U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Emery Berger University of Massachusetts Amherst Operating Systems CMPSCI 377 Lecture 21: Distributed File Systems

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 2 Distributed File Systems Most common use of distributed systems Idea: Given set of disks attached to different nodes, share as if all were attached to every node Examples: Edlab: one server, workstations on LAN AppleShare: nodes are servers with disk & client

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 3 Distributed File Systems: Issues Naming & transparency Remote file access Caching Server with state or without Replication

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 4 Naming & Transparency Issues How are files named? Do filenames reveal location? Do filenames change if file moves? Do filenames change if user moves?

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 5 Transparency Location transparency: filename does not reveal physical storage location Location independence: filename need not change if file’s storage location changes In practice: Most naming schemes do not have location independence Many have location transparency

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 6 Naming Strategies: Absolute Names Disadvantages: User must know complete name – aware of which files are local & which are remote File is location dependent (cannot move) Makes sharing harder Not fault-tolerant Advantages: Easy to find fully specified filename Easy to add & delete new names No global state Scales easily Examples: AppleShare, Windows NT

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 7 Naming Strategies: Mount Points Mount points (NFS – Network File System) Each host has set of local names for remote locations Mount table ( /etc/fstab ): specifies At boot: bind local name to remote Users refer to local pathnames NFS manages mapping

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 8 Mount Points: Pros & Cons Advantages: Location transparent Remote name can change across reboots Disadvantages: Single unified strategy hard to maintain Same file can have different names

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 9 NFS Example Partial contents of /etc/fstab for Edlab:

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 10 NFS Example

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 11 Naming Strategies: Global Name Space Single name space: Examples: AFS (CMU’s Andrew File System) Sprite (Berkeley) No matter which node you are on, filenames remain the same Client: gets filename structure from server(s) When users access files, server sends copies to workstation, where they are cached

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 12 Global Name Space: Pros & Cons Advantages: Naming – consistent Ensures all files are same regardless of where you login Late binding of names ) moving them is easier Disadvantages: Difficult for OS to keep files consistent (caching) Global name space may limit flexibility Performance issues

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 13 Distributed File Systems: Issues Naming & transparency Remote file access Caching Server with state or without Replication

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 14 Remote File Access & Caching Can access files: Remotely: returns results using RPC = remote service Transfer part of file, perform local access = caching Caching issues: Where & when are file blocks cached? When are modifications propagated back to remote file? What happens when multiple clients cache same file?

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 15 Remote File Caching Local disk:  Reduces access time (compared to remote)  Safe if node fails – Difficult to keep local copy consistent with remote copy – Requires client to have disk! Local memory:  Quick access time  Works without disks – Difficult to keep local copy consistent with remote copy – Smaller cache size – Not fault-tolerant

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 16 Cache Update Policies Write-through: write to remote disk  Reliable – Low-performance = remote service for all writes Write-back: write only to cache Write to disk on evictions, periodic synch  Quick  Reduces network traffic (repeated writes to same block) – User machine crashes ) data loss

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 17 Cache Consistency Client-initiated consistency: client contacts server and checks consistency Can check every access, at given intervals, only upon opening a file Server-initiated consistency: server detects potential conflicts, invalidates caches Server needs to know which clients have cached which parts of which files which clients are readers & which are writers

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 18 Case Study: Sun’s Network File System NFS: standard for distributed UNIX file access Designed to run on LANs Nodes are both servers & clients Servers have no state Uses mount protocol to make global name local /etc/exports : lists local names server willing to export /etc/fstab : lists global names that local nodes import Corresponding global name must be in /etc/exports on server

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 19 NFS Implementation NFS defines set of RPC operations for remote file access: 1. Directory search, reading directory entries 2. Manipulating links & directories 3. Accessing file attributes 4. Reading/writing files Does not rely on node homogeneity Heterogeneous nodes support NFS mount & remote access protocols using RPC Users may need to know different names depending upon which node they log on

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 20 NFS Implementation

U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science 21 Summary Distributed File Systems Naming & transparency Remote file access Caching Server with state or without Replication