chapter6 2 omar

8/3/2019 Chapter6 2 Omar

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CIT-315

OPERATING SYSTEM ADMINISTRATION

The Network File System


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Background2

A network file system is any computer file system

that supports sharing of files, printers and other

resources as persistent storage over acomputer

network. The first file servers were developed in the1970s, and in 1985 Sun Microsystems created the

file system called "Network File System" (NFS)

which became the first widely used network file

system. [Wikipedia]


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Transparency3

Ideally, anetwork file system should appear to its

users to be aconventional, centralized file system.

The multiplicity and dispersion of its servers and

storage devices should be made invisible, i.e. no

distinction between local and remote files.

It is up to the network file system to locate the files

and to arrange for the transport of the data.[Wikipedia]


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Performance4

The most important performance measurement of a

network file system is the amount of time needed to

satisfy service requests.

In conventional systems, this time consists of a disk-

access time and a small amount of CPU-processing

time.

[Wikipedia]


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Performance (contd)5

In a network file system, a remote access has

additional overhead due to the distributed

structure. This includes the time to deliver the

request to a server, the time to deliver the responseto the client, and for each direction, a CPU

overhead of running the communication protocol

software. [Wikipedia]


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Concurrent File Updates6

A network file system should allow multiple client

processes on multiple machines to access and update

the same files.

Updates to the file from one client should notinterfere with access and updates from other clients.

Concurrency control or locking may be either built

into the file system or be provided by an add-on

protocol. [Wikipedia]


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NFS Server Configuration Files7

There are 3 main configuration files to edit in

order to set up an NFS server:

/etc/exports

/etc/hosts.allow (security)

/etc/hosts.deny (security)


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The /etc/exports File8

This file contains a list of entries where each entry indicates:

Which volume to share

How is the volume to be shared

Format:

/dir1/dir2 host1(options) host2(options)

Example:

/another/exported/directory 192.168.0.3(rw,sync)


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Specifying the Host Name9

Either one of theses ways can be used to specify the

specify the host name:

The direct hostname or IP

Wildcards in the hostname, such as *.zu.ac.ae will

match all hosts that end in .zu.ac.ae


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Specifying the Options10

rw - Read and write access

ro - Read-only access

sync - Writes are committed to the disk without

delays. The data is safer; puts more strain on thesystem; By default it is turned off

no_root_squash - Trust the client's root account


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Rules for Exporting File Systems11

1. Any FS (or subset of an FS) can be exported

2. You cannot export a subdirectory of an FS where the FS

is already exported, unless the subdirectory is on a

different physical device.3. You cannot export the parent directory of an exported

FS, unless the parent is on different physical device.

4. You can only export local FS.


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NFS Daemons12

3 programs that provide NFS services:

rpc.portmapper - Maps calls made from other

machines to the correct NFS daemons. (Server & client)

rpc.nfsd - Translates NFS requests into actual requests

on the local filesystem. (server)

rpc.mountd - Services requests to mount and unmount

filesystems. (server)


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NFS Processes13

The following programs should be running:

portmapper , mountd, and nfs

To check for this:

$ rpcinfo pProgram vers proto port

100000 2 tcp 111 portmapper

100000 2 udp 111 portmapper

100012 1 udp 821 mountd

100012 1 tcp 823 mountd100003 2 udp 2049 nfs

100003 2 tcp 2049 nfs


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Starting/Stopping NFS Processes14

To start NFS:

# /etc/init.d/nfs start

To stop NFS

# /etc/init.d/nfs stop

To check status:

# /etc/init.d/nfs status


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Re- exporting the File Systems15

# /usr/sbin/exportfs r

sends the appropriate signals to the rpc.nfsd and

rpc.mountd daemons to re-read the /etc/exports file

and update their internal tables

/usr/sbin/exportfs

Lists all exported file system


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Mounting an Exported File System16

To mount a filesystem, use the mount command:

# mount server:/exported/dir /dir/to/mount

serveris the name of the server from which you want to mounta filesystem,

/exported/dir is the directory listed in its /etc/exports file,and

/dir/to/mount is the location on your local machine where youwant to mount the filesystem.


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Options with the Mount Command17

Characteristic Description

rw Read/write

ro Read-only

bg Background mount intr Interruptible mount

soft client can return failure

retrans maximum retried transm

wsize write size rsize read size


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Example of Mount Command18

mount -o rw,bg,intr,soft,retrans=6 ser:/export/bin /bin

bg: should the initial mount fail, the mount process

places itself in the background and continue untilsuccessful

intr: if mount process is pending I/O on a mounted

partition, it is ok to interrupt


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Finding Mounted File Systems19

On the NFS server, use the following command to list

the set of clients who are mounting filesystems from it

# /usr/sbin/showmount

From the client, to find out the list of exported file

systems by a server:

# /usr/sbin/showmount e citserver


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Unmounting a File System20

For example, to unmount /mounted/fsys, you

can use:

# umount /mounted/fsys

If files are being used on a mounted filesystem, you

cannot unmount it.

All files must be closed before the unmount can be

done.


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Unmounting with Open Files?21

Use lsof N to find out information about files

opened by processes. Then you have 2 options:

Use umount f to force the filesystem to unmount

Bring system to single-user mode and then unmount the

filesystem


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Mounting File systems Automatically

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You can use the file /etc/fstab to specify all the

partitions that need to be mounted at boot time

Example of /etc/fstab line entry:

server:/usr/local/pub /pub nfs

rsize=8192,wsize=8192,timeo=14,intr


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Remounting File Systems23

To remount the filesystems that are already listed in

the fstab file:

# mount -a

To remount a single filesystem that already has an

entry in the fstab file:

# mount /dir/to/mount


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The Automount24

One drawback to using /etc/fstab:

Regardless of how infrequently a user accesses theNFS mounted file system, the system must dedicateresources to keep the mounted file system.

Overall system performance may degrade whenthe system is maintaining mounts to a large numberof systems.

Alternatively, we can use the kernel-based

automount utility, to automatically mount andunmount NFS file systems, based on demand.


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To Setup Automount on a Client25

Adding this line to /etc/auto.master:

/export/home /etc/auto.home --timeout=120

Create a new file called /etc/auto.home and add

the following line:* -fstype=nfs,rw,hard,intr,rsize=2048,

wsize=2048,nosuid,nfsvers=3

citserver:/export/home/&


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Setting up Automount (contd)26

To start the auto-mounter daemon on the client:

# /etc/init.d/autofs start

Example of what the mount command will display:

automount(pid4633) on /export/home type autofs

(rw,fd=5,pgrp=4633,miniproto=2,maxproto=3)

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