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Coda (Constant Data Avaialabilty) is a distributed file system developed at Carnegie Mellon University . This presentation explains how it works and different aspects of it.
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• 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.