Distributed File Systems:RPC, NFS, and AFS

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Announements

• Homework 6 available later tonight– Due next Tuesday, December 2nd

• See me after class to pick up prelim

• Upcoming Agenda– No class on Thursday—Happy Thanksgiving!– Next week last week of classes—December 2nd and 4th

– Final—Thursday, December 18th at 2pm• Room 131 Warren Hall

• Length is 2hrs

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Goals for Today

• Distributed file systems (DFS)

• Network file system (NFS)– Remote Procedure Calls (RPC)

• Andrew file system (AFS)

Distributed File Systems (DFS)

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Distributed File Systems

• Goal: view a distributed system as a file system– Storage is distributed– Web tries to make world a collection of hyperlinked documents

• Issues not common to usual file systems– Naming transparency– Load balancing– Scalability– Location and network transparency– Fault tolerance

• We will look at some of these today

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Transfer Model

• Upload/download Model:– Client downloads file, works on it, and writes it back on server– Simple and good performance

• Remote Access Model: – File only on server; client sends commands to get work done

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Naming transparency

• Naming is a mapping from logical to physical objects• Ideally client interface should be transparent

– Not distinguish between remote and local files– /machine/path or mounting remote FS in local hierarchy are not

transparent

• A transparent DFS hides the location of files in system• 2 forms of transparency:

– Location transparency: path gives no hint of file location• /server1/dir1/dir2/x tells x is on server1, but not where server1 is

– Location independence: move files without changing names• Separate naming hierarchy from storage devices hierarchy

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File Sharing Semantics• Sequential consistency: reads see previous writes

– Ordering on all system calls seen by all processors– Maintained in single processor systems– Can be achieved in DFS with one file server and no caching

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Caching

• Keep repeatedly accessed blocks in cache– Improves performance of further accesses

• How it works:– If needed block not in cache, it is fetched and cached– Accesses performed on local copy– One master file copy on server, other copies distributed in DFS– Cache consistency problem: how to keep cached copy

consistent with master file copy

• Where to cache?– Disk: Pros: more reliable, data present locally on recovery– Memory: Pros: diskless workstations, quicker data access, – Servers maintain cache in memory

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File Sharing Semantics

• Other approaches:– Write through caches:

• immediately propagate changes in cache files to server

• Reliable but poor performance

– Delayed write:• Writes are not propagated immediately, probably on file close

• Session semantics (AFS): write file back on close

• Alternative (NFS): scan cache periodically and flush modified blocks

• Better performance but poor reliability

– File Locking:• The upload/download model locks a downloaded file

• Other processes wait for file lock to be released

Network File System (NFS)

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Network File System (NFS)

• Developed by Sun Microsystems in 1984– Used to join FSes on multiple computers as one logical whole

• Used commonly today with UNIX systems• Assumptions

– Allows arbitrary collection of users to share a file system– Clients and servers might be on different LANs– Machines can be clients and servers at the same time

• Architecture:– A server exports one or more of its directories to remote clients– Clients access exported directories by mounting them

• The contents are then accessed as if they were local

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Example

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NFS Mount Protocol

• Client sends path name to server with request to mount– Not required to specify where to mount

• If path is legal and exported, server returns file handle– Contains FS type, disk, i-node number of directory, security info– Subsequent accesses from client use file handle

• Mount can be either at boot or automount– Using automount, directories are not mounted during boot– OS sends a message to servers on first remote file access– Automount is helpful since remote dir might not be used at all

• Mount only affects the client view!

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NFS Protocol

• Supports directory and file access via remote procedure calls (RPCs)

• All UNIX system calls supported other than open & close• Open and close are intentionally not supported

– For a read, client sends lookup message to server– Server looks up file and returns handle– Unlike open, lookup does not copy info in internal system tables– Subsequently, read contains file handle, offset and num bytes– Each message is self-contained

• Pros: server is stateless, i.e. no state about open files• Cons: Locking is difficult, no concurrency control

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NFS Implementation

• Three main layers:• System call layer:

– Handles calls like open, read and close

• Virtual File System Layer: – Maintains table with one entry (v-node) for each open file– v-nodes indicate if file is local or remote

• If remote it has enough info to access them

• For local files, FS and i-node are recorded

• NFS Service Layer: – This lowest layer implements the NFS protocol

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NFS Layer Structure

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How NFS works?

• Mount:– Sys ad calls mount program with remote dir, local dir– Mount program parses for name of NFS server– Contacts server asking for file handle for remote dir– If directory exists for remote mounting, server returns handle– Client kernel constructs v-node for remote dir – Asks NFS client code to construct r-node for file handle

• Open:– Kernel realizes that file is on remotely mounted directory– Finds r-node in v-node for the directory– NFS client code then opens file, enters r-node for file in VFS,

and returns file descriptor for remote node

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Cache coherency

• Clients cache file attributes and data– If two clients cache the same data, cache coherency is lost

• Solutions:– Each cache block has a timer (3 sec for data, 30 sec for dir)

• Entry is discarded when timer expires

– On open of cached file, its last modify time on server is checked• If cached copy is old, it is discarded

– Every 30 sec, cache time expires• All dirty blocks are written back to the server

Remote Procedure Call (RPC)

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Procedure Call• More natural way is to communicate using procedure calls:

– every language supports it– semantics are well defined and understood– natural for programmers to use

• Basic idea: define server as a module that exports a set of procedures callable by client programs.

• To use the server, the client just does a procedure call, as if it were linked with the server

call

return

Client Server

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(Remote) Procedure Call

• So, we would like to use procedure call as a model for distributed communication.

• Lots of issues:– how do we make this invisible to the programmer?– what are the semantics of parameter passing?– how is binding done (locating the server)?– how do we support heterogeneity (OS, arch., language)– etc.

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Remote Procedure Call

• The basic model for Remote Procedure Call (RPC) was described by Birrell and Nelson in 1980, based on work done at Xerox PARC.

• Goal to make RPC as much like local PC as possible.• Used computer/language support.• There are 3 components on each side:

– a user program (client or server)– a set of stub procedures– RPC runtime support

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RPC• Basic process for building a server:

– Server program defines the server’s interface using an interface definition language (IDL)

– The IDL specifies the names, parameters, and types for all client-callable server procedures

– A stub compiler reads the IDL and produces two stub procedures for each server procedure: a client-side stub and a server-side stub

– The server writer writes the server and links it with the server-side stubs; the client writes her program and links it with the client-side stubs.

– The stubs are responsible for managing all details of the remote communication between client and server.

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RPC Stubs

• Client-side stub is a procedure that looks to the client as if it were a callable server procedure.

• Server-side stub looks like a calling client to the server

• The client program thinks it is calling the server; – in fact, it’s calling the client stub.

• The server program thinks it’s called by the client; – in fact, it’s called by the server stub.

• The stubs send messages to each other to make RPC happen.

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RPC Call Structure

call foo(x,y)

proc foo(a,b) call foo(x,y)

proc foo(a,b)

begin foo...

end foo

clientprogram

clientstub

RPCruntime

RPCruntime

serverstub

serverprogram

Call

client makeslocal call tostub proc.

stub builds msgpacket, insertsparams

runtime sendsmsg to remotenode

server iscalled byits stub

stub unpacksparams andmakes call

runtimereceives msgand calls stub

call foo

send msg

call foo

msg received

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RPC Return Structure

call foo(x,y)

proc foo(a,b) call foo(x,y)

proc foo(a,b)

begin foo...

end foo

clientprogram

clientstub

RPCruntime

RPCruntime

serverstub

serverprogram

return

client continues

stub unpacksmsg, returns to caller

runtimereceives msg, calls stub

server procreturns

stub buildsresult msgwith outputargs

runtime respondsto originalmsg

return

msg received

return

send msg

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RPC Information Flow

Client(caller)

Server(callee)

RPCRuntime

RPCRuntime

call

return

send

receive

send

receive

return

call

Netw

ork

Netw

ork

ClientStub

marshalargs

marshalreturn values

unmarshalret vals

ServerStub

unmarshalargs

Machine A

Machine B

mbox1

mbox2

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RPC Binding

• Binding is the process of connecting the client and server• The server, when it starts up, exports its interface,

– identifying itself to a network name server and – telling the local runtime its dispatcher address.

• The client, before issuing any calls, imports the server,– which causes the RPC runtime to lookup the server through the

name service and – contact the requested server to setup a connection.

• The import and export are explicit calls in the code.

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RPC Marshalling

• Marshalling is packing of procedure params into message packet.

• RPC stubs call type-specific procedures to marshall (or unmarshall) all of the parameters to the call.

• On client side, client stub marshalls parameters into call packet;

– On the server side the server stub unmarshalls the parameters to call the server’s procedure.

• On return, server stub marshalls return parameters into return packet;

– Client stub unmarshalls return params and returns to the client.

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Problems with RPC• Non-Atomic failures

– Different failure modes in distributed system than on a single machine– Consider many different types of failures

• User-level bug causes address space to crash• Machine failure, kernel bug causes all processes on same machine to fail• Some machine is compromised by malicious party

– Before RPC: whole system would crash/die– After RPC: One machine crashes/compromised while others keep

working– Can easily result in inconsistent view of the world

• Did my cached data get written back or not?• Did server do what I requested or not?

– Answer? Distributed transactions/Byzantine Commit• Performance

– Cost of Procedure call « same-machine RPC « network RPC– Means programmers must be aware that RPC is not free

• Caching can help, but may make failure handling complex

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Cross-Domain Comm./Location Transparency

• How do address spaces communicate with one another?– Shared Memory with Semaphores, monitors, etc…– File System– Pipes (1-way communication)– “Remote” procedure call (2-way communication)

• RPC’s can be used to communicate between address spaces on different machines or the same machine– Services can be run wherever it’s most appropriate– Access to local and remote services looks the same

• Examples of modern RPC systems:– CORBA (Common Object Request Broker Architecture)– DCOM (Distributed COM)– RMI (Java Remote Method Invocation)

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Microkernel operating systems• Example: split kernel into application-level servers.

– File system looks remote, even though on same machine

• Why split the OS into separate domains?– Fault isolation: bugs are more isolated (build a firewall)– Enforces modularity: allows incremental upgrades of pieces of software

(client or server)– Location transparent: service can be local or remote

• For example in the X windowing system: Each X client can be on a separate machine from X server; Neither has to run on the machine with the frame buffer.

App App

file systemWindowingNetworkingVM

Threads

App

Monolithic Structure

App Filesys windows

RPC addressspaces

threadsMicrokernel Structure

Andrew File System (AFS)

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Andrew File System (AFS)

• Named after Andrew Carnegie and Andrew Mellon– Transarc Corp. and then IBM took development of AFS– In 2000 IBM made OpenAFS available as open source

• Features:– Uniform name space– Location independent file sharing– Client side caching with cache consistency– Secure authentication via Kerberos– Server-side caching in form of replicas– High availability through automatic switchover of replicas– Scalability to span 5000 workstations

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AFS Overview

• Based on the upload/download model– Clients download and cache files– Server keeps track of clients that cache the file– Clients upload files at end of session

• Whole file caching is central idea behind AFS– Later amended to block operations– Simple, effective

• AFS servers are stateful– Keep track of clients that have cached files– Recall files that have been modified

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AFS Details

• Has dedicated server machines• Clients have partitioned name space:

– Local name space and shared name space– Cluster of dedicated servers (Vice) present shared name space– Clients run Virtue protocol to communicate with Vice

• Clients and servers are grouped into clusters– Clusters connected through the WAN

• Other issues:– Scalability, client mobility, security, protection, heterogeneity

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AFS: Shared Name Space

• AFS’s storage is arranged in volumes– Usually associated with files of a particular client

• AFS dir entry maps vice files/dirs to a 96-bit fid– Volume number– Vnode number: index into i-node array of a volume– Uniquifier: allows reuse of vnode numbers

• Fids are location transparent– File movements do not invalidate fids

• Location information kept in volume-location database– Volumes migrated to balance available disk space, utilization– Volume movement is atomic; operation aborted on server crash

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AFS: Operations and Consistency

• AFS caches entire files from servers– Client interacts with servers only during open and close

• OS on client intercepts calls, and passes it to Venus– Venus is a client process that caches files from servers– Venus contacts Vice only on open and close

• Does not contact if file is already in the cache, and not invalidated

– Reads and writes bypass Venus

• Works due to callback:– Server updates state to record caching– Server notifies client before allowing another client to modify– Clients lose their callback when someone writes the file

• Venus caches dirs and symbolic links for path translation

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AFS Implementation

• Client cache is a local directory on UNIX FS– Venus and server processes access file directly by UNIX i-node

• Venus has 2 caches, one for status & one for data– Uses LRU to keep them bounded in size

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Summary

• RPC– Call procedure on remote machine– Provides same interface as procedure– Automatic packing and unpacking of arguments without user

programming (in stub)

• NFS: – Simple distributed file system protocol. No open/close– Stateless server

• Has problems with cache consistency, locking protocol

• AFS:– More complicated distributed file system protocol– Stateful server

• session semantics: consistency on close

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Prelim II• Prelims graded

– Mean 73 (Median 76), Stddev 13.3, High 98 out of 100!– Good job!

• Re-grade policy– Submit written re-grade request to Nazrul.

• Entire prelim will be re-graded.

• We were generous the first time…

– If still unhappy, submit another re-grade request. • Nazrul will re-grade herself

– If still unhappy, submit a third re-grade request. • I will re-grade. Final grade is law.

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Grade distribution

Question #3

• Hardlinks– Need a count in inode/fileheader– Remove decrements count and removes file if count is 0– New syscall: Link(char *src, char *dst)

• Softlinks– Need to file type indicated in inode/fileheader– Also, need path to target file– Open needs to change to perform recursive lookup– New syscall: SymLink(char *src, char *dst)

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Question #4

• Concurrent writers– RAID 0, 1+0, 5, 6– Not 1 or 4 because cannot perform independent writes

• Concurrent readers (but not concurrent writers)– RAID 1 and 4

• 2*k-1 disks and want concurrent readers– Unavailable: RAID 1 — requires 2*k disks– Undesirable: RAID 4, 5, and 6 — requires complex controllers

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Happy Thanksgiving!!!