DISTRIBUTED FILE SYSTEM Nhóm báo cáo :

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DISTRIBUTED FILE SYSTEM

Nhóm báo cáo : Lê Tuấn Anh

Nguyễn Hải DuyĐặng Thanh LinhTrần Trung Hiếu 50500892Nguyễn Hoàng Nam

Computer Science Distributed file system.

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Content:

I. Distributed file system design.II. Distributed file system ImplementationIII. Network file system (NFS)IV. Trends in distributed file system.


What’s Distributed File System?

Distributed File System (DFS) is a mechanism for sharing filesDFS is used to make files distributed across multiple servers appear to users as if they reside in one place on the networkDFS provides a mechanism to create logical views of folders and files regardless of where those files are physically located on the network


What’s Distributed File System?(cont.)


File Service

Specify what the file system offers to its clients to manipulate on shared files ex: read,write…on files

Implemented by a user/kernel process called file serverA system may have one or several file servers running at the same time


File Service (cont.)

Two models for file servicesupload/download: files move between server and clients, few operations (read file & write file), simple, requires storage at client, good if whole file is accessedremote memory access: files stay at server, reach interface for many operations, less space at client, efficient for small accesses


File Service (cont.)


Directory Service

Provide operations for : creating and deleting directories naming and renaming files moving files from one directory to another entering, removing, looking up files in one

directory


Naming Transparency

Naming is the mapping between logical and physical objects. Ex: 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 replications.


Naming

Transparency(cont.)

Location transparency: the path name gives no hint as to where the file (or other object) is located.ex: /server1/dir1/x specifies x is located on server1 but

it does not tell where that server1 is located -> server can move the file in the network without changing the path

Location independence: possible to remove one file among servers which not change the path name.


Naming Schemes

Machine + path naming, such as /machine/path

Mounting remote file system onto the local file hierarchy

A single name space that looks the same on all machines


Two level naming

Symbolic name (external), e.g. prog.c; binary name (internal), e.g. local i-node number as in Unix

Directories provide the translation from symbolic to binary names

Binary name formati-node: no cross references among servers(server, i-node): a directory in one server can refer to a

file on a different server{binary_name}: binary names refer to the original file

and all of its backups when looking up


File Sharing Semantics

UNIX semantics: total ordering of R/W events easy to achieve in a non-distributed systemin a distributed system with one server and multiple clients with no caching at client, total ordering is also easily achieved since R and W are immediately performed at server

Session semantics: writes are guaranteed to become visible only when the file is closed

if two or more clients simultaneously write: one file (last one or non-deterministically) replaces the other


File Sharing Semantics (cont.)

Immutable files: create and read file operations (no write)

writing a file means to create a new one and enter it into the directory replacing the previous one with the same name: atomic operationstwo processes try to replace the same file at the same time: last copy or nondeterministicallywhat happens if a file is replaced while another process is busy reading it

Transaction semantics: mutual exclusion on file accesses; either all file operations are completed or none is. Good for banking systemsComputer Science Distributed file system.

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II.DFS Implementation

File usage- Measurements.- File Usage Pattern(Observed in a study by Satyanarayanan ).

System Structure- File-server and Directory-server Organization.- Special attention to alternative approaches.


File usage- Measurements

- Static measurements: * Represent a snapshot of the system at a certain instant.* Made by examining the disk to see what is on it.

- Dynamic measurements: * Modifying the file system to record all operations to a log for subsequent analysis


File usage- Measurements

- Static measurements: The distribution of files size.The distribution of file types. The amount of storage occupied by files of various types and size.- Dynamic measurements: The relative frequency of various operationsThe number of files open at any momentThe amount of sharing that takes place


File Usage- Measurement Problems

- How typical the observed user population is?Satyanarayanan's measurements were made at a university -> Also apply to industrial research lab or office automation project or banking system?- Watching out for artifacts of the system being measuredEx: Distribution of file names in an MS-DOS system- File names are never more than 8 characters( plus an optional three- characters extension)- Made on more-or-less traditional UNIX systems. Whether or not they can be transferred or extrapolated to distributed systems


File Usage- File Usage Pattern

Observed in a study by Satyanarayanan (1981)- Most files are small (< 10K)- Reading is much more frequent than writing- Most R&W accesses are sequential (random access is rare)- Most files have a short lifetime -> create the file on the client- File sharing is unusual -> caching at client- The average process uses only a few files


Server System Structure

Are client and server different?- Some system, all machines run the same basic software -> any machine can offer file-service to the public- offer names of selected directories so that other machines can access them.- The other systems, the file server and directory server are just user programs-> run client and server software on the same machines or no



Are client and server different?- The other extreme systems have clients and server are on different machine.



File + directory service: combined or not ?- Combine file service and directory service into a single server that handles all the directory and file calls.- Keep file service and directory service separate: Directory-server map symbolic name onto its binary name.File-server with the binary name to read or write the file.



Separating File + directory service-AdvantageProduce simpler software-DisadvantageRequire more communications



Separating File + directory service Example: Look-up a/b/c

Client sends a symbolic name to the directory-server-> binary name given by file-serverDirectory-hierarchy be partitioned among multiple servers: -1st directory on sever 1 contain an entry a for another directory on server 2.- 2nd directory on sever 2 contain an entry b for another directory on server 3.- 3rd directory on sever 3 contain an entry c for a file.- File with its binary name.



-Client send a message -> server 1-Server 1 finds a and sees the binary name refers to another server -> (1) tell the client which hold b•Requires the client to know which server holds which directory -> require more messages.




-Client send a message -> server 1-Server 1 finds a and sees the binary name refers to another server -> (2) forward the remainder of the request to server 2.• Efficient • Can not use RPC (Remote Procedure Call) because the process which the client sends the message to is not one that sends the reply




Separating File + directory service

ProblemPath names look up, especially with multiple directory servers can be expensive.

Cache directory hints at client to accelerate the path name look up – directory and hints must be kept coherent



Another question

Whether or not file, directory and other servers should keep state information about clients ?

- Yes Stateful server.

- No Stateless server.



requests are self-contained better fault tolerance open/close at client (fewer

messages) no space reserved for tables thus, no limit of open files no problem if client crashes

shorter messages better performance (info in

memory until close) open/close at server file locking possible read ahead possible

Stateless Server Stateful ServersStateless vs. Stateful

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Main memory

Caching

Definition: A cache is a block of memory for temporary storage of data likely to be used again.

Index Data 0 xyz 1 pdq 2 abc 3 ght

Cache Memory

Index Tag Data 0 2 abc 1 0 xyz


Caching

There are four potential places to store files, or parts of files: -The Server’s disk. -The Server’s main memory. -The Client disk. -The Client ‘s main memory.

These different storage locations all have different properties .


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Caching


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Caching-Store all file in the server’s disk.

Advantages:-Plenty of space.-The file are accessible to all clients .-Have one copy of each file ->no consistency problems arises.Problem:-Performance: the file must be transferred from the server’s disk to the server’s main memory,and then again over the network to the client’s main memory.


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Caching files in the server's main memory.Advantages:-Eliminates the disk transfer.-Keep its memory and disk copies synchronizedProblems:-Network transfer still has to be done.-What is the unit the cache manages?(whole files or disk blocks ).-What to do when the cache fills up and something must be evicted.(one of algorithm :LRU).


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Caching at client’s disk (if available):

-The disk holds more but is slower.- If large amounts of data are being used, a client disk cache may be better.- This method isn’t used in practice.- In any event, most systems that do client caching do it in the client's main memory.


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Cache in the client's main memory:

There are three options to decide where to put files:-Inside each process address space: no sharing at client, it is effective only if individual processes open and close files repeatedly

-In the kernel: kernel involvement on hits, a kernel call is needed in all cases

-In a separate user-level cache manager: flexible and efficient if paging can be controlled from user-level


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Cache in the client's main memory


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Cache Consistency.

-Two clients simultaneously read the same file and then both modify it.-Two files are written back to the server, the one written last will overwrite the other one.- Client caching has to be thought out fairly carefully-There are several ways to solve the consistency problem:

- Write through; Delayed write; Write on close; Centralized control


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Cache Consistency- Write-through algorithm

-When a cache entry (file or block) is modified, the new value is kept in the cache, but is also sent immediately to the server -> high traffic, requires cache managers to check (modification time) with server before can provide cached content to any client


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Cache Consistency -Delayed write

-Delayed write: coalesces multiple writes; better performance but ambiguous semantics . *the client just makes a note that a file has been updated. Once every 30 seconds or so, all the file updates are gathered together and sent to the server all at once.*entire sequence happens before time to send all modified files back to the server


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Cache Consistency -Write-on-close

-Write-on-close: implements session semantics, write a file back to the server only after it has been closed.


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Cache Consistency -Central control

-Central control: file server keeps a directory of open/cached files at clients -> Unix semantics, but problems with robustness and scalability; problem also with invalidation messages because clients did not solicit them


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Replication:

-Multiple copies of selected files.1. To increase reliability by having independent backups of each file. 2. To allow file access to occur even if one file server is down. A server crash should not bring the entire system down until the server can be rebooted. 3. To split the workload over multiple .By having files replicated on two or more servers, the least heavily loaded one can be used.


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Replication transparency

Replication transparency-explicit file replication: programmer controls replication-lazy file replication: copies made by the server in background-use group communication: all copies made at the same time in the foreground



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Replication-Update protocols:

Updating all replicas using a coordinator works but is not robust (if coordinator is down, no updates can be performed) => Voting: updates (and reads) can be performed if some specified # of servers agree.Voting Protocol:

A version # (incremented at write) is associated with each fileTo perform a read, a client has to assemble a read quorum of Nr servers; similarly, a write quorum of Nw servers for a writeIf Nr + Nw > N, then any read quorum will contain at least one most recently updated file versionFor reading, client contacts Nr active servers and chooses the file with largest version #For writing, client contacts Nw active servers asking them to write. Succeeds if they all say yes.


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Replication-Update protocols:

Nr is usually small (reads are frequent), but Nw is usually close to N (want to make sure all replicas are updated). Problem with achieving a write quorum in the presence of server failuresVoting with ghosts: allows to establish a write quorum when several servers are down by temporarily creating dummy (ghost) servers (at least one must be real)Ghost servers are not permitted in a read quorum (they don’t have any files)When server comes back it must restore its copy first by obtaining a read quorum


III.Network file system (NFS)

Three aspects of NFS:The architecture The protocolThe implementation



NFS Architecture

Basic idea NFS: An arbitrary collection of clients and servers.Server export one or more directory for access by remote client.List of director is maintained /etc/exports/


NFS Architecture

Clients access exported directories by mounting them. Clients diskless can mount on remote root directory and else.To programs running on clients is no difference between a file located.So, the basic architectural characteristic NFS is server exported directory and clients mount them remotely.


NFS Protocol

The goal of NFS is to support heterogeneous system.To accomblishing that must to define two client-server protocol.The first NFS protocol handle mounting.The second NFS protocol is for directory and file access.


NFS Protocol: Mounting

Clients send the path name to a server and request to mount. If legal, server return handle file to client else.Handle file contains all information of file and directory.Many clients contain /etc/rc to not manual intervention.


NFS Protocol: Automounting

Allows a set remote directories to be associated with the local directory. First time client sent a message to each of server and first one to reply wins. Advantages:

-If server down, it is possible to bring client up.-allowing client to try to a set of servers in parallel.

Other, automounting most often used for read-only file and rarely change.


NFS Protocol: Accessing

Clients send the message to server to manipulate and read and write file.Most of UNIX system calls supported NFS exception OPEN and CLOSE.To READ, clients send message to server and receive file handle.To WRITE, clients only need a file handle, offset and the number of file desired.


NFS Protocol: Accessing

AdvantagesServers don’t remember any information between calls to open connectionStateless, not efficient when server crashes and recoversIn contrast, statefull


NFS Protocol: Security

Problem: in stateless, locks can’t associated with open fileNFS uses UNIX protection mechanism with “rwx” bit Other, use public key cryptographyInformation about all of keys are maintained by NIS (Network Information Services)NIS’s function is to store (key, value) and mapping between user name to password, machine name to network address


NFS Inplementation


NFS Inplementation

System call layerThis handle calls like OPEN, READ and CLOSE.

Virtual file system layer (VFS)Maintain a table with one entry for each open fileEntry is v-node (virtual, i-node)


NFS Inplementation: Usage v-node

Mount The system administrator Call mount programMake a MOUNT system callKernel asked NFS client to create r-node (remote, i-node) in internal table to hold the file handleV-node point to r-node



OPENKernel base on some point during parsing the name.Kernel asked NFS client code to OPEN file NFS client lookup in remain table and report back to VFS layerPut in its table a v-node that point to r-node



READThe caller is given a file descriptor for the remote fileVFS locates the corresponding v-node

Transfers between client and serverMake in large chunks, normally 8192 bytescaching


IV.Trends In Distributed File Systems

Some Problem make changes in File System :New Hardware ScalabilityWAN Mobile UsersFault ToleranceMulimedia


New Hardware

Well Designed Hardware can help solve problem :


Scalability

Distributed file systems is toward lager . Old algorithm may not work and may cause bottle neck problemA general way to solve this problem is partition the systems into smaller units which are relatively independent


WAN

Most current work now on distributed systems focuses on LAN-based systems but it will be interconnected to form transparent distributed systems covering countries and continent . So what kind of file system would be need to serve all the world ?A larger system lead to a large variety encounter for example what format one should use for files containing floating-pint numbers .


Mobile Users

Laptop ,pocket pc , smart phone can be found every where these days and they are multiplying like rabbits . However the connection may not good at all .And solution is based on caching.Remote control


Fault Tolerance

If the a system goes down for an hour there are many serious problem so the demand for systems that essentially never fail will grow.File replication become an essential requirement .


Multimedia

Real time conference , video on demand or multimedia will need completely different file system .

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