Introduction to a ``Network File System'' (NFS)
What was life like before NFS?
Introduction to the Network File System (NFS)
NFS is built on top of:
- UDP - User Datagram Protocol (unreliable delivery)
- XDR - eXternal Data Representation (machine independent data format)
- RPC - Remote Procedure Call
NFS overview
Two protocols, mount and nfs: ``mount'' protocol establishes initial link between client and server machines
NFS overview
NFS protocols provide a set of RPCs for remote file operations
- Searching a directory
- Reading a set of directory entries
- Manipulating links and directories
- Accessing file attributes
- Read and writing files
- Notably missing are open() and close()
NFS overview
There was no equivalent to traditional UNIX file table on the server side -- NFS was designed to be stateless on the server side, and each request for data included a full set of arguments rather than assuming persistent state. However, this stateless wasn't all that practical and we soon ended up with caching and file handles.
Each request must provide full set of arguments including a unique file identifier and offset
NFS problems
Performance
- Modified data may be cached locally on the client
- Once the cache flushes to the server, the data must be written to disk before results are returned to the client and the cache is flushed
- NFS is close to UNIX semantics, but...
- there are two client caches: file blocks and file attributes
- cached attributes are validated with server on an open()
- the old biod/nfsiod process implemented read-ahead and delayed-write techniques on the client-side, but is not seen much (if at all) these days
- newly created files may not be visible to other sites for up to 30 seconds
- it is indeterminant whether writes to a file will be immediately seen by other clients who have the file open for reading
- If a single NFS stat() request hangs, it can hang up UNIX commands, like ``df''!
- ``magic cookies'' (random numbers) used to short-cut future validations. Given to client from server, client can use it to re-connect whenever a server comes back up after a service interruption.
NFS request
Network File System, FSINFO Call DH:0x75867c04
Program Version: 3
V3 Procedure: FSINFO (19)
object
length: 12
hash: 0x75867c04
type: Linux knfsd (new)
version: 1
encoding: 0 0 0
auth_type: no authentication (0)
fsid_type: major/minor/inode (0)
fileid_type: root (0)
authentication: none
file system ID: 3,2 (inode 4112441)
major: 3
minor: 2
inode: 4112441
file ID: root inode
NFS security problems
- The original NFS protocol can be spoofed (no encryption nor authentication). The first attempts to add authentication
were not all that good (see USAH p. 492).
- Note that ``stale cookies'' can make a client hang (solution: remount the filesystem on the client to make it get a new, fresh cookie).
- RPCSEC is supposed to cure all manner of security problems, but depends on kerberos infrastructure.
What are the differences in v2 and v3?
See RFC1813 for a full description of v3. There is a good summary at nfs.sourceforge.net of the differences in v2 and v3:
- In v2, clients can access only 2 gigabytes of a file. In v3, much larger (64 bit)
- v3 supports larger reads and writes
- Idea of ``Weak Cache Consistency'' introduced in v3 to help detect if modifications are happening to an object (file or directory).
- Server-based access checks
- v3 supports ``safe asynchronous writes'', where a server is permitted to reply before it has synced data to the drive.
Starting NFS on Linux
[root@sophie root]# more /etc/exports
#
/home/exports monet.cs.fsu.edu(ro,no_root_squash,insecure)
[root@sophie root]# /etc/init.d/nfs start
Starting NFS services: [ OK ]
Starting NFS quotas: [ OK ]
Starting NFS daemon: [ OK ]
Starting NFS mountd: [ OK ]
[root@sophie root]# /etc/init.d/iptables stop
Flushing firewall rules: [ OK ]
Setting chains to policy ACCEPT: filter [ OK ]
Unloading iptables modules: [ OK ]
Starting NFS on Linux
On the client side:
mount sophie:/etc/exports /mnt-tmp
Starting NFS on Linux
What is actually done when on a Linux machine when you run /etc/init.d/nfs start?
Here's what to look for:
portmap # maybe, usually not these days
exportfs # /etc/exports
rpc.rquotad
rpc.nfsd
rpc.mountd
Starting NFS on Solaris
shareall # /etc/dfs/dfstab, not /etc/dfs/sharetab
mountd
nfsd
NFS Security
- Don't export to hosts for which non-trusted users have root access.
- If you don't control root on the remote machine then don't export the file system as a writeable one
- ALWAYS block NFS traffic at your router/firewall
Tuning NFS
NFS probably will work just fine "out of the box", but if you feel the urge to tinker, look at perhaps:
- Adjusting the number of nfsd processes
- Use nfsstat -c to see client-side NFS traffic
- Use nfsstat -s to see server-side NFS traffic
Tuning NFS
/usr/sbin/nfsstat -s
Server rpc stats:
calls badcalls badauth badclnt xdrcall
28 0 0 0 0
Server nfs v3:
null getattr setattr lookup access readlink
2 7% 10 35% 0 0% 2 7% 3 10% 0 0%
read write create mkdir symlink mknod
8 28% 0 0% 0 0% 0 0% 0 0% 0 0%
remove rmdir rename link readdir readdirplus
0 0% 0 0% 0 0% 0 0% 0 0% 1 3%
fsstat fsinfo pathconf commit
0 0% 2 7% 0 0% 0 0%
Tuning NFS clientside
Tuning with mount command:
- rsize=n → Set the read buffer size to n bytes.
- wsize=n → Set the write buffer size to n bytes.
- timeo=n → Set the NFS timeout to n tenths of a second.
- retrans=n → The number of NFS retransmissions.
Tuning NFS on Linux
On a Linux server, you can try sysctl:
- Tuning with sysctl command:
- Do sysctl -a | egrep '(r|w)mem'
- Increasing both net.core and net.ipv4 memory settings seems to help
Automounting
Original implementations were buggy, and some required reboots to straighten out problems.
For most production environments, the reasons for automounting are less of an issue from server-to-server since
this is not done a great deal in practice and almost never to random hosts as auto-mounting assumes; for server-to-client,
this would be only a benefit where a number of distinct NFS servers needed to be accessed on an irregular basis by a
given client — not all common these days. A better solution for this problem is likely sshfs
Beyond NFS v3
NFS v4 (RFC3530)
- adds state (remember, NFS was originally stateless)
- file delegation -- the client can work on a local copy of a file until another client requests the same file
- multiple RPCs in a single request
- better security with RPCSEC/GSS
- improved ACL support
- consolidates disparate parts into a single NFS mechanism (no longer lock, mount, stat, nfs)
Variations on NFS
AFS — Andrew File System
- was in development since the late 1980s
- better security than NFS, but never saw the success that NFS did and seems to be on the retreat
- AFS has been used in global configurations; Morgan Stanley, for instance, has a global AFS network (25,000+ hosts over
6 continents (good slide presentation at http://www-conf.slac.stanford.edu/AFSBestPractices/Slides/MorganStanley.pdf)
- OpenAFS — IBM released a branch for open source development, but has dropped all commercial support
