Distributed File Systems Chapter 10

NFS Architecture (1) a)The remote access model. b)The upload/download model

NFS Architecture (2) The basic NFS architecture for UNIX systems.

File System Model An incomplete list of file system operations supported by NFS. Operationv3v4Description CreateYesNoCreate a regular file CreateNoYesCreate a nonregular file LinkYes Create a hard link to a file SymlinkYesNoCreate a symbolic link to a file MkdirYesNoCreate a subdirectory in a given directory MknodYesNoCreate a special file RenameYes Change the name of a file RmdirYesNoRemove an empty subdirectory from a directory OpenNoYesOpen a file CloseNoYesClose a file LookupYes Look up a file by means of a file name ReaddirYes Read the entries in a directory ReadlinkYes Read the path name stored in a symbolic link GetattrYes Read the attribute values for a file SetattrYes Set one or more attribute values for a file ReadYes Read the data contained in a file WriteYes Write data to a file

Communication a)Reading data from a file in NFS version 3. b)Reading data using a compound procedure in version 4.

Naming (1) Mounting (part of) a remote file system in NFS.

Naming (2) Mounting nested directories from multiple servers in NFS.

Automounting (1) A simple automounter for NFS.

Automounting (2) Using symbolic links with automounting.

File Attributes (1) Some general mandatory file attributes in NFS. AttributeDescription TYPEThe type of the file (regular, directory, symbolic link) SIZEThe length of the file in bytes CHANGE Indicator for a client to see if and/or when the file has changed FSIDServer-unique identifier of the file's file system

File Attributes (2) Some general recommended file attributes. AttributeDescription ACLan access control list associated with the file FILEHANDLEThe server-provided file handle of this file FILEIDA file-system unique identifier for this file FS_LOCATIONSLocations in the network where this file system may be found OWNERThe character-string name of the file's owner TIME_ACCESSTime when the file data were last accessed TIME_MODIFYTime when the file data were last modified TIME_CREATETime when the file was created

Semantics of File Sharing (1) a)On a single processor, when a read follows a write, the value returned by the read is the value just written. b)In a distributed system with caching, obsolete values may be returned.

Semantics of File Sharing (2) Four ways of dealing with the shared files in a distributed system. MethodComment UNIX semanticsEvery operation on a file is instantly visible to all processes Session semanticsNo changes are visible to other processes until the file is closed Immutable filesNo updates are possible; simplifies sharing and replication TransactionAll changes occur atomically

File Locking in NFS (1) NFS version 4 operations related to file locking. OperationDescription LockCreates a lock for a range of bytes LocktTest whether a conflicting lock has been granted LockuRemove a lock from a range of bytes RenewRenew the leas on a specified lock

File Locking in NFS (2) The result of an open operation with share reservations in NFS. a)When the client requests shared access given the current denial state. b)When the client requests a denial state given the current file access state. Current file denial state NONEREADWRITEBOTH READSucceedFailSucceed WRITESucceed FailSucceed BOTHSucceed Fail (a) Requested file denial state NONEREADWRITEBOTH READSucceedFailSucceed WRITESucceed FailSucceed BOTHSucceed Fail (b) Request access Current access state

Client Caching (1) Client-side caching in NFS.

Client Caching (2) Using the NFS version 4 callback mechanism to recall file delegation.

RPC Failures Three situations for handling retransmissions. a)The request is still in progress b)The reply has just been returned c)The reply has been some time ago, but was lost.

Security The NFS security architecture.

Secure RPCs Secure RPC in NFS version 4.

Access Control The classification of operations recognized by NFS with respect to access control. OperationDescription Read_dataPermission to read the data contained in a file Write_dataPermission to to modify a file's data Append_dataPermission to to append data to a file ExecutePermission to to execute a file List_directoryPermission to to list the contents of a directory Add_filePermission to to add a new file t5o a directory Add_subdirectoryPermission to to create a subdirectory to a directory DeletePermission to to delete a file Delete_childPermission to to delete a file or directory within a directory Read_aclPermission to to read the ACL Write_aclPermission to to write the ACL Read_attributesThe ability to read the other basic attributes of a file Write_attributesPermission to to change the other basic attributes of a file Read_named_attrsPermission to to read the named attributes of a file Write_named_attrsPermission to to write the named attributes of a file Write_ownerPermission to to change the owner SynchronizePermission to to access a file locally at the server with synchronous reads and writes

The Coda File System The various kinds of users and processes distinguished by NFS with respect to access control. Type of userDescription OwnerThe owner of a file GroupThe group of users associated with a file EveryoneAny user of a process InteractiveAny process accessing the file from an interactive terminal NetworkAny process accessing the file via the network Dialup Any process accessing the file through a dialup connection to the server BatchAny process accessing the file as part of a batch job AnonymousAnyone accessing the file without authentication AuthenticatedAny authenticated user of a process ServiceAny system-defined service process

Overview of Coda (1) The overall organization of AFS.

Overview of Coda (2) The internal organization of a Virtue workstation.

Communication (1) Side effects in Coda's RPC2 system.

Communication (2) a)Sending an invalidation message one at a time. b)Sending invalidation messages in parallel.

Naming Clients in Coda have access to a single shared name space.

File Identifiers The implementation and resolution of a Coda file identifier.

Sharing Files in Coda The transactional behavior in sharing files in Coda.

Transactional Semantics The metadata read and modified for a store session type in Coda. File-associated dataRead?Modified? File identifierYesNo Access rightsYesNo Last modification timeYes File lengthYes File contentsYes

Client Caching The use of local copies when opening a session in Coda.

Server Replication Two clients with different AVSG for the same replicated file.

Disconnected Operation The state-transition diagram of a Coda client with respect to a volume.

Secure Channels (1) Mutual authentication in RPC2.

Secure Channels (2) Setting up a secure channel between a (Venus) client and a Vice server in Coda.

Access Control Classification of file and directory operations recognized by Coda with respect to access control. OperationDescription ReadRead any file in the directory WriteModify any file in the directory LookupLook up the status of any file InsertAdd a new file to the directory DeleteDelete an existing file AdministerModify the ACL of the directory

Plan 9: Resources Unified to Files General organization of Plan 9

Communication Files associated with a single TCP connection in Plan 9. FileDescription ctlUsed to write protocol-specific control commands dataUsed to read and write data listenUsed to accept incoming connection setup requests localProvides information on the caller's side of the connection remoteProvides information on the other side of the connection statusProvides diagnostic information on the current status of the connection

Processes The Plan 9 file server.

Naming A union directory in Plan 9.

Overview of xFS. A typical distribution of xFS processes across multiple machines.

Processes (1) The principle of log-based striping in xFS.

Processes (2) Reading a block of data in xFS.

Naming Main data structures used in xFS. Data structureDescription Manager mapMaps file ID to manager ImapMaps file ID to log address of file's inode InodeMaps block number (i.e., offset) to log address of block File identifierReference used to index into manager map File directoryMaps a file name to a file identifier Log addressesTriplet of stripe group, ID, segment ID, and segment offset Stripe group mapMaps stripe group ID to list of storage servers

Overview of SFS The organization of SFS.

Naming A self-certifying pathname in SFS. /sfsLOCHIDPathname /sfs/sfs.vu.sc.nl:ag62hty4wior450hdh63u623i4f0kqere/home/steen/mbox

Summary A comparison between NFS, Coda, Plan 9, xFS. N/S indicates that nothing has been specified. IssueNFSCodaPlan 9xFSSFS Design goalsAccess transparencyHigh availabilityUniformityServerless systemScalable security Access modelRemoteUp/DownloadRemoteLog-basedRemote CommunicationRPC SpecialActive msgsRPC Client processThin/FatFatThinFatMedium Server groupsNoYesNoYesNo Mount granularityDirectoryFile system Directory Name spacePer clientGlobalPer processGlobal File ID scopeFile serverGlobalServerGlobalFile system Sharing sem.SessionTransactionalUNIX N/S Cache consist.write-back write-throughwrite-back ReplicationMinimalROWANoneStripingNone Fault toleranceReliable comm. Replication and caching Reliable comm.StripingReliable comm. RecoveryClient-basedReintegrationN/S Checkpoint & write logs N/S Secure channels Existing mechanisms Needham-Schroeder No pathnamesSelf-cert. Access controlMany operationsDirectory operationsUNIX based NFS BASED