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Network File System
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  1. Network File System Phil Segel Muhammad Kamran Arain Ala F. Alnawaiseh Group 3

  2. Outline • Introduction • Network File System (NFS) • Windows Distributed File System (DFS)

  3. Introduction • A Distributed File System (DFS): is a File System, that supports sharing of files and resources in the form of persistent storage over a network. • The first file servers were developed in the 1970s. • Sun’s Network File System (NFS) became the first widely used distributed file system after its introduction in 1985.

  4. Clients and servers • A file server provides file services to clients. • A client interface for a file service is formed by a set of primitive file operations: • Creating a file. • Deleting a file. • Reading from a file. • and Writing to a file.

  5. Distribution • A DFS is a file system whose clients, servers, and storage devices are dispersed among the machines of a Distributed System or intranet. • Accordingly, service activity has to be carried out across the network, and instead of a single centralized data repository, the system has multiple and independent storage devices. • The distinctive features of a DFS are the multiplicity and autonomy of clients and servers in the system.

  6. Transparency • Ideally, a DFS should appear to its clients to be a conventional, centralized file system. • The multiplicity and dispersion of its servers and storage devices should be made invisible.

  7. Performance • The most important performance measurement of a DFS is the amount of time needed to satisfy service requests. • In conventional systems, this time consists of a disk-access time and a small amount of CPU-processing time. • In a DFS, however, a remote access has the additional overhead attributed to the distributed structure.

  8. Concurrent File Updates • A DFS should provide for multiple client processes on multiple machines not just accessing but also updating the same files. • Concurrency control or locking may be either built into the file system or be provided by an add-on protocol

  9. Distributed Data Store • A Distributed Data Store is a network in which a user stores his or her information on a number of peer network nodes. • Most of the peer to peer networks do not have distributed data stores in that the user's data is only available when their node is on the network.

  10. Distributed Data-Store Networks • FreeNet. • MNet. • Andrew File System (AFS). • NNTP. • BitTorrent • The Mnesia Database. • GNUnet. • Secure File system (SFS) • Global File System (GFS) • The Chord Project. • SVK – Distributed Version Control. • Groove shared workspace, used for DoHyki.

  11. Windows Distributed File Systems

  12. What is the purpose of Windows DFS? • To unite files on different computers into a single namespace • Make it easy to build a single, hierarchal view of multiple file servers and file server shares on your network • To display files in a single directory structure regardless of what server the files are on

  13. Comparison • Windows Distributed File Systems do for servers what a file system does for a hard disk.

  14. DFS File Protocols • Not limited to a single protocol • Regardless of client used, can support mapping of: • Servers • Shares • Files • Supports these provided that the client supports the native server and share

  15. History • The UNC (Universal Naming Convention) was required to specify the physical server and share to access file information • i.e. \\Server\share\path\filename • Could be used directly by drive mapping • i.e. X:\path\fileame • As network continues to grow mapping shares individually scales poorly

  16. Solution to historical problems • Windows DFS solves these problems by linking physical storage into logical representation. • Permits shares to be hierarchally connected to other Windows shares • Make physical location of data transparent to users and applications

  17. DFS Features and Benefits • Feature: Custom hierarchical view of shared network resources • Description: By linking shares together, administrators can create a single hierarchical volume that behaves as though it were one giant hard drive. Individual users can create their own Dfs volumes, which in turn can be incorporated by other Dfs volumes. These are called inter-Dfs links. • Benefit: Provides a simplified view of network shares that can be customized by the administrator.

  18. DFS Features and Benefits (Continued) • Feature: Flexible volume administration • Description: Individual shares participating in the Dfs volume can be taken offline without affecting the remaining portion of the volume name space. • Benefit: Allows administrators to manage physical network shares, independent of their logical representation to users.

  19. DFS Features and Benefits (Continued) • Feature: Graphical administration tool • Description: Each Dfs root is administered with an easy-to-use graphical administration tool that permits browsing, configuration of volumes, alternates, and inter-Dfs links, as well as administration of remote Dfs roots. • Benefit: Requires little training, reducing the need for trained, full-time server administrators.

  20. DFS Features and Benefits (Continued) • Feature: Higher data availability • Description: Multiple copies of read-only shares can be mounted under the same logical Dfs name to provide alternate locations for accessing data. If one of the copies becomes unavailable, an alternate is automatically selected. • Benefit: Important business data is always available, even if a server, disk drive, or file occasionally fails.

  21. DFS Features and Benefits (Continued) • Feature: Load balancing • Description: Multiple copies of read-only shares on separate disk drives or servers can be mounted under the same logical Dfs name, thus permitting limited load balancing between drives or servers. As users request files from the Dfs volume, they are transparently referred to one of the network shares comprising the Dfs volume. • Benefit: Automatically distributes file access across multiple disk drives or servers to balance loads and improve response time during peak usage periods.

  22. DFS Features and Benefits (Continued) • Feature: Name transparency • Description: End users navigate the logical name space without consideration to the physical locations of their data. Physical data can be relocated to any server and the logical Dfs name space can be reconfigured so that the end user‘s perspective of the Dfs name space is unaffected (that is, it is transparent to users that their data has changed location). • Benefit: Increased administrative flexibility. Administrators can move network shares between servers or disk drives without affecting users’ ability to access the data.

  23. DFS Features and Benefits (Continued) • Feature: Integration with Windows NT security model • Description: No additional administrative or security issues. Any user who connects to a Dfs volume is only permitted to access files for which he or she has appropriate rights on that share. • Benefit: Uses the existing Windows NT security model for easy administration and secure access.

  24. DFS Features and Benefits (Continued) • Feature: Dfs client integrated into Windows NT Workstation 4.0, available for Windows 95 and Windows 98 • Description: The Dfs Windows NT Workstation client has been incorporated into Windows NT Workstation 4.0. This integration with the SMB redirector allows the extra Dfs features to be fully pageable and does not affect memory needs or standard client access performance. • Benefit: Dfs functionality requires no additional resources on client systems.

  25. DFS Features and Benefits (Continued) • Feature: Intelligent client caching • Description: A Dfs volume can potentially connect hundreds or thousands of published shares. The client software makes no assumptions about what portion of Dfs published information a user might access. As a result, the first access of a published directory caches certain information locally. The next time a client accesses that portion of the Dfs name space, the cached referral is accessed, rather than obtaining a new referral. • Benefit: Allows high-performance access to complex hierarchies of network volumes.

  26. DFS Features and Benefits (Continued) • Feature: Windows 95 and Windows 98 Client • Description: Dfs includes a service to permit Windows 95 and Windows 98 users to navigate the Dfs name space. With the current release of Dfs, Windows clients can only access non-SMB volumes through a server-based gateway (for example, Microsoft Gateway Services for NetWare, which is included with Windows NT Server). • Benefit: Extends Dfs benefits to Windows 95 and Windows 98 users.

  27. DFS Features and Benefits (Continued) • Feature: Interoperates with other network file systems • Description: Any volume that is accessible through a redirector on Windows NT Workstation can participate in the Dfs name space. This can be through either client redirectors or server-based gateway technology. • Benefit: Administrators can create a single hierarchy incorporating heterogeneous network file systems.

  28. Administration • DFS Provides tools to add and remove shares as necessary

  29. Administration (Continued) • Easy to replace servers since each node in the Dfs is assigned a logical name that points to a file share • Can point a particular share to a new node while the current node is being replaced

  30. User View • Maps just like a regular Windows drive

  31. Load Balancing • If volumes are unavailable, Dfs will hand off request to an alternate volume if available • Example: • If 300 users require access to one volume, Dfs can split users among copies of 2 or more servers to balance the load

  32. Name Transparency • Eliminates the need for end users to know where the information is physically stored • Eases updating to accommodate additional storage • Example: • Users do not need to know the location of physical storage, so it can be swapped out behind the scenes to accommodate additional storage

  33. Technical Overview of DFS • DFS Root • Serves as a starting point and host to other shares

  34. Post-Junction Junctions • This is a junction that has child junctions • Inter-Dfs Links • Can join separate Dfs volumes together • Example: Organizations having their own Dfs, and then one large Dfs to encompass the smaller Dfs’s

  35. Post-Junction Junctions (Continued) • Midlevel Junctions • Planned for future versions of Dfs • Unlimited hierarchical junctioning • Reduces points of failures • Does not require inter-dfs links • Minimizes the number of referrals to deeply nested paths • Maintained by the Root

  36. Example

  37. Example (Continued)

  38. Alternate Volumes • Keeping exact replicas of the same volume for redundancy • Can be mounted to the same point • Limit of 32 alternates for any given junction point

  39. Down-Level Volumes • Legacy support for all older Windows operating systems • Can participate in Dfs but cannot host the Dfs tree

  40. Partition Knowledge Table (PKT) • Maintains knowledge of all of the junction points • Approximately 300 bytes per entry containing: • Dfs Path • [Server + Share] (a list) • Time to Live

  41. Illustration

  42. Resolving Junctions • Logical names into physical names is done by searching the PKT. • Maintained in a tree • Top-down search

  43. Example

  44. Fail over to between volumes • When alternates are available they are provided to the client during name resolution • Choosing which volume among alternates is arbitrary and selected by the client

  45. Fail-over Scenario 1 A client is browsing an alternate volume. The computer hosting the alternate loses power or drops completely from the network for any reason. To fail-over, the client must first detect that the hosting computer is no longer present. How long this takes depends on which protocol the client is using. Many protocols account for slow and loosely connected WAN links, and therefore may have retry counts of up to two minutes before the protocol itself times out. Once that occurs, Dfs immediately selects a new alternate. If none are available from the local cache, the Dfs client consults with the Dfs root to see if the administrator has modified any PKT entries. If no alternates are available at the root, a failure occurs; otherwise, Dfs initiates a fresh alternate selection and session setup.

  46. Fail-over Scenario 2 A client is browsing an alternate volume. The computer hosting the alternate loses a hard disk hosting the alternate, or the share is deactivated. In this situation, the server hosting the alternate is still responding to the client request; the fail-over to a fresh alternate is nearly instantaneous.

  47. Fail-over Scenario 3 • A client has open files. The computer hosting the alternate loses power or drops completely from the network for any reason. In this scenario, the same protocol fail-over process described in Scenario #1 occurs, but the application that previously had file locks from the previous alternate must detect the change and establish new locks. • New attempts to open files trigger the same fail-over process described in Scenario #1. Operations on already open files fail with appropriate errors.

  48. Fail-over Scenario 4 A client has open files. The computer hosting the alternate loses a hard disk hosting the alternate, or the share is deactivated. In this scenario, the same very quick fail-over process described in Scenario #2 occurs, but the application that previously had file handles from the previous alternate must detect the change and establish new handles.

  49. Security • Allows for special handling of security issues at session startup using ACL’s. • The ACL’s are not consistent system wide • ACL’s maintained on each server share

  50. Administered at each physical share • There is no mechanism to administer system wide security from the Dfs root • There is no attempt made to keep the ACL’s consistent between alternate volumes