1. Operating Systems
CMPSCI 377
Distributed File Systems
Emery Berger
University of Massachusetts Amherst
UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science

2. Distributed File Systems
Numerous drawbacks of local file systems:

Inconvenient

Administrative overhead

Single point-of-failure

Solution: distributed file systems

FS appears to be local, but data is remote

Two major implementations:

Windows

NFS (Sun’s Network File System)

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3. Complications
Distributed file systems add complexity

& many design tradeoffs
Naming – absolute vs. relative (to server)

Remote access vs. caching

Stateless or stateful server

Single image or replication

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4. Naming & Transparency
Issues

How are files named?

Do filenames reveal location?

Do filenames change if file moves?

Do filenames change if user moves?

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5. Location naming
Location transparency:

filename does not reveal
physical storage location
Normal in Unix

Compare to Windows - C:foobar

Provides location independence:

no change if file’s storage location changes
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6. Windows: Absolute Names
lokihomeemery
machine nameremote pathname
Advantages: Disadvantages:
 
Easy to find fully User must know
 
specified filename complete name –
local & remote
Easy to add & delete

different
new names
Location dependent
No global state 

(cannot move file)
Scales easily

Makes sharing harder

Not fault-tolerant

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7. NFS: Relative Names
/nfs/sting/users1/emery
Advantages: Disadvantages:
 
Location  Admin

transparent overhead
Remote name

can change
across reboots
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8. NFS: Relative Names
/courses/cs300/cs377
Implemented via mount points

one level of indirection!

Each host: local names ! remote locations

Mount table (/etc/fstab)

<remote pathname @ machine, local pathname>

% cat /etc/fstab
elsrv4:/courses /courses nfs intr,hard,rw 0 0
elsrv4:/courses/cs100_200 /courses/cs100_200 nfs intr,hard,rw 0 0
elsrv4:/courses/cs300 /courses/cs300 nfs intr,hard,rw 0 0
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9. NFS Example
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10. URLs Viewed as File System
Uniform Resource Locator names

increasingly standard way to access data
protocol://machine/path/to/file
Good? Bad?

Looks like Windows… same?

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11. Distributed File Systems: Issues
Naming & transparency

Remote file access & caching

Server with state or without

Replication

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12. Remote File Access & Caching
Can access files two ways

Remotely: returns results using RPC

Locally: transfer part of file = caching

Caching issues:

Performance: Where & when to cache file

blocks?
Correctness:

When to propagate updates back to remote file?

What happens when multiple clients cache same file?

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13. Remote File Caching
Local disk:

Reduces access time (compared to remote)

Safe if node fails

Difficult to keep copy consistent with remote file
–
Requires client to have disk (…)
–
Local memory:

Quick

Works without disks

Difficult to keep copy consistent with remote file
–
Smaller cache size
–
Not fault-tolerant
–
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14. Cache Update Policies
Write-through: always write to remote disk

Reliable

Low-performance = remote service for all writes
–
Write-back: write only to cache

Write to disk on evictions, periodic sync

Quick

Reduces network traffic (n writes to same block)

User machine crashes ) data loss
–
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15. Cache Consistency
Client-initiated consistency:

client contacts server and checks consistency
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, plus

which clients are readers & which are writers

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16. Case Study: Network File System
NFS: standard for distributed UNIX file access

Designed to run on LANs

Nodes: both servers & clients

Servers have no state = no info about clients

Uses mount protocol to make global name local

/etc/exports

local names server willing to export

/etc/fstab

global names that local nodes import

global name must be in /etc/exports on server

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17. NFS Implementation
Set of RPC operations for remote file access:
Directory search, reading directory entries

Manipulating links & directories

Accessing file attributes

Reading/writing files

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18. NFS Implementation
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19. The End
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20. 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
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21. 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

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