Data Storage Network Technologies -WILP MTech. sem 3 BITS pilani

1. BITS Pilani
Pilani Campus
Data Storage Technologies
& Networks
Dr. Virendra Singh Shekhawat
Department of Computer Science and Information Systems

2. BITS Pilani, Pilani Campus
Topics
• Network File System protocol (NFS)
– Design goals
– Interface
– NFS Transport
– Operation
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3. BITS Pilani, Pilani Campus
What is NFS?
• A remote file system protocol
– Designed and implemented by Sun Microsystems
– Provides Unix-like file system interface and
semantics
• It is implemented as a Client-Server application
– Client part imports file systems (from other
computers)
– Server part exports (local) file systems
• i.e. makes it visible on the network and enables access
from other computers
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4. BITS Pilani, Pilani Campus
NFS Design Goals
• Stateless protocol
– Increased robustness – easy recovery from failures
• Support for Unix filesystem semantics
– Also supports other semantics such as MS-DOS
• Protection and Access Control same as Unix
filesystem semantics
• Transport-independent protocol
– Originally implemented on top of UDP
– Easily moved to TCP later
– Has been implemented over many non-IP-based
protocols.
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5. BITS Pilani, Pilani Campus
NFS Design Limitations
• Design for clients and servers connected on a
locally fast network.
– Does not work well over slow links nor between
clients and servers with intervening gateways
• Stateless protocol
– Session state is not maintained
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6. BITS Pilani, Pilani Campus
NFS- Transport[1]
• Uses a Request – Response model
– Server receives RPC – Remote Procedure Call –
requests from clients
– Can be run on top of stream (e.g. TCP) or datagram
protocol (e.g. UDP)
• Each RPC message may need to be broken into
multiple packets to be sent across the network.
– May typically require up to 6 packets
– May cause problems in UDP – in case of failure,
entire message must be retransmitted
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7. BITS Pilani, Pilani Campus
NFS – Transport[2]
• Remote Procedure Call (RPC)
– Client sees a procedure call interface (akin to a local
procedure call)
– But the call (along with the parameters) is marshalled
into a message and sent over the network
• Marshalling may involve serializing
• Server unmarshalls the message (i.e. separates it
into pieces) and processes it as a local operation.
• The result is similarly marshalled by the server and
sent to the client which in turn unmarshalls it.
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8. BITS Pilani, Pilani Campus
NFS- Interface
• NFS RPC requests
– Idempotent Operations(i.e. NFS client may send the
same request one or more times without any harmful
side effects)
• GETATTR, SETATTR, LOOKUP, READLINK, READ, WRITE, CREATE,
SYMLINK, READDIR, STATFS
– Non-Idempotent Operations
• REMOVE, RENAME, LINK, MKDIR, RMDIR
• Idempotency is significant because of
– slow links and lost RPC acks.
• Each file on the server is identified by a globally
unique file handle
– created when a lookup is sent by client to server
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9. BITS Pilani, Pilani Campus
NFS- Operation[1]
• NFS protocol is stateless
– Each request is independent
• Server need not maintain information about clients and their
requests
• In practice, server caches recently accessed file
data.
– It improves performance
– and is useful for detecting retrials of previously serviced
requests
• Transaction-Commit semantics implies high
overhead:
– Synchronous writes are required for write operations.
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10. BITS Pilani, Pilani Campus
NFS- Operation[2]
• NFS is a client-server protocol
– No discovery or registry is used.
• i.e. client must know the server and the filesystem
– Server’s filesystem has to be exported (by
mounting).
• Refer to mount system call on Unix
• Each server has a globally unique id (guid)
• Client application need not distinguish
between local and remote files
– i.e. once a filesystem is mounted applications use
the same interface for both types of files
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11. BITS Pilani, Pilani Campus
NFS - Operation[3]
• NFS is a client-server protocol
– File manager part of client file system remains the same
as that for the local file system
• Only the file store is local or remote
• Each remote file that is active manifests as an
nfsnode in the client’s filesystem
– Recall the active file data structures:
• Per-Process File Descriptor
– Kernel File Entry
– vnode
– file-store-specific data structure
» inode for local file store and
» nfsnode for remote filestore
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12. BITS Pilani, Pilani Campus
NFS - Operation
• Basic Protocol (Mounting): Client C, Server S
– C (mount process) -----> S(portmap daemon)
• request port address of mountd
– S (portmap daemon) ----> C
• return the port address of its mountd
– C (mount process) ----> S(mountd daemon)
• request mounting of filesystem [pathname]
– S (mountd daemon):
• get file handle for desired mount point from kernel
– S (mountd daemon) ---> C
• return filehandle of mountpoint OR error
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13. BITS Pilani, Pilani Campus
NFS - Operation
• Basic Protocol (I/O Operation): Client C, Server S
– C (app. process): write system call
• Data is copied into kernel buffer and call returns
• nfsiod daemon wakes up
– C (nfsiod daemon) -----> S(nfs_rcv)
• Request write [buffer contents]
– S delegates request to nfsd daemon
• nfsd daemon invokes write on disk and waits for I/O
– S (nfsd daemon) ---> C (nfsiod daemon)
• return ack for the write operation
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14. BITS Pilani, Pilani Campus
NFSv3 vs NFSv4
• NFSv3
– One RPC per RTT
– Port mapper service required to determine which network
port to listen or connect
– No caching
• NFSv4 features
– Access control lists (similar to Windows ACLs)
– Compound RPCs
– Client side caching
– Operation over TCP port 2049
– Mandating Strong security (Kerberos v5 for cryptographic
services)
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15. BITS Pilani, Pilani Campus
Thank You!
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