Information Management NTU Distributed File Systems

Information Management NTU Purposes of a Distributed File System Sharing of storage and information across a network Convenience (and efficiency) of a conventional file system Persistent storage that most other services (e.g., Web servers) need

Information Management NTU Properties of Storage Systems Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition. Other properties include availability, timing guarantees, etc.

Information Management NTU Files Files are an abstraction of permanent storage. A file is typically defined as a sequence of similar-sized data items along with a set of attributes. A directory is a file that provides a mapping from text names to internal file identifiers.

Information Management NTU File Attributes Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU File Systems Responsible for the (a) organization, (b) storage, (c) retrieval, (d) naming, (e) sharing, and (f) protection of files. Provide a set of programming operations that characterize the file abstraction, particularly operations to read and write subsequences of data items beginning at any point of a file.

Information Management NTU File System Modules Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition. A basic distributed file system implements all of the above plus modules for client-server communication and distributed naming and location of files.

Information Management NTU UNIX File Operations Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU Distributed File System Requirements Transparency: access, location, mobility, performance, and scaling transparency. Concurrency (and Consistency) Replication/Caching (and Consistency) Hardware/operating system heterogeneity Fault-Tolerance Security (Access Control, Authentication) Efficiency

Information Management NTU A File Service Architecture Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition. Note: The modules communicate with one another by remote procedure calls.

Information Management NTU File Service Components Flat file service: implementing operations on the contents of files, which are referred to by unique file identifiers (UFIDs) Directory service: mapping text names of files (including directories) to their UFIDs Client module: integrating and extending the previous two services under a single application programming interface * Why is this structure more open and configurable?

Information Management NTU Flat File Service Operations Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU Difference from UNIX Immediate access to files using UFIDs (without open or close) Read or write starts at the position indicated by a parameter All operations, except create, are repeatable Allows a stateless implementation

Information Management NTU Access Control Conventional access rights checks (at open calls) not feasible Two ‘stateless’ approaches: * Capability (by manipulating the UFID) * User identity sent with every request (adopted in NFS and AFS) Main problem: forged requests; some authentication mechanism is needed

Information Management NTU Capabilities and UFIDs A capability is a binary value that acts as an access key; it can be encoded in the UFID. Basic construction of a UFID: file group id + file number + random number Additional field: permissions Additional field: encryption of the permission field

Information Management NTU Directory Service Operations Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition. Note: Each directory is stored as an ordinary file with a UFID.

Information Management NTU The Network File System (NFS) Introduced by Sun Microsystems in 1985, now an Internet standard Runs on top of RPC (RFC 1831) Implemented on most operating systems Version described here: UNIX implementation of NFS Version 3 (RFC 1813, June 1995) Most recent version: NFS Version 4 (RFC 3010, December 2000)

Information Management NTU NFS Architecture Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition. Note: Each computer can act as both a client and a server.

Information Management NTU The Virtual File System Module Access transparency File handles (file identifiers):  ‘ filesystem indentifier’ + ‘i-node number’ + ‘i-node generation number’ One VFS structure for each mounted filesystem  relates a remote filesystem (identified by its file handle obtained at mount time) to a local directory on which it is mounted One v-node per open file  indicates whether a file is local or remote, etc.

Information Management NTU The NFS Client Module in UNIX Integrated with the kernel Emulates the UNIX file system primitives A single client module serves all user-level processes The encryption key for authentication stored in the kernel Caches file blocks

Information Management NTU NFS Server Operations Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU NFS Server Operations (cont’d) Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU Remote File Acceses Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU File System Information in UNIX saturn:~ 35 % df -k Filesystemkbytes capacityMounted on /dev/dsk/c0t3d0s %/ /dev/dsk/c0t3d0s %/usr /dev/dsk/c0t3d0s %/tmp galaxy:/usr/local.real %/usr/local lucky:/var/mail.real %/var/mail cosmos:/home.real/student/xxx %/home/xxx galaxy:/home.real/faculty/yyy %/home/yyy * Note: The output of ‘df -k’ has been edited.

Information Management NTU Caching Server caching  read-ahead  write-through  delayed-write with the commit operation Client caching  cache validation (freshness interval and validation timestamp, modification timestamp and getattr, …)  bio-daemon (for read-ahead and delayed-write caching at the client side)

Information Management NTU Achievements of NFS Access and location transparency Mobility transparency (partially) Read-only file replication: the automounter Fault-tolerance: stateless servers, the automounter Efficiency: caching of disk blocks (main problem: frequent use of getattr) Nonachievements: scalability, concurrency and consistency, security (Kerberos),...

Information Management NTU The Andrew File System (AFS) Developed at CMU Current versions: AFS-2, AFS-3 Compatible with NFS Main achievement over (older) NFS: better scalability by minimizing client-server communication Key characteristics: whole-file serving and caching (partial file caching allowed in AFS-3)

Information Management NTU Observations on UNIX File Usage Files are mostly small Read operations are more common Sequential accesses are more common Most files are written by one user Files are referenced in burst

Information Management NTU AFS Architecture Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU AFS File Name Space Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU System Call Interception in AFS Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU AFS System Calls Implementation Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU Cache Consistency A callback promise is provided when Vice supplies a copy of file to a Venus process The callback promise stored with the cached copy is in either valid or cancelled state When Venus handles an open, it checks the cache.

Information Management NTU The Vice Service Interface Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Information Management NTU Enhancements to NFS and AFS Spritely NFS  add open and close, use callbacks NQNFS (Not Quite NFS)  use callbacks and leases WebNFS  allow browsers and other applications to interact with an NFS server directly NFS Version 4 (RFC 3010, December 2000)  incorporating all of the above and more DCE/DFS (based on AFS)  use callbacks and write tokens (with a lifetime)

Information Management NTU New Features of NFS Version 4 Adoption of the RPCSEC_GSS (RFC 2203) security protocol Multiple operations in one request Better migration and replication abilities  A client may query the location(s) of a file system. Introduction of open and close operations Lease-based file locking Callback-based delegation of files

Information Management NTU New Design Approaches Background  high-performance storage technology (e.g., RAID)  log-structure file systems (e.g., Sprite, BSD LFS)  high-performance switched networks (e.g., ATM, high-speed Ethernet) Goals: high scalability and fault-tolerance Main ideas: distribute file data among many nodes, separate responsibilities, … Constraints: high level of trust

Information Management NTU More Recent File System Designs xFS  Serverless: all data, metadata, and control can be located anywhere in the system; any machine can take over the responsibilities of a failed one Frangipani  Two-layer structure the Petal distributed virtual disk system the Frangipani server module Both designs utilize RAID-style striping, log- structured file storage, etc.

Information Management NTU Log-based Striping in xFS Source: T.E. Anderson et al., Serverless Network File Systems, ACM TOCS 1996

Information Management NTU An xFS Configuration Source: T.E. Anderson et al., Serverless Network File Systems, ACM TOCS 1996

Information Management NTU A Frangipani Configuration Source: C.A. Thekkath et al., Frangipani: A Scalable Distributed File System, ACM SOSP 1997