Tuesday, September 15, 2009

Segment-Based Recovery: Write-ahead logging revisited

Large-scale distributed systems have challenges that are not adequately addressed by previous recovery systems, or previous architectures based on single systems in general. This paper proposes segment-oriented recovery to enable distributed recovery architectures that are not hindered by the tight coupling of components required by page-oriented recovery. The distributed variations are quite flexible and enable recovery to be a large-scale distributed service.
Page-oriented recovery leads to a tight coupling between the application, the buffer manager and the log manager. The tight coupling might be fine on a traditional single core machine, but it leads to performance issues when distributing the components to different machines and different cores. Segment-oriented recovery enables simpler and looser coupling among components. Write back caching reduces communication between the buffer manager and application, since the communication occurs only on cache eviction. Since there is no shared state, calls to the buffer manager and log manager can be asynchronous. The use of natural layouts for large objects allows DMA and zero-copy I/O in the local case. In the distributed case, this allows application data to be written without copying the data and the LSNs to the same machine. This allows for very flexible large-scale write-ahead logging as a service for cloud computing.
For small transactions, the networked version is roughly ten times slower than the local versions, but approximately 20 times faster than a distributed, page-oriented approach. As transaction sizes increase, segment-based recovery is better able to amortize network round trips due to log and buffer manager requests, and network throughput improves to more than 400 times that of the page-based approach. As above, the local versions of these benchmarks are competitive with local page-oriented approaches, especially for long transactions.

Analysis: The asynchronous approach is necessary for large-scale distributed systems. Also, recovery based on the granularity of application requests is more in line with the transactions in present large-scale Internet systems. However, the focus on large transaction size may not be valid for the cloud computing that this approach targets. The retrieval of a single web page can require communication with hundreds of small sub-services running on remote nodes. Perhaps some sort of lightweight version (we should consider letting go of some constraints if the application does not require them) would be preferable for the short transactions.

This up to date paper looking at modern challenges was well placed in the course
I hope we have more such updated papers throughout the course.


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