show that BabuDB is up to twice as fast as the ext4-based backend and outperforms a Berkeley DB setup by an order of magnitude.

Flease - Lease Coordination Without a Lock Server

Large-scale distributed systems often require scalable and fault-tolerant mechanisms to coordinate exclusive access to shared resources such as files, replicas or the primary role. The best known algorithms to implement distributed mutual exclusion with leases, such as Multipaxos, are complex, difficult to implement, and rely on stable storage to persist lease information. In this paper we present {\bf F}LEASE, an algorithm for fault-tolerant lease coordination in distributed systems that is simpler than Multipaxos and does not rely on stable storage. The evaluation shows that {\bf F}LEASE can be used to implement scalable, decentralized lease coordination that outperforms a central lock service implementation by an order of magnitude.

Loosely time-synchronized snapshots in object-based file systems

A file system snapshot is a stable image of all files and directories in a well-defined state. Local file systems offer point-in-time consistency of snapshots, which guarantees that all files are frozen in a state in which they were at the same point in time. However, this cannot be achieved in a distributed file system without global clocks or synchronous snapshot operations. We present an algorithm for distributed file system snapshots that overcomes this problem by relaxing the point-intime consistency of local file system snapshots to a time span-based consistency. Built on loosely synchronized server clocks, it makes snapshots available within milliseconds, without any kind of locking or synchronization. Our evaluation demonstrates that enabling and accessing snapshots involves a read/write throughput penalty of no more than 1% under normal conditions.

Efficient Management of Consistent Backups in a Distributed File System

Setting up backup infrastructures for large-scale data management systems that can be operated cheaply and accessed with low latency has emerged as a practical problem. As a solution, we present a highly scalable and cost-efficient architecture for backup management in a distributed file system. We describe techniques for the creation of consistent backups at runtime, as well as approaches to resource management in connection with an integrated backup architecture.

FaTLease: scalable fault-tolerant lease negotiation with Paxos

A lease is a token which grants its owner exclusive access to a resource for a defined span of time. In order to be able to tolerate failures, leases need to be coordinated by distributed processes. We present FaTLease, an algorithm for fault-tolerant lease negotiation in distributed systems. It is built on the Paxos algorithm for distributed consensus, but avoids Paxos’ main performance bottleneck of requiring persistent state. This property makes our algorithm particularly useful for applications that can not dispense any disk bandwidth. Our experiments show that FaTLease scales up to tens of thousands of concurrent leases and can negotiate thousands of leases per second in both LAN and WAN environments.