Distributed System and Middleware Distributed Systems Distributed File System Dr. Sunny Jeong. [email protected] Mr. Coling Zhang [email protected] With Thanks to Prof. G. Coulouris, Prof. A.S. Tanenbaum and Prof. S.C Joo
Dec 13, 2015
Distributed System and Middleware
Distributed Systems
Distributed File System
Dr. Sunny Jeong. [email protected]
Mr. Coling Zhang [email protected]
With Thanks to Prof. G. Coulouris, Prof. A.S. Tanenbaum and Prof. S.C Joo
Distributed System and Middleware
Overview
Requirements for distributed file systems transparency, performance, fault-tolerance, Consistency...
Design issues possible options, architectures file sharing, concurrent updates Caching
Examples Sun Network File System Andrew File System
2
Distributed System and Middleware
Distributed Services
3
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
• A Distributed File System ( DFS ) is simply a classical model of a file system ( as discussed before ) distributed across multiple machines. The purpose is to promote sharing of dispersed files.
• This is an area of active research interest today.
• The resources on a particular machine are local to itself. Resources on other machines are remote.
• A file system provides a service for clients. The server interface is the normal set of file operations: create, read, etc. on files.
Definitions
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
Clients, servers, and storage are dispersed across machines. Configuration and implementation may vary -
a) Servers may run on dedicated machines, ORb) Servers and clients can be on the same machines.c) The OS itself can be distributed (with the file system a part of
that distribution.a) A distribution layer can be interposed between a
conventional OS and the file system.
Clients should view a DFS the same way they would a centralized FS; the distribution is hidden at a lower level.
Performance is concerned with throughput and response time.
Definitions
Distributed System and Middleware
Distributed file service
Basic services persistent file storage of data and programs operations on files (create, open, read, write…) multiple remote clients within intranet file sharing typically one-copy update semantics over RPC
Many new developments persistent object stores (storage of objects)
Persistent Java, CORBA, … replication, whole-file caching distributed multimedia (Tiger video file server)
6
Distributed System and Middleware
Storage system and their properties
Sharing Persis-tence
Distributedcache/replicas
Consistencymaintenance
Example
Main memory RAM
File system UNIX file system
Distributed file system Sun NFS
Web Web server
Distributed shared memory Ivy (Chap. 16)
Remote objects (RMI/ORB) CORBA
Persistent object store 1 CORBA PersistentObject Service
Persistent distributed object store PerDiS, Khazana
1
1
1
* “1” is for one-copy consistency
7
Distributed System and Middleware
Characteristics of file systems
Operations on files ( =data + attributes) create/delete query/modify attributes open/close read/write access control
Storage organization directory structure (hierarchical, pathnames) metadata (= file management information, data about data)
file attributes directory structure information, etc
8
Distributed System and Middleware
Characteristics of file systems
Persistently stored data sets( files = data + attributes) Hierarchic name space visible to all processes API with the following characteristics:
access and update operations on persistently stored data sets sequential access model (with additional random facilities)
Sharing of data between users, with access control Concurrent access:
certainly for read-only access what about updates?
9
Distributed System and Middleware
File attribute record structure
File length
Creation timestamp
Read timestamp
Write timestamp
Attribute timestamp
Reference count
Owner
File type
Access control list(ACL)E.g. for UNIX: rw-rw-r--
User controlled
updated by system:
updated by owner:
10
Distributed System and Middleware
File system Modules
Concentrate on higher levels.
Directory module: relates file names to file IDs
File module: relates file IDs to particular files
Access control module: checks permission for operation requested
File access module: reads or writes file data or attributes
Block module: accesses and allocates disk blocks
Device module: disk I/O and buffering
11
Distributed System and Middleware
Distributed file system requirements [1/4]
Facilities support the sharing of persistent storage and information enable user programs to access files without copying them to a local disk
Transparency (clients unaware of the distributed nature) access transparency - client unaware of distribution of files, same interface
for local/remote files location transparency - uniform file name space from any client workstation mobility transparency - files can be moved from one server to another
without affecting client performance transparency - client performance not affected by load on
service scaling transparency - expansion possible if numbers of clients increase
12
Distributed System and Middleware
[Distributed file system requirements –ctd[2/4]
Concurrent file updates changes by one client do not affect another Isolation File-level or record-level locking Other forms of concurrency control to minimise contention (Minimum
Competition)
File replication File service maintains multiple identical copies of files
Load-sharing between servers makes service more scalable Local access has better response (lower latency) Fault tolerance
Full replication is difficult to implement Caching (of all or part of a file) gives most of the benefits (except fault
tolerance)13
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
Naming is the mapping between logical and physical objects.
Example: A user filename maps to <cylinder, sector>.
In a conventional file system, it's understood where the file actually resides; the system and disk are known.
In a transparent DFS, the location of a file, somewhere in the network, is hidden.
File replication means multiple copies of a file; mapping returns a SET of locations for the replicas.
Location transparency -
a)The name of a file does not reveal any hint of the file's physical storage location.a)File name still denotes a specific, although hidden, set of physical disk
blocks.b)This is a convenient way to share data.c) Can expose correspondence between component units and machines.
Naming and Transparency
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
Location independence -
The name of a file doesn't need to be changed when the file's physical storage location changes. Dynamic, one-to-many mapping.
Better file abstraction. Promotes sharing the storage space itself. Separates the naming hierarchy from the storage devices
hierarchy.
Most DFSs today:
Support location transparent systems. Do NOT support migration; (automatic movement of a file from
machine to machine.) Files are permanently associated with specific disk blocks.
Naming and Transparency
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
The ANDREW DFS AS AN EXAMPLE:
Is location independent. Supports file mobility. Separation of FS and OS allows for disk-less systems. These have lower
cost and convenient system upgrades. The performance is not as good.
NAMING SCHEMES:
There are three main approaches to naming files: 1. Files are named with a combination of host and local name.
• This guarantees a unique name. NOT location transparent NOR location independent.
• Same naming works on local and remote files. The DFS is a loose collection of independent file systems.
Naming and Transparency
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
NAMING SCHEMES:
2. Remote directories are mounted to local directories.
• So a local system seems to have a coherent directory structure.
• The remote directories must be explicitly mounted. The files are location independent.
• SUN NFS is a good example of this technique. 3. A single global name structure spans all the files in the system.
• The DFS is built the same way as a local filesystem. Location independent.
Naming and Transparency
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
IMPLEMENTATION TECHNIQUES:
Can Map directories or larger aggregates rather than individual files.
A non-transparent mapping technique:
name ----> < system, disk, cylinder, sector > A transparent mapping technique:
name ----> file_identifier ----> < system, disk, cylinder, sector >
So when changing the physical location of a file, only the file
identifier need be modified. This identifier must be "unique" in the universe.
Naming and Transparency
Distributed System and Middleware
File Service Design Options
State full server holds information on open files, current position, file locks open before access, close after access better performance
shorter message, read-ahead possible server failure
lose state client failure
tables fill up can provide file locks
19
Distributed System and Middleware
File Service Design Options -ctd
Stateless no state information held by server file operations(idempotent) must contain all information needed
(longer message) simpler file server design can recover easily from client or server crash locking requires extra lock server to hold state
20
Distributed System and Middleware
File Service Architecture
Client Side
File serverSide
21
Distributed System and Middleware
File Service Architecture
Client computer Server computer
Applicationprogram
Applicationprogram
Client module
Flat file service
Directory service
LookupAddNameUnNameGetNames
ReadWriteCreateDeleteGetAttributesSetAttributes
22
Distributed System and Middleware
File Server Architecture -ctd
Components (for openness): Flat file service
Flat file service operations below on file contents Have unique file identifiers (UFIDs) translates UFIDs to file locations
Read(FileId, i, n) -> Data — throws BadPosition
If 1 ≤ i ≤ Length(File): Reads a sequence of up to n items
from a file starting at item i and returns it in Data.
Write(FileId, i, Data) — throws BadPosition
If 1 ≤ i ≤ Length(File)+1: Writes a sequence of Data to a
file, starting at item i, extending the file if necessary.
Create() -> FileId Creates a new file of length 0 and delivers a UFID for it. Delete(FileId) Removes the file from the file store.
GetAttributes(FileId) -> Attr Returns the file attributes for the file. SetAttributes(FileId, Attr) Sets the file attributes (only those attributes that are not
shaded in ).
23
Distributed System and Middleware
File Server Architecture -ctd
Directory service mapping between text-(file) names to UFIDs
Client module API for file access, one per client computer holds states: open files, positions knows network location of flat file & directory server
Flat file service
Read(FileId, i, n) -> Data
Write(FileId, i, Data)
Create() -> FileId
Delete(FileId)
GetAttributes(FileId) -> Attr
SetAttributes(FileId, Attr)
Directory service
Lookup(Dir, Name) -> FileId
AddName(Dir, Name, FileId)
UnName(Dir, Name)
GetNames(Dir, Pattern) -> NameSeq
24
Distributed System and Middleware
Flat file service RPC interface
Used by client modules, not user programs FileId (UFID) uniquely identifies file invalid if file not present or inappropriate access Read/Write; Create/Delete; Get/SetAttributes
No open/close! (unlike UNIX) access immediate with FileId Read/Write identify starting point
Improved fault-tolerance operations idempotent except Create, can be repeated (at-least-once RPC
semantics) stateless service
25
Distributed System and Middleware
Access control
In UNIX file system access rights are checked against the access mode (read, write, execute) in
open user identity checked at login time, cannot be tampered(=changed) with in
non-distributed implementations.
In distributed (file) systems Access rights must be checked at server
RPC unprotected Forging identity possible, a security risk
user id typically passed with every request (e.g. Sun NFS) stateless
26
Distributed System and Middleware
Directory structure
Hierarchical tree-like, pathnames from root (in UNIX) several names per file (link operation)
Naming system implemented by client module, using directory service root has well-known UFID locate file following path from root
big bobjon
people
export
(root)
. . .
27
Distributed System and Middleware
File names
Text name = directory pathname+file name hostname:local name
not mobility transparent
uniform name structure (the same name space for all clients)
remote mount (e.g. Sun NFS) remote directory inserted into local directory relies on clients maintaining consistent naming conventions across all clients
all clients must implement same local tree must mount remote directory into the same local directory
28
Distributed System and Middleware
File names
Mount operation:
mount(remotehost, remotedirectory, localdirectory)
A server maintains a table of clients who have mounted file systems at that server.
Each client maintains a table of mounted file systems holding: < IP address, port number, file handle>
Hard versus soft mounts
29
Distributed System and Middleware
Remote mount
jim jane joeann
usersstudents
usrvmunix
Client Server 2
. . . nfs
Remote
mountstaff
big bobjon
people
Server 1
export
(root)
Remote
mount
. . .
x
(root) (root)
Note: The file system mounted at /usr/students in the client is actually the sub-tree located at /export/people in Server 1; the file system mounted at /usr/staff in the client is actually the sub-tree located at /nfs/users in Server 2. server-side : /export/people, : /nfs/users
client-side : mount -t nfs server1:/export/people /usr/students /* client: /usr/students(=people)/jon,… */client-side : mount -t nfs server2:/nfs/users /usr/staff /* client:/usr/staff(=users)/jane, … */
30
Distributed System and Middleware
Directory service
Directory conventional file (client of the flat file service) mapping from text names to UFIDs
Operations require FileId, machine readable UFID as parameter locate file (LookUp) add/delete file (AddName/UnName) match file names to regular expression (GetNames)
31
Distributed System and Middleware
Directory service operations
Lookup(Dir, Name) -> FileId— throws NotFound
Locates the text name in the directory and returns therelevant UFID. If Name is not in the directory, throws an
exception.
AddName(Dir, Name, File) — throws NameDuplicate
If Name is not in the directory, adds (Name, File) to thedirectory and updates the file’s attribute record.If Name is already in the directory: throws an exception.
UnName(Dir, Name) — throws NotFound
If Name is in the directory: the entry containing Name isremoved from the directory.
If Name is not in the directory: throws an exception.
GetNames(Dir, Pattern) -> NameSeq Returns all the text names in the directory that match theregular expression Pattern.
32
Distributed System and Middleware
File sharing
Multiple clients share the same file for read/write access.
One-copy update semantics every read sees the effect of all previous writes a write is immediately visible to clients who have the file open for reading
Problems! caching: maintaining consistency between several copies difficult to achieve serialize access by using file locks (affects performance ) trade-off between consistency and performance
33
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
CACHING
Reduce network traffic by retaining recently accessed disk blocks in a cache, so that repeated accesses to the same information can be handled locally.
If required data is not already cached, a copy of data is brought from the server to the user.
Perform accesses on the cached copy.
Files are identified with one master copy residing at the server machine, Copies of (parts of) the file are scattered in different caches.
Cache Consistency Problem -- Keeping the cached copies consistent with the master file.
Remote File Access
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
CACHING
A remote service ((RPC) has these characteristic steps:
a) The client makes a request for file access.b) The request is passed to the server in message format.c) The server makes the file access.d) Return messages bring the result back to the client.
This is equivalent to performing a disk access for each request.
Remote File Access
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
CACHE LOCATION:
Caching is a mechanism for maintaining disk data on the local machine. This data can be kept in the local memory or in the local disk. Caching can be advantageous both for read ahead and read again.
The cost of getting data from a cache is a few HUNDRED instructions; disk accesses cost THOUSANDS of instructions.
The master copy of a file doesn't move, but caches contain replicas of portions of the file.
Caching behaves just like "networked virtual memory".
Remote File Access
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
CACHE LOCATION:
What should be cached? << blocks <---> files >>. Bigger sizes give a better hit rate; Smaller give better transfer times.
Caching on disk gives:— Better reliability.
Caching in memory gives:— The possibility of diskless work stations,— Greater speed,
Since the server cache is in memory, it allows the use of only one mechanism.
Remote File Access
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
CACHE UPDATE POLICY:
A write through cache has good reliability. But the user must wait for writes to get to the server. Used by NFS.
Delayed write - write requests complete more rapidly. Data may be written over the previous cache write, saving a remote write. Poor reliability on a crash.
Flush sometime later tries to regulate the frequency of writes.
Write on close delays the write even longer.
Which would you use for a database file? For file editing?
Remote File Access
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMSExample: NFS with Cachefs
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
CACHE CONSISTENCY:
The basic issue is, how to determine that the client-cached data is consistent with what's on the server.
Client - initiated approach -
The client asks the server if the cached data is OK. What should be the frequency of "asking"? On file open, at fixed time interval, ...?
Server - initiated approach -
Possibilities: A and B both have the same file open. When A closes the file, B "discards" its copy. Then B must start over. The server is notified on every open. If a file is opened for writing, then disable caching by other clients for that file. Get read/write permission for each block; then disable caching only for particular blocks.
Remote File Access
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
COMPARISON OF CACHING AND REMOTE SERVICE: Many remote accesses can be handled by a local cache. There's a
great deal of locality of reference in file accesses. Servers can be accessed only occasionally rather than for each access.
Caching causes data to be moved in a few big chunks rather than in many smaller pieces; this leads to considerable efficiency for the network.
Cache consistency is the major problem with caching. When there are infrequent writes, caching is a win. In environments with many writes, the work required to maintain consistency overwhelms caching advantages.
Caching requires a whole separate mechanism to support acquiring and storage of large amounts of data. Remote service merely does what's required for each call. As such, caching introduces an extra layer and mechanism and is more complicated than remote service.
Remote File Access
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
STATEFUL VS. STATELESS SERVICE:
Stateful: A server keeps track of information about client requests.
It maintains what files are opened by a client; connection
identifiers; server caches. Memory must be reclaimed when client closes file or when client dies.
Stateless: Each client request provides complete information needed by the server (i.e., filename, file offset ).
The server can maintain information on behalf of the client, but it's not required.
Useful things to keep include file info for the last N files touched.
Remote File Access
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
STATEFUL VS. STATELESS SERVICE:
Performance is better for stateful.
Don't need to parse the filename each time, or "open/close" file on every request.
Stateful can have a read-ahead cache. Fault Tolerance: A stateful server loses everything when it crashes.
Server must poll clients in order to renew its state. Client crashes force the server to clean up its encached
information. Stateless remembers nothing so it can start easily after a crash.
Remote File Access
Distributed System and Middleware
DISTRIBUTED FILE SYSTEMS
FILE REPLICATION:
Duplicating files on multiple machines improves availability and performance. Placed on failure-independent machines ( they won't fail together ).
Replication management should be "location-opaque". The main problem is consistency - when one copy changes, how do
other copies reflect that change? Often there is a tradeoff: consistency versus availability and performance.
Example: "Demand replication" is like whole-file caching; reading a file causes it
to be cached locally. Updates are done only on the primary file at which time all other copies are invalidated.
Atomic and serialized invalidation isn't guaranteed ( message could
get lost / machine could crash. )
Remote File Access
Distributed System and Middleware
Example: Sun NFS (1985)
An industry standard for file sharing on local networks since the 1980s
An open standard with clear and simple interfaces Closely follows the abstract file service model defined above Supports many of the design requirements already mentioned:
transparency heterogeneity efficiency fault tolerance
Limited achievement of: concurrency replication consistency security
45
Distributed System and Middleware
Example: Sun NFS (1985)
Structure of flat file, client & directory service NFS protocol
RPC based, OS independent (originally UNIX) NFS server
stateless (no open/close) no locks or concurrency control no replication with updates
Virtual file system, Remote mount Access control (user id with each request)
security loophol modify RPC to impersonate users
Client and Server caching
46
Distributed System and Middleware
Sun NFS architecture
UNIX kernel
protocol
Client computer Server computer
system calls
Local Remote
UNIXfile
system
NFSclient
NFSserver
UNIXfile
system
Applicationprogram
Applicationprogram
NFS
UNIX
UNIX kernel
Virtual file systemVirtual file system
Oth
er f
ile s
yste
m
Operationson remote files
47
Distributed System and Middleware
File identifier (FileId)
Simple Solution i-node (number identifying file
within file system) file migration requires finding and
changing all FileIds UNIX reuses i-node numbers after
file is deleted (i-generation. no)
NFS file handle Virtual file system uses i-node if local, file handle(fh) if remote.
Server address Index
IP address.socket i-node number
File system identifier i-node gener. no.i-node no.File handle(fh)
fh = file handle:
Filesystem identifier i-node number i-node generation no
48
Distributed System and Middleware
NFS Server Operations (simplified)
• read(fh, offset, count) -> attr, data• write(fh, offset, count, data) -> attr• create(dirfh, name, attr) -> newfh, attr• remove(dirfh, name) -> status• getattr(fh) -> attr• setattr(fh, attr) -> attr• lookup(dirfh, name) -> fh, attr• rename(dirfh, name, todirfh, toname)• link(newdirfh, newname, dirfh, name)• readdir(dirfh, cookie, count) -> entries• symlink(newdirfh, newname, string) -> status• readlink(fh) -> string• mkdir(dirfh, name, attr) -> newfh, attr• rmdir(dirfh, name) -> status• statfs(fh) -> fsstats
fh = file handle:
Filesystem identifier i-node number i-node generation no
•i-node contains information of files
49
Distributed System and Middleware
Caching in NFS
Indispensable for performance (necessary) Caching
Retains recently the used data (file pages, directories, file attributes) in cache
updates data in cache for speed block size typically 8kbytes
Server caching cache in server memory (UNIX kernel)
Client caching cache in client memory, local disk
50
Distributed System and Middleware
Server caching
Store data in server memory Read-ahead: anticipate which pages to read Delayed write
update in cache; write to disk periodically (UNIX sync to synchronize cache) or when space needed
which contents seen by users depends on timing
Write through cache and write to disk (reliable, poor performance), whenever updated
Write on close write to disk only when commit received (fast but problems with files open
for a long time)
51
Distributed System and Middleware
Client caching
Potential consistency problems! different versions, portions of files, … since writes delayed clients poll server to check if copy still valid
Timestamp method Tag with latest time of validity check and modification time copy valid if time since last check less than freshness interval, or
modification time on server the same choose freshness interval adaptively, 3~30 sec for files, 30~60 sec for
directories for small freshness interval, potential heavy load on Network
52
Distributed System and Middleware
Client caching ctd
Reads perform validity check whenever cache entry(input) used if not valid, request data from server several optimizations to reduce traffic Recent updates not always visible (timing!)
Writes when page modified, marked as dirty dirty pages flushed asynchronously, periodically (client’s synch) and on
close
Not truly one-copy update semantics...
53
Distributed System and Middleware
NFS summary
Transparency Access transparency
providing application programming interface(= local system interface) Location transparency
supporting a single network file name space Mobility transparency
migration transparency Scalability
To handle very large-world loads efficiently File replication
NSF : read-only replica supporting file replication with updates
Hardware and operating system - heterogeneity Fault tolerance
54
Distributed System and Middleware
Example: Andrew File System(AFS)
Overview
A distributed computing environment (Andrew) under development since 1983 at Carnegie-Mellon University, purchased by IBM and released as Transarc DFS, now open sourced as OpenAFS.
Information sharing on a large scale via transparencyNFS compatible(called NSF-2)
File reference by NFS-style file handle
55
Distributed System and Middleware
AFS tries to solve complex issues such as uniform name space, location-independent file sharing, client-side caching (with cache consistency), secure authentication (via Kerberos)
Also includes server-side caching (via replicas), high availability Can span 5,000 workstations
Scalable Whole-file serving (> 64kbytes) Whole-file caching (on local client disk, 100s of recently used files)
Characteristics of AFS local-cached copy providing sufficient cache storage UNIX based on file size and referencing locality
DISTRIBUTED FILE SYSTEMS
Distributed System and Middleware
AFS Software architecture
Venus
Workstations(clients) Servers
Venus
VenusUserprogram
Network
UNIX kernel
UNIX kernel
Vice
Userprogram
Userprogram
ViceUNIX kernel
UNIX kernel
UNIX kernel
Two software components
Vice(user-level UNIX processing running in server, server module)
Venus( user-level process running in a client, client module)
57
Distributed System and Middleware
SHARED NAME SPACE:
The server file space is divided into volumes. Volumes contain files of only one user. It's these volumes that are the level of granularity attached to a client.
A vice file can be accessed using a fid = <volume number, vnode >. The fid doesn't depend on machine location. A client queries a volume-location database for this information.
Volumes can migrate between servers to balance space and utilization. Old server has "forwarding" instructions and handles client updates during migration.
Read-only volumes ( system files, etc. ) can be replicated. The volume database knows how to find these.
DISTRIBUTED FILE SYSTEMSAndrew File System
Distributed System and Middleware
FILE OPERATIONS AND CONSISTENCY SEMANTICS:
Andrew caches entire files form serversA client workstation interacts with Vice servers only during opening
and closing of files Venus – caches files from Vice when they are opened, and stores
modified copies of files back when they are closed Reading and writing bytes of a file are done by the kernel without
Venus intervention on the cached copy Venus caches contents of directories and symbolic links, for path-
name translation Exceptions to the caching policy are modifications to directories that
are made directly on the server responsibility for that directory
DISTRIBUTED FILE SYSTEMSAndrew File System
Distributed System and Middleware
Clients have a partitioned space of file names: a local name space and a shared name space
Dedicated servers, called Vice, present the shared name space to the clients as an homogeneous, identical, and location transparent file hierarchy
Workstations run the Virtue protocol to communicate with Vice.
Are required to have local disks where they store their local name space
Servers collectively are responsible for the storage and management of the shared name space
DISTRIBUTED FILE SYSTEMSAndrew File System
Distributed System and Middleware
Clients and servers are structured in clusters interconnected by a backbone LAN
A cluster consists of a collection of workstations and a cluster server and is connected to the backbone by a router
A key mechanism selected for remote file operations is whole file caching
Opening a file causes it to be cached, in its entirety, on the local disk
DISTRIBUTED FILE SYSTEMSAndrew File System
Distributed System and Middleware
IMPLEMENTATION – Flow of a request: Deflection of open/close: The client kernel is modified to detect references to vice files.
The request is forwarded to Venus with these steps:
Venus does pathname translation.
Asks Vice for the file
Moves the file to local disk
Passes inode of file back to client kernel.
Venus maintains caches for status ( in memory ) and data ( on local disk.)
A server user-level process handles client requests.
A lightweight process handles concurrent RPC requests from clients.
State information is cached in this process.
Susceptible to reliability problems.
DISTRIBUTED FILE SYSTEMSAndrew File System
Distributed System and Middleware
New developments -ctd
AFS enhancements DCE/DFS standards, adopts a similar Spritely NFS and NQNFS to callbacks improving in storage organization
Redundant array of inexpensive(RAID) Log-structure file storage(LFS)
New design approaches(UC of Berkeley) xFS (serverless network architecture, file serving responsibility distributed
across LAN) Frangipni( high scalable distributed file system, Digital System Research
Center, 1997)
63
Distributed System and Middleware
Summary
File service crucial to the running of a distributed system performance, consistency and easy recovery essential
Design issues separate flat file service from directory service and client module stateless for performance and fault-tolerance caching for performance concurrent updates difficult with caching approximation of one-copy update semantics
Case studies SUN-NFS AFS Recent advances
64