Unit VI: Adding New Users and Storage

Index The /etc/passwd file, The /etc/shadow,

/etc/security/passwd files and /etc/group file,

Adding users, Adding users with useradd,

Storage: Adding a hard Disk, Storage Hardware, Storage hardware Interfaces,

Software aspects of storage, Formatting, Disk Partitioning, RAID, LVM, Linux,

File System, mkfs, fsck, file system mounting, setup

for automatic mounting, USB drive, mounting, Enabling swapping.

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THE /etc/passwd FILE

When you use mkuser command to add new user to your system,the command updates the /etc/passwd file.

Each line in the file represents one user and contains seven fields separated by colons: Login name Encrypted password placeholder UID (user ID) number Default GID (group ID) number “GECOS” information: full name, office,

extension, home phone Home directory Login shell

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An entry in /etc/passwd file has the following form:

Name:password:UserID:PrincipleGroup:Geco

s:HomeDirectory:Shell

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THE /ETC/PASSWD FILE

THE /ETC/SHADOW AND /ETC/SECURITY/PASSWD FILES

/etc/passwd is world readable. Encrypted password is in /etc/security/passwd file /etc/shadow file read only by root. The shadow file is not a superset of the passwd

file, and the passwd file is not generated from it. Like /etc/passwd, /etc/shadow contains one line

for each user. Each line contains nine fields, separated by colons :

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Login name Encrypted password Date of last password change Minimum number of days between password

changes Maximum number of days between password

changes Number of days in advance to warn users about

password expiration Linux: Days after password expiration that

account is disabled Solaris/HP-UX: Days before account automatically

expires Account expiration date A reserved field that is currently always empty,

except on Solaris

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THE /ETC/GROUP FILE The /etc/group file contains the names of

UNIX groups and a list of each group’s members.

Each line represents one group and contains four fields separated by colon(:)

group_name:password:GID:user_list

Cdrom:x:24:raj,student1,abc

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Commands to manage the groups

chgrp chgrpmem chuser lsgroup mkgroup mkuser rmgroup rmuser

ADDING USERS: THE BASIC STEPS

Required: Have the new user sign your policy agreement. Edit the passwd and shadow files to define the user’s

account. Add the user to the /etc/group file Set an initial password. Create, chown, and chmod the user’s home directory. Configure roles and permissions.For the user:

Copy default startup files to the user’s home directory. Set the user’s mail home and establish mail aliases.For you:

Verify that the account is set up correctly. Add the user’s contact information and account status to

your database.

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ADDING USERS WITH USERADD Syntax: useradd<username> To setup a new password: passwd<username> e.g. # useradd comp # passwd technical

Ubuntu provides two ways to add users: adduser and useradd.

adduser is configured in /etc/adduser.conf, which includes options such as these: Rules for locating home directories: by

group, by username, etc. Permission settings for new home

directories UID and GID ranges for system users and

general users An option to create individual groups for

each user Disk quotas (Boolean only, unfortunately) Regex-based matching of usernames and

group names

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ADDING USERS on UBUNTU

Storage

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Adding a hard Disk Run sudo fdisk -l to list the system’s

disks and identify the new drive. Then run any partitioning utility to create

a partition table for the drive.

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STORAGE HARDWARE Hard disks

A typical hard drive contains several rotating platters coated with magnetic film.

They are read and written by tiny skating heads that are mounted on a metal arm that swings back and forth to position them.

The heads float close to the surface of the platters but do not actually touch.

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Hard disk

Moving-head disk mechanism

STORAGE HARDWARE Solid state disks

SSDs spread reads and writes across banks of flash memory cells, which are individually rather slow in comparison to modern hard disks.

But because of parallelism, the SSD as a whole meets or exceeds the bandwidth of a traditional disk.

The great strength of SSDs is that they continue to perform well when data is read or written at random, an access pattern that’s predominant in real-world use.

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STORAGE HARDWARE INTERFACES• If a system supports several different

interfaces, use the one that best meets your requirements for speed, redundancy, mobility, and price. ATA (Advanced Technology Attachment), known

in earlier revisions as IDE (Integrated Drive Electronics), was developed as a simple, low-cost interface for PCs.

It put the hardware controller in the same box as the disk platters and used a relatively highlevel protocol for communication between the computer and the disks.

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STORAGE HARDWARE INTERFACES The PATA interface

PATA (Parallel Advanced Technology Attachment), also called IDE, was designed to be simple and inexpensive. It is most often found on PCs or low-cost workstations.

The original IDE became popular in the late 1980s.

Most PCs have two PATA buses, each of which can host two devices.

If you have more than one device on a PATA bus, you must designate one as the master and the other as the slave.

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STORAGE HARDWARE INTERFACES The SATA interface

As data transfer rates for PATA drives increased, the standard’s disadvantages started to become obvious. Electromagnetic interference and other electrical issues caused reliability concerns at high speeds.

Serial ATA, SATA, was invented to address these problems. It is now the predominant hardware interface for storage.

It improves transfer rates (potentially to 750 MB/s with the upcoming 6 Gb/s SATA) and includes superior error checking.

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STORAGE HARDWARE INTERFACES Parallel SCSI

SCSI, the Small Computer System Interface, defines a generic data pipe that can be used by all kinds of peripherals.

In the past it was used for disks, tape drives, scanners, and printers, but these days most peripherals have abandoned SCSI in favor of USB.

It uses terminator and has a SCSI address or “target number”

Serial SCSI From the hardware perspective, SAS

improves just about every aspect of traditional parallel SCSI.

SAS does not use terminators and has a Fibre-Channel-style 64-bit World Wide Name (WWN) assigned by the manufacturer.

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THE SOFTWARE SIDE OF STORAGEcntd..

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Details of pieces in Exhibit B represent:

Details of pieces in Exhibit B represent: A storage device is anything that looks

like a disk. It can be a hard disk, a flash drive, an SSD, an external RAID array implemented in hardware, or even a network service that provides block-level access to a remote device.

A partition is a fixed-size subsection of a storage device. Each partition has its own device file and acts much like an independent storage device.

A RAID array (a redundant array of inexpensive/independent disks) combines multiple storage devices into one virtualized device.

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THE SOFTWARE SIDE OF STORAGEcntd..

Details of pieces in Exhibit B represent: Volume groups and logical volumes are

associated with logical volume managers (LVMs). These systems aggregate physical devices to form pools of storage called volume groups. The administrator can then subdivide this pool into logical volumes in much the same way that disks of yore were divided into partitions.

A filesystem mediates between the raw bag of blocks presented by a partition, RAID array, or logical volume and the standard filesystem interface expected by programs: paths such as /var/spool/mail, UNIX file types, UNIX permissions, etc. The filesystem determines where and how the contents of files are stored, how the filesystem namespace is represented and searched on disk, and how the system is made resistant to (or recoverable from) corruption.

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THE SOFTWARE SIDE OF STORAGEcntd..

Partitioning and logical volume management are both ways of dividing up a disk into separate chunks of known size.

Most of the systems support logical volume management, but only Linux, Solaris, and sometimes HP-UX allow traditional partitioning.

Both partitions and logical volumes make backups easier, prevent users from poaching each other’s disk space, and confine potential damage from runaway programs.

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DISK PARTITIONING

RAID, “redundant arrays of inexpensive disks,” is a system that distributes or replicates data across multiple disks.

RAID not only helps avoid data loss but also minimizes the downtime associated with hardware failures (often to zero) and potentially increases performance.

RAID can be implemented by dedicated hardware that presents a group of hard disks to the operating system as a single composite drive.

It can also be implemented simply by the operating system’s reading or writing multiple disks according to the rules of RAID.

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RAID: REDUNDANT ARRAYS OF INEXPENSIVE DISKS

RAID levels : RAID can do two basic things First, it can improve performance by

“striping” data across multiple drives, thus allowing several drives to work simultaneously to supply or absorb a single data stream.

Second, it can replicate data across multiple drives, decreasing the risk associated with a single failed disk.

Replication assumes two basic forms: mirroring, in which data blocks are reproduced

bit-for-bit on several different drives, and parity schemes, in which one or more drives

contain an error-correcting checksum of the blocks on the remaining data drives.

Mirroring is faster but consumes more disk space.

Parity schemes are more disk-space-efficient but have lower performance.

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RAID: REDUNDANT ARRAYS OF INEXPENSIVE DISKS

A logical volume manager lets you reallocate space dynamically from the greedy partition to the needy partition.

It groups individual storage devices into “volume groups.”

The blocks in a volume group can then be allocated to logical volumes,”

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LOGICAL VOLUME MANAGEMENT

filesystems: Even after a hard disk has been

conceptually divided into partitions or logical volumes, it is still not ready to hold files.

The Filesystem, must be implemented in terms of raw disk blocks.

The filesystem is the code that implements these, and it needs to add a bit of its own overhead and data.

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FILE SYSTEMS

Linux filesystems: the ext family ext2: “second extended filesystem,” was

written specifically for Linux, it is functionally similar to the Berkeley Fast File System.

ext3: adds journaling capability to the existing ext2 code, a conceptually simple modification that increases reliability enormously.

ext4: is a comparatively incremental update that raises a few size limits, increases the performance of certain operations, and allows the use of “extents” (disk block ranges) for storage allocation rather than just individual disk blocks. Use of ext4 over the previous versions is recommended as of Linux kernel 2.6.28. It is the default on Ubuntu and SUSE; Red Hat remains on ext3.

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FILE SYSTEMS

Filesystem terminology filesystems share some descriptive

terminology. “Inodes” are fixed-length table entries

that each hold information about one file. They were originally preallocated at the

time a filesystem was created, but some filesystems now create them dynamically as they are needed.

Filesystem polymorphism Filesystems are software packages with

multiple components. One part lives in the kernel and implements the

nuts and bolts of translating the standard filesystem API into reads and writes of disk blocks.

Other parts are user-level commands that initialize new volumes to the standard format, check filesystems for corruption.

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FILE SYSTEMS

mkfs: format filesystems The general recipe for creating a new

filesystem is mkfs [-T fstype ] [-o options]

rawdevice The default fstype may be hard-coded

into the wrapper, or it might be specified in /etc/default/fs.

fsck: check and repair filesystems filesystem data structures can potentially

become self-inconsistent (Because of block buffering and the fact that disk drives are not really transactional devices).

If these problems are not corrected quickly, they propagate.

The original fsck scheme worked surprisingly well it could take hours on a large drive. an early optimization: a “filesystem clean” bit

that could be set in the superblock when the filesystem was properly unmounted.

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FILE SYSTEMS

Filesystem mounting A filesystem must be mounted before it

becomes visible to processes. The mount point for a filesystem can be any directory, but the files and subdirectories beneath it are not accessible while a filesystem is mounted there. After installing a new disk, you should mount new filesystems. For example, the command

$ sudo mount /dev/sda1 /mnt/temp

mounts the filesystem in the partition represented by the device file /dev/sd1a on a subdirectory of /mnt, which is a traditional path used for temporary mounts. You can verify the size of a filesystem with the df

command.$ df -h /mnt/web1

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FILE SYSTEMS

Setup for automatic mounting The command mount -a mounts all

regular filesystems listed in the filesystem catalog;

it is usually executed from the startup scripts at boot time.

$ sudo mount -at ext4 mounts all local ext4 filesystems. The mount command reads fstab sequentially.

USB drive mounting USB drives: Friendly. Faster & fun flavors: personal “thumb” drives, digital

cameras, iPods, and large external disks

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FILE SYSTEMS

Enabling swapping Raw partitions or logical volumes, rather

than structured filesystems, are normally used for swap space.

Instead of using a filesystem to keep track of the swap area’s contents, the kernel maintains its own simplified mapping from memory blocks to swap space blocks.

On Linux systems, swap areas must be initialized with mkswap, which takes the device name of the swap volume as an argument.

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FILE SYSTEMS