GLOBAL EDGE SOFTWERE LTD1 R EMOTE F ILE S HARING - Ardhanareesh Aradhyamath

GLOBAL EDGE SOFTWERE LTD2 The arc of the presentation  Introduction distributed file system  Stateful and stateless operation  RFS architecture  RFS Implementation  Client-side caching  Cache consistency  Conclusion

GLOBAL EDGE SOFTWERE LTD3 Introduction Distributed file system A Distributed File System is simply a classical model of a file system 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.

GLOBAL EDGE SOFTWERE LTD4 Properties of Distributed File Systems Network transparency. Location transparency. Location independence. User mobility. Fault tolerance. Scalability. File Mobility

GLOBAL EDGE SOFTWERE LTD5 Design Considerations Different Distributed File Systems can be compared according to how they deal with these issues: Name Space. Stateful or stateless operation. Semantics of sharing. Remote access methods.

GLOBAL EDGE SOFTWERE LTD6 Remote File Sharing (RFS)‏ Introduced by AT&T. RFS uses the client-server model. The design goal is to provide transparent access to remote files by preserving UNIX semantics. RFS can be used both LANs and WANs.

GLOBAL EDGE SOFTWERE LTD7 RFS Architecture RFS based on client server model. Server advertises the directories, and the client mount them. RFS is completely stateful architecture, which necessary to correctly provide Unix open file semantics. RFS uses a reliable, virtual circuit transport service such as TCP/IP. A virtual circuit is established between a client and server during the first mount operation.

GLOBAL EDGE SOFTWERE LTD8 Client application System call interface RFS client code Local disk RFS server code stream Virtual circuit ClientServer Communication in RFS

GLOBAL EDGE SOFTWERE LTD9 Remote Message Protocol RFS uses a remote system call model. The server creates the clients environment executes the system call. The result is send back to the clients system call and client interprets the result. This implementation was called the RFS1.0 protocol. SVR4 introduces a new protocol RFS2.0. It provides a set of requests that mapped vnode and vfs operation, and provides integration with vnode/vfs interface.

GLOBAL EDGE SOFTWERE LTD10 Stateful Operation Server maintains state about the clients. The server records which file has been opened by the client, and increments the reference counts. Server keep track of file/record locks. Server also maintains a table of all clients that have mounted its file system. The table stores the client Internet address, the mounted file system and parameters of the virtual circuit connection.

GLOBAL EDGE SOFTWERE LTD11 Remote Mount An RFS server can advertise a directory using the advfs system call. The arguments to advfs are path name of exported directory, the resource name associated with the directory and a list of client machines authorized to the resource.

GLOBAL EDGE SOFTWERE LTD12 vnode ufsnode v_data v_op Resource list head Ptr to auth clients Root vnode ptr Resource name Ptr to mnt list Mount list for resource List of authorized clients Resource entry RFS resource list

GLOBAL EDGE SOFTWERE LTD Server registers name with name server Client gets server location from name server Client sends mount request to server Name server RFS server RFS Client Mounting an RFS file system

GLOBAL EDGE SOFTWERE LTD14 Continued … The server invokes the adv(1) command to register its its advertised resource with the name server. Some times later, a client mounts an RFS resource by a command mount -d /mnt Mount system call sends mount request to server, passing the symbolic resource name. The response contains mount ID.

GLOBAL EDGE SOFTWERE LTD15 Continued … When client receive mount response it sets up the vfs entry, vnode for root directory of resource. V_data field of an RFS vnode points to a data structure called a send descriptor, which contains information about the virtual circuit as well as a file handle. The mount operation initiates a connection to a deamon process on the server. Once the connection established the client and server negotiate the run time parameters.

GLOBAL EDGE SOFTWERE LTD16 RFS Clients and Servers The client can access an RFS file either through its path name or through file descriptor. If mount point comes while traversing. Server returns handle to vnode which client stores in private data of vnode. The vnode operations invokes RFS client functions. Then RFS server runs daemon process, which listen for the incoming requests and services each request before attempting next. The RFS daemon may sleep if they need to wait for resources and scheduled just like normal processes.

GLOBAL EDGE SOFTWERE LTD17 Client Server RFS send descriptor Local vnode of file vnode of exported directory V_node... V_node... V_node... vnode Vfs_data... Struct vfs Mount_id... Struct rf_vfs Virtual circuit file handler Resource list RFS data structures

GLOBAL EDGE SOFTWERE LTD18 Crash Recovery Client and server need to detect and handle the failures i.e. when either machine crashes or virtual path between them breaks. Client crashes: server must undo all states related to that client i.e. 1. Decrements the inode reference count. 2. releases any reader/writer locks on named pipes in use by the client. 3. releases any file/record locks held by the client.

GLOBAL EDGE SOFTWERE LTD19 Continued... Server crash: All client processes waiting for responses from the server are woken up. System calls return an ENOLINK error message. All RFS inode referring to files on that server are flagged such that subsequent operations on them return an error.

GLOBAL EDGE SOFTWERE LTD20 Other Issues Process pool: Whenever a client request arrives and all the server processes are busy, it creates a new process. This pool has upper bound, and the last process is not allowed to sleep. Remote signal: Unix allows a signal to abort a system call, and this facility must be extended to RFS environments. Data transfer: For a local read or write system call, the copyin() and copyout() routines move data.

GLOBAL EDGE SOFTWERE LTD21 Client-side Caching Client cache is activated at the mount time. The cache is strictly write-through. Clients send all writes to server immediately, after updating the local cached copy. A read system call returns cached data if and if all the data requested is available in the cache. If data not in the local cache then the client fetches all the data from server. Buffers are reused in least recently used order.

GLOBAL EDGE SOFTWERE LTD22 Cache Consistence Any modification to a file, either by a user on the server, or by one of the clients, invalidates the cached copies on all other clients. Consistence protocols are only required for files that are shared by multiple files. When server receive the first write request for a file open on more than one client.

GLOBAL EDGE SOFTWERE LTD23 CLIENT C1 SERVERCLIENT C2 Write to file1 Sends message to server Has opened file1 time Returns from write call Block write, sends invalidate message to C2 Complete write, Responds to C1 Has opened file1 Invalidates cache entries for files1 Replay to server RFS cache consistency algorithm

GLOBAL EDGE SOFTWERE LTD24 Continued.. Another case there may be some clients who closed the file, if opened again. Solution for this is version number with each file which is incremented each time the file is modified. Server return the version number in the response to each open request. Its will be compared with old one, if mismatch then flush all data blocks of that file.

Conclusion  RFS provides Unix semantics and also provides device sharing.  As it uses stateful operation the network traffic is reduced.  The strong cache consistency model guarantee that a read always returns data that is identical to server image of the file at the time of access.  So Reliability and efficiency is more.

GLOBAL EDGE SOFTWERE LTD26 Thank you.