Emery Berger

1
Operating SystemsCMPSCI 377Lecture 21
Distributed File Systems

• Emery Berger
• University of Massachusetts Amherst

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

3
Distributed File Systems Issues

• Naming transparency
• Remote file access
• Caching
• Server with state or without
• Replication

4
Naming Transparency

• Issues
• How are files named?
• Do filenames reveal location?
• Do filenames change if file moves?
• Do filenames change if user moves?

5
Transparency

• Location transparency filename does not reveal
  physical storage location
• Location independence filename need not change
  if files storage location changes
• In practice
• Most naming schemes do not have location
  independence
• Many have location transparency

6
Naming StrategiesAbsolute Names
ltmachine name, pathnamegt Examples AppleShare,
Windows NT

• Advantages
• Easy to find fully specified filename
• Easy to add delete new names
• No global state
• Scales easily

• 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

7
Naming StrategiesMount Points

• Mount points (NFS Network File System)
• Each host has set of local names for remote
  locations
• Mount table (/etc/fstab) specifies ltremote
  pathname _at_ machine name, local pathnamegt
• At boot bind local name to remote
• Users refer to local pathnames
• NFS manages mapping

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

9
NFS Example

• Partial contents of /etc/fstab for Edlab
• /usr1/mail_at_elux3.cs.umass.edu/var/spool/mail
• /users/users1_at_elsrv1/users/users1
• /courses/cs300_at_elsrv3/courses/cs300
• /rcf/common_at_elsrv1/exp/rcf/common

10
NFS Example
11
Naming StrategiesGlobal Name Space

• Single name space
• Examples
• AFS (CMUs 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

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

13
Distributed File Systems Issues

• Naming transparency
• Remote file access
• Caching
• Server with state or without
• Replication

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?

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

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

17
Cache Consistency

• Client-initiated consistencyclient contacts
  server and checks consistency
• Can check every access,
• at given intervals,
• only upon opening a file
• Server-initiated consistencyserver 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

18
Case StudySuns 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

19
NFS Implementation

• NFS defines set of RPC operations for remote file
  access
• Directory search, reading directory entries
• Manipulating links directories
• Accessing file attributes
• 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

20
NFS Implementation
21
Summary

• Distributed File Systems
• Naming transparency
• Remote file access
• Caching
• Server with state or without
• Replication