coda file system

• Introduction to Coda File System• Naming and Location• Architecture• Caching and Replication• Synchronization• Communication• Fault Tolerance• Security• Summary

• Coda (constant data availability) is a distributed file system that was developed as a research project at Carnegie Mellon University in 1987 under the direction of Mahadev Satyanarayan.

• Coda’s design goals:• Scalability• Constant data availability• Transparency• Security• Consistency

• The name space in Coda is hierarchically structured as in UNIX and is partitioned into disjoint volumes.

• A volume consists of a set of files and directories located on one server, and is the unit of replication in Coda.

• Each file and directory is identified by a 96-bit-long unique file identifier (FID). Replicas of a file have the same FID.

• An FID has 2 components:

1. A 32-bit RVID (Replication Volume Identifier) of the

logical volume that the file is part of.

2. A 64-bit file handle, i.e. vnode, that uniquely identifies

the file within a volume.

Each file in Coda belongs to exactly one volume

Volume may be replicated across several servers

Multiple logical (replicated) volumes map to the same physical volume

It works by implementing the following functionalities :

1. Availability of files by replicating a file volume across

many servers

2. Disconnected mode of operation by caching files at the

client machine

Coda File System is divided into two types of nodes:

1. Vice nodes: dedicated file servers 2. Virtue nodes: client machines

The internal organization of a Virtue workstation: is designed to allow access to files even if

server is unavailable and uses Virtual File System to intercept calls

from client application

Coda uses RPC2: a sophisticated reliable RPC system Start a new thread for each request,

server periodically informs client it is still working on the request

• Coda servers allow clients to cache whole files• Modifications by other clients are notified through

invalidation messages which require multicast RPCa) Sending an invalidation message one at a timeb) Sending invalidation messages in parallel

Client

Server

Client

Time

Open (RD)

Open (WR)

Session A

Session B

File f

File fClose

CloseInvalidate

Client A

Server

Client B

Open (RD) File f

Close

Open (WR) Close

Invalidate (Callback Break)

Open (WR)

Open (RD) File f

Close

Close

OK(no file transfer)

Time

Session A

Session B

Session C

Session D

File f

• Scalability• Fault Tolerance

Data structures:

• VSG (Volume Storage Group):• Set of servers storing replicas of a volume

• AVSG (Accessible Volume Storage Group):• Set of servers accessible to a client for every

volume the client has cached

Versioning vector (Coda Version Vector) when partition happens: [1,1,1]

Client A updates file versioning vector in its partition: [2,2,1]

Client B updates file versioning vector in its partition: [1,1,2]

Partition repaired compare versioning vectors: conflict!

HOARDING: File cache in advance with all files that will be accessed when disconnected

EMULATION: when disconnected, behavior of server emulated at client

REINTEGRATION: transfer updates to server; resolves conflicts

• Hoard database

• Cache equilibrium:• There is no uncached file with a higher priority than

any cached file.• The cache is full, or no uncached file has nonzero

priority.• Each cached file is a copy of the one maintained in

the client’s AVSG.• Hoard walk

Coda’s security architecture consists of two parts:

•The first part deals with setting up a secure channel between a client and a server using secure RPC and system-level authentication.

•The second part deals with controlling access to files.

Clie

nt (

Ve

nus

)

Vic

e S

erv

er

Operation Description

Read Read any file in the directory

Write Modify any file in the directory

Lookup Look up the status of any file

Insert Add a new file to the directory

Delete Delete an existing file

Administer Modify the ACL of the directory

• Peter J. Braam, The Coda File System, www.coda.cs.cmu.edu.