WAFL - The Write Anywhere File Layout System(WAFL Written...

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The Write Anywhere File Layout System (WAFL) Written by Sebastian Scholz, based on “File System Design for an NFS File Server Appliance” by Dave Hitz, James Lau & Michael Malcolm, Network Appliance, Inc. 1. Abstract The patented Write Anywhere File Layout (WAFL) File System is a fully NFS compatible File System, which Strengths are High Performance NFS Processing, Support for large Disks by using RAID Systems and quick Startup after an unclean Shutdown without checking the File System for Consistency. Data and Metadata is arranged in a Tree of Blocks, which allows Backups to be created rapidly with negligible Effort. Non Volatile RAM is used to cache NFS Requests in order to minimize the File Server Response Time to an NFS Request. 2. Introduction Since File Servers become more and more important today, a robust, reliable and fast File System is necessary. WAFL is a Unix compatible File System optimized for network File Access, which was designed to meet four Requirements: It should provide fast NFS Service It should support large File Systems that grow dynamically as disks are added It should provide high Performance while supporting RAID It should restart quickly without checking the File System for Consistency. The Need for fast NFS Service is obvious, given WAFL’s intended use in an NFS Appliance. RAID strain Write Performance, because of the Read Modify Write Sequence it uses to maintain Parity. Because a Write on such a System typically consumes four Disk IO Operations (Data Load, Parity Load, Data Write and Parity Write), one Design Goal was to minimize the Amount of small Writes and, in Contrast, to maximize the Amount of Writes which affect all Stripes in a RAID System.[1, 2] WAFL uses a special Treatment of Data Blocks (Discussed in 3.1) and non-volatile Ram (NVRAM – discussed in 3.3) to optimize Write Performance. Because Data is treated in a different way compared to other File Systems, Snapshots (which are consistent Back Ups of the File System) can be created fast without notable Effort. (Discussed in 3.2) Large File Systems require special Techniques for fast Restart, because checking the File System for Consistency at startup is not a convenient Way, since it can become unacceptable slow as the Dimension of the File System grows. We will see that WAFL uses a mechanism similar to a log Structured File System [4] to avoid checking the File System at Startup. (Discussed in 3.3) 3. WAFL Implementation 3.1. Data Block Arrangement
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WAFL uses 4 KB Data Blocks without fragments to store Data. Each Inode contains 16 Block Pointers, which means that a Single Inode can address a File smaller or equal than 64 KB. If a File exceeds this Limit, indirect Blocks will be used to point to actual Data, while small Files are stored directly in the Inode File. The Way in which Data Blocks are arranged is quite
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WAFL - The Write Anywhere File Layout System(WAFL Written...

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