Technical Report 3002Rev. C 3/95File System Designfor an NFSFile Server AppliancebyDave Hitz, James Lau, and Michael MalcolmNetworkAppliancePresented January 19, 1994USENIX Winter 1994 — San Francisco, CaliforniaCopyright © 1994 The USENIX Association. Reproduced by permission.2 File System Design for An NFS File Server Appliance - Rev. C 3/95© 1995 Network Appliance Corporation-Printed in USA.319 North Bemardo Avenue, Mountain View, CA 94043All rights reserved. No part of this publication covered by copyright may be reproduced inany form or by any means-graphic, electronic or mechanical, including photocopying,recording, taping, or storage in an electronic retrieval system-without prior writtenpermission of the copyright owner.Network Appliance reserves the right to change any products described herein at any time,and without notice. Network Appliance assumes no responsibility or liability arisingfrom the use of products described herein, except as expressly agreed to in writing byNetwork Appliance. The use and purchase of this product do not convey a license user anypatent rights, trademark rights, or any other intellectual property right of NetworkAppliance.The product described in this publication may be protected by one or more U.S. patents,foreign patents, and/or pending applications.RESTRICTED RIGHTS LEGEND: Use, duplication, or disclosure by the government issubject to restrictions as set forth in subparagraph (c)(1)(ii) of the Rights in TechnicalData and Computer Software clause at DFARS 252.277-7103 (October 1988) and FAR52-227-19 (June 1987).Trademark AcknowledgmentFAServer, WAFL, and Snapshot are trademarks of Network Appliance. SunOS, NFS andPC-NFS are registered trademarks of Sun Microsystems, Inc.UNIX is a registered trademark in the United States and other countries, licensedexclusively through X/Open Company Limited.Ethernet is a registered trademark of Xerox Corporation.Intel 486 is a registered trademark of Intel Corporation.All other products or services mentioned in this document are identified by the trademarks,service marks, or product names as designated by the companies who market theseproducts. Inquiries concerning such trademarks should be made directly to thosecompanies.File System Design for An NFS File Server Appliance - Rev. C 3/95 3Table of ContentsAbstract...................................................................................................41. Introduction.................................................................................52. Introduction To Snapshots........................................................62.1. User Access to Snapshots........................................................62.2. Snapshot Administration ........................................................73. WAFL Implementation...............................................................83.1. Overview ................................................................................83.2. Meta-Data Lives in Files .........................................................83.3. Tree of Blocks........................................................................93.4. Snapshots.............................................................................103.5. File System Consistency and Non-Volatile RAM..................123.6. Write Allocation ...................................................................144. Snapshot Data Structures And Algorithms ...........................164.1. The Block-Map File .............................................................164.2. Creating a Snapshot..............................................................175. Performance...............................................................................196. Conclusion................................................................................ 20Bibliography........................................................................................22Biographies..........................................................................................234 File System Design for An NFS File Server Appliance - Rev. C 3/95AbstractNetwork Appliance recently began shipping a new kind of network server called anNFS file server appliance, which is a dedicated server whose sole function is to provideNFS file service. The file system requirements for an NFS appliance are different fromthose for a general-purpose UNIX system, both because an NFS appliance must beoptimized for network file access and because an appliance must be easy to use.This paper describes WAFL (Write Anywhere File Layout), which is a file systemdesigned specifically to work in an NFS appliance. The primary focus is on thealgorithms and data structures that WAFL uses to implement Snapshots™, which areread-only clones of the active file system. WAFL uses a copy-on-write technique tominimize the disk space that Snapshots consume. This paper also describes how WAFLuses Snapshots to eliminate the need for file system consistency checking after anunclean shutdown.File System Design for An NFS File Server Appliance - Rev. C 3/95 51. IntroductionAn appliance is a device designed to perform a particular function. A recent trend innetworking has been to provide common services using appliances instead of general-purpose computers. For instance, special-purpose routers from companies like Ciscoand Bay Networks have almost entirely replaced general-purpose computers for packetrouting, even though general purpose computers originally handled all routing. Otherexamples of network appliances include network terminal concentrators, network FAXservers, and network printers.A new type of network appliance is the NFS file server appliance. The requirementsfor a file system operating in an NFS appliance are different from those for a generalpurpose file system: NFS access patterns are different from local access patterns, andthe special-purpose nature of an appliance also affects the design.WAFL (Write Anywhere File Layout) is the file system used in Network ApplianceCorporation's FAServer™ NFS appliance. WAFL was designed to meet four primaryrequirements:1. It should provide fast NFS service.2. It should support large file systems (tens of GB) that grow dynamically asdisks are added.3. It should provide high performance while supporting RAID (Redundant Arrayof Independent Disks).4. It should restart quickly, even after an unclean shutdown due to power failureor system crash.The requirement for fast NFS service is