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MBFS Overview

Our objective was to replace Derby's (and conventional file systems') one-size-fits-all persistence model with a configurable persistence model thereby removing the need for non-volatile memory and the other overheads of disk persistence. The new design, called MBFS (Memory Based File System), is motivated by the fact that current distributed systems contain massive amounts of memory storage - in many cases tens or hundreds of gigabytes. The aggregate memory space, consisting of both local and remote memory, can be used to provide high-speed file storage for a large portion of file traffic that does not require conventional disk persistence. These files are often short lived and require fast access. In MBFS, memory rather than disk serves as the primary storage location for these files. Disks, tapes, writable CD's, and other high-latency devices are relegated to the role of archival storage, out of the critical path of most file operations.

MBFS supports a continuum of persistence guarantees and allows applications to select the desired persistence on a per-file basis. Weaker persistence guarantees result in better performance than strong persistence guarantees, allowing applications with weak persistence requirements to avoid the performance penalties and overheads of conventional disk-persistent models.

Because systems cannot guarantee persistence (data will not be lost under any circumstance), they actually only guarantee ``the probability of persistence'' or ``the probability the data will not be lost.'' In conventional file systems, persistence is defined as (1-(probability of disk failure)) , whereas MBFS supports any persistence probability. The difficulty in supporting such a continuum lies in an application's need to know exactly what each position in the continuum means. For example, what does it mean to select a persistence at the midpoint of the continuum, halfway between ``will lose the data immediately'' and ``will keep multiple almost indestructible copies''? The midpoint definition depends on the the devices used to store the data, their failure characteristics, and if or when data is archived to more persistent storage. Also, the same persistence probability can be implemented different ways. For example, storing data in two NVRAM's may have an equivalent persistence probability to storing the data on an inexpensive disk. Exposing applications to the details of the storage devices and their failure rates is undesirable as it complicates the file system interface and ties the application to the environment, thereby limiting application portability. As a result, MBFS attempts to hide these details from the application.

next up previous
Next: Storage Hierarchy Up: An Application-Aware Data Storage Previous: Enhancing Derby
Todd Anderson