addin a disk

8/2/2019 Addin a Disk

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Chapter 8 Adding a Disk

By Vickie Shen


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Content :

A general discussion of the SCSI and IDE standards

The structure of modern hard disks

The general mechanisms by which disks are formatted andpartitioned

The procedure for initial file systems

Proprietary commands for setting up new disks supported

by individual vendors Systems installation procedure for one particular disk


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How do most systems connect their disks?

Two major methods are being used =>

SCSI (the small Computer Systems Interfaces)

IDE (Integrated Drive Electronics)


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Disk Interfaces

The common-use interface standards:

1. SCSI

2. IDE

3. Fibre Channel

4. USB


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The features of the SCSIinterface:

One of the most common and widely supported disk

interfaces

Support multiple disks on a bus

Various speeds support

Support different communication styles

* SCSI will be described in more detail in the next section


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The features of theIDEinterface:

Developed as a simple, low-cost interface for PCs

Now support all modern disks (why?)

Because it put the hardware controller in the same box as

the disk platters and used a relatively high-leave protocol

for communication between the computer and the disks.

Medium in speed

High in capacity

Very cheap

Design only for workstations with four or fewer devices


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The features of the Fibre Chennel interface:

High bandwidth

Large number of devices can be attached to the Fibre

Channel at once

Devices connect together with a fiber optic or twinaxialcopper cable

High speed: can reach to 100MB/s and up

Two common topologies:

1) FC-AL (Fibre Channel Arbitrated Loop):Can speak with several protocols, including SCSI and even IP /

Channel devices are identified by hardwired ID number called

World Wide Name.

2) Fabrics : Construct with Fibre Channel Switches


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The features of the USB interface:

Stand for Universal Serial Bus

A popular interface for connecting devices such as

keyboard and mice Good for PCs

Easier for users to remover a disk among disks

Enough bandwidth to support slower disk devices such

as removable HDs and CD-ROM drives


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Introduction to SCSI Interfaces

SCSI standard is composed of several chip sets

Vendors sometimes put SCSI support on the CPU or

peripheral board.

Generic data pipe is being used by all kinds peripheral

- disks

- tape drives

- scanners- printers


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The summarize the different SCSI version and their

associated bus bandwidths and cable length :

(Table 8.1- p.121)

Version Freq. Wedth Speed Length Diff.length

SCSI-1 5 MH 8 bits 5 MB/s 6m 25m

SCSI-2 5 MH 8 bits 5 MB/s 6m 25mFast SCSI-2 10 MH 8 bits 10 MB/s 3m 25m

Fast/wide SCSI-2 10 MH 16 bits 20 MB/s 3m 25m

Ultra SCSI 20 MH 8 bits 20 MB/s 1.5m 25m

Wide Ultra SCSI 20 MH 16 bits 40 MB/s 1.5m 25m

Wide Ultra2 SCSI 40 MH 16 bits 80 MB/s - 25m(HVD)

12m(LVD)

Wide Ultra3 SCSI 80 MH 16 bits 160 MB/s - 12m(LVD)


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Common SCSI connectors: (p122)

- Centronics(50 pins, SCSI-1/2, external)-Bibbon connector(female, 50 pins, SCSI-1/2, internal)

-Mini-micro, aka HD50(50 pins, SCSI-2, external)

- Wide mini-micro, aka HD68(68 pins, SCSI-2/3, int/ext)

- SCA-2(80 pins, SCSI-3, internal)

SCSI address/ target number

Each device has a SCSI address or target number that distinguishes it from the

other devices on the bus. Target number starts at 0 and go up to 7-15,

depending on whether the bus is narrow or wide. The SCSI controller itself

counts as a device and is usually target 7. All other devices must have their

target numbers set to unique values.It is a common error to forget that SCSI

controller has a target number and to set a device to the same target number as

the controller.


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SCSI buses configuration

- Easy to configure- The variety of things need to pay attention during the

installation process

1. Many workstations have internal SCSI devices. Check the listing

of current devices before you reboot to add a new device.Remember that most tape systems and some floppy drives are

SCSI.

2. Make sure for the following things when setup :

- a differential SCSI controller has only differential devicesand differential terminator connected to it.

- a single chain does not contain any differential devices.

- the single ended and differential singling technique are

incompatible.


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- After you added a new SCSI devices, check the listing of devices

discovered by the OS when it reboots to make sure that everything youexpect is there. Most SCSI drivers will not detect multiple devices that

have the same SCSI address, which is an illegal configuration. SCSI

address conflicts can lead to very strange behavior.

- Some expansion boxes terminate the bus inside the box. If devices

are attached after the expansion box, you can have reliability problems

with any of the devices on the SCSI chain. Always double-check that

you have exactly two terminators and that they are both at the ends of

the bus.

-The thumbwheel used to set a devices SCSI address is sometimes

connected backwards. When this happens, the thumbwheel will change

the SCSI address, but not to the displayed value.


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- When figuring the length of your SCSI-2 bus, make sure you countthe cables inside devices and expansion boxes. They can be quite long.

Also remember that the maximum length can be reduced if order SCSI

devices are added to a newer SCSI bus.

- Never forget that your SCSI controller uses one of the SCSIaddresses!


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Which is better, SCSI or IDE?

For a signal-user workstation, a good IDE disk is simple ,

high-capacity, dirt-cheap solution that provides 85% of theperformance of a SCSI setup. In most cases, upgrading a

single-user workstation to SCSI will not increase the

systems perceived performance.


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In some situations SCSI is advisable or even mandatory:

1. If you absolutely must have the best possible performance,

go SCSI.

2. For the Servers and multiuser systems, it requires SCSI.

3. If you want to connect many devices, SCSI wins again.

4. You might need some particular feature that only SCSIprovides


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Introduction to IDE Interfaces

IDE, also called ATA (for AT Attachment)

IDE was designed for simple and inexpensive purposes

The controller is built into disk, which reduces interface

cost and simplifier the firmware.

The IDE connector is a 40-pin header that connects the

drive to the interface card with a ribbon cable.


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IDE devices are accessed in a connected manner -> means

that only one device can be active at the same time.Therefore, performance is best if you spread the devices

out over multiple buses. SCSI handles multiple devices on

a bus much better than does IDE

Designate one as master and the other one as the slave

when more then one device on an IDE bus.


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The revolution of IDE inferfaces:

ATA-1

ATA-2

The following features are added to ATA-2:- Faster Programmed I/O (PIO)

- Direct Memory Access mode(DMA)

- Extend the buss Plug and Play features

- Logical Block Addressing (LBA) - overcomes a problem thatprevented BIOSs from accessing more than the first 1024 cylinders of

a disk. The constraint formerly limited disk si e t0 504MB


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ATA-3

- Additional reliability, more sophisticated powermanagement, and self-monitoring capabilities.

ATA-4 (still being developed)

- merge ATA-3 with ATA Packet Interface(ATAPI)

*APAPI is a protocol that allows CD-ROM and tape drivers towork on an IDE bus.

Ultra-ATA

- is designed to bridge the gap between ATA-3 and ATA-4

Ultra DMA-33 & Ultra DMA/66

- extend the bus bandwidth from 16MB/s to 33MB/s to

66MB/s, respectively.


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An overview of the disk installation procedure

Connecting the disk to the computer

Creating device file through which the disk can be

accessed

Formatting the disk Labeling and partitioning the disk

Establishing logical volumes

Creating UNIX filesystems whithin disk partitions

Setting up automatic mounting

Setting up swapping


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Connecting the disk

Depends mostly on the interface that is used.

If the disk is IDE => configure the system with only one

IDE disk per bus. Check the cable orientation and the

master/ slave settings on each disk.If the disk is SCSI => double-check the following things...

1) you have properly terminated both ends of the SCSI bus

2) the cable length is less then the maximum appropriate

for the SCSI variant you are using3) new SCSI target does not conflict with the controller or

another device on the bus


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Creating device entrices

Before you can access a new disk, you need device files in

/dev that point to it. You will need both block device files

and character device files. Many versions of UNIX

automatically create files for all possible SCSI devices(detail on p.138)

* Block device files generally used for mounting

filesystems.

* Character device files used for backing up and checkingthe integrity of filesystems.


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Formattingthe disk The formatting process does two things:

- Writes address information and timing marks on the

patters to delineate each sector.

- Identify bad blocks.(Imperfactions in the media that

result in areas that cannot be reliably read or written.

On order disks, the UNIX device driver is responsible for

understanding bad blocks and mapping them to

replacement blocks elsewhere on the disk. SCSI disks have

bad block management built in. Users and driver itself do

not need to worry about it

Encounter read or write when formatting, first check for

cabling, termination, and address problems, which can

cause symptoms similar to those of a bad block .


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IDE disks are usually not designed to be formatted outside

of the factory. However, you may be able to get special

formatting software from the manufacturer, usually for

Window. Make sure the software matches the drive you

plan to format and follow the manufacturers directions

carefully.

SCSI disks format in response to a command that you send

from the host computer. The procedure for sending this

command varies from system to system. On PCs, you canoften send the command from the SCSI controllers BIOS.(format command on Solaris; mediainit command on HP-UX)


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Labeling and partitioning the disk

What is partitions?

After a disk has been formatted and its bad sectors

remapped, it must be divided into chunks called partitions

or slices.


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What does the partitioning do?

1. Partitioning allows the disk to be treated as a group of

independent data areas rather ten as one vast expanse of

blocks.

2. Partitioning also allows the boot blocks and the partition

table itself to be hidden from high-level software (e.g.,

filesystem)

3. Partitions make backups easier, prevent users from

poaching each others disk space.

4. Partitions can improve performance of the entire system.

5. Partitions confine potential damage from runaway

programs.


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What is called label?

On most operation systems, the partition table is kept on

the disk in a record called the label. The label usually

occupies the first few blocks of the disk. Basically, what it

does is to get the system booting.


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Some systems allow users to define multiple overlappingsets of partitions. In real life, such overlapping partitions

invite operator errors and a common cause of random data

corruption.

Modern system tend to have fewer partitions then theirpredecessor. Most systems will have at least the following

three partition

- The root partition

- The swap partition- The user partition


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The root partition :everything needed to bring the

system up to single user mode is kept here. A second copyof this partition is often stored on another disk for

emergencies.


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The swap partition : A swap area stores pages of virtual

memory when not enough physical memory is available tohold them. Every system should have at least one swap

partition.


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The user partition : Home directories, data files, source

code libraries, and other random data files find a homehere.


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Some hints for splitting disks into partitions:

If you have multiple disks, make a copy of the root

filesystem of one of them and verity that you can boot

from it.

As you add memory to your machine, you should also

add swap space.

Splitting swap space among several disks increases

performance

If you intend to back up a partition, dont make it

bigger than the capacity of your backup device.


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It is a good idea to create a separate filesystem (/tmp) for

temporary files because it limits the files to a finite si e and

save you from having to back them up.

If your system keeps long files in/var, it is a good idea for/var to be a separate disk partition. Many systems ship

with/var as part of a very small root partition, making it

easy to fill the root and bring the machine to a halt.


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Establishing logical volume

Some system have a logical volume manager that provide asort of supercharged version of disk partitioning. The

volume managers let user group multiple disks or

partitions into a logical volume.

The common features supported by volume managers:

Striping

RAID5 (Redundant Array of Inexpensive Disks) Mirroring


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Volume Manager and the supported vendors

Veritas..Solaris & HP-UXSolstice DiskSuite....Sun

Vinum(Open source volume manager)Free BSD

LinuxLVM &

implementation of kernel RAID..Linux


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Creating UNIX filesystems

After a hard disk has been conceptually divided into partitions.The filesystem needs to add a little of its own overhead

before the disk is ready for use.

Use newfs or mkfs toinstall a filesystem within adisk partition.


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Setting up automatic mounting

A system must be mounted before it becomes available on

UNIX processes. The mount point for a filesystem can be

any directory, but the files and subdirectories beneath it

will not be accessible while a filesystem is mounted there.

*mount, unmount, swapon and fsck all read the fstab file

The command used to mount new filesystems: mount

The command used for unmounting filesystems: unmount


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Enabling swapping

Why do we need swapping?This feature allows the operation system to pretend that the

machine has more memory then it actually does.

The more swap space you have, the more virtual memory

your process can allocate. The best swap performance is achieved when the swap

area is split among several drives.

Users can manually enable swapping to a particular device,

but you will generally want to have this functionperformed automatically at boot time.

On most systems, swap areas can be listed in the fstab file.


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Check and repair filesystems

The UNIX filesystem is surprisingly reliable, and it does a

remarkable job of coping with unexpected system crashes

and flaky hardware. However, filesystems can becomedamaged or inconsistent in a number of ways.


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Problem :

Any time the kernel panics or power fails, small

inconsistencies may be introduced to the filesystems that

will active immediately preceding the crash.

Additional information:

Since the kernel buffers both data blocks and summary

information, the most recent image of the filesystem is

split between disk and memory. During a crash, the

memory portion of the image is lost. The buffered blocks

are effectively overwritten with the version that were

most recently saved to disk.


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Solutions:

There are a couple of approaches to fixing this problem.

1) fsck command : can be used to solved minor damage. It

isnt a very architechurally elegant way of approaching the

issue, but it works pretty well for all the common

inconsistencies.

2)Journaling filesystems: write metadata out to a sequential

log file that is flushed to disk before each command returns.

If the system crashes, the log can be rolled up to the most

recent consistency point.

* Journaling filesystems is available on Solariss UFS

filesystem and VXFS for HP-UX.


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If some form of journaling is not available, you must wait forfsck to work.

The five common types of damage are:

Unreferenced inodes

Inexplicably large link counts

Unused data blocks not recorded in the block maps

Data blocks listed as free that are also used in a file

Incorrect summary information in the superblock

* fsck can safely and automatically fix these five problems.


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fsck -pbesides correct the errors, it also examines all localfilesystems listed in /etc/fstab. It can also be run on

particular filesystem


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Vendor Specifies

Adding a disk to different vendor systems:

SPARC-based Solaris system

Red Hat Linux

Free BSD

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