UNIT-V
MASS STORAGE STRUCTURE
Mass Storage Structure:
Overview of Mass-Storage Structure: we present a general overview of the physical
structure of secondary and tertiary storage devices.
Magnetic Disks: It provide the bulk of secondary storage for modern computer systems.
Conceptually, disks are relatively simple in given Figure.
Each disk platter has a flat circular shape, like a CD. Common platter diameters range
from 1.8 to 3.5 inches.
The two surfaces of a platter are covered with a magnetic material. We store information
by recording it magnetically on the platters.
A read – write head “flies” just above each surface of every platter. The heads are
attached to a disk arm that moves all the heads as a unit. The surface of a platter is
logically divided into circular tracks, which are subdivided into sectors.
The set of tracks that are at one arm position makes up a cylinder. There may be
thousands of concentric cylinders in a disk drive, and each track may contain hundreds
of sectors. The storage capacity of common disk drives is measured in gigabytes.
When the disk is in use, a drive motor spins it at high speed. Most drives rotate 60 to 250
times per second, specified in terms of rotations per minute (RPM).
The transfer rate is the rate at which data flow between the drive and the computer. The
positioning time, or random-access time, consists of two parts: the time necessary to
move the disk arm to the desired cylinder, called the seek time, and the time necessary
for the desired sector to rotate to the disk head, called the rotational latency.
1
,Because the disk head flies on an extremely thin cushion of air (measured in microns),
there is a danger that the head will make contact with the disk surface. Although the
disk platters are coated with a thin protective layer, the head will sometimes damage
the magnetic surface. This accident is called a head crash.
A head crash normally cannot be repaired; the entire disk must be replaced.
A disk can be removable, allowing different disks to be mounted as needed.
Removable magnetic disks generally consist of one platter, held in a plastic case to
prevent damage while not in the disk drive.
Other forms of removable disks include CDs, DVDs, and Blu-ray discs as well as removable
flash-memory devices known as flash drives.
A disk drive is attached to a computer by a set of wires called an I/O bus.
Several kinds of buses are available, including advanced technology attachment
(ATA), serial ATA (SATA), eSATA, universal serial bus (USB), and fibre channel
(FC).
The data transfers on a bus are carried out by special electronic processors called
controllers.
⮚ The host controller is the controller at the computer end of the bus.
⮚ A disk controller is built into each disk drive.
Solid-State Disks – New:
● As technologies improve and economics change, old technologies are often used
in different ways. One example of this is the increasing use of solid-state disks,
or SSDs.
● SSD is nonvolatile memory that is used like a hard drive.
● SSDs use memory technology as a small fast hard disk. Specific implementations
may use either flash memory or DRAM chips protected by a battery to sustain
the information through power cycles.
● Because SSDs have no moving parts, they are much faster than traditional hard
drives, and certain problems such as the scheduling of disk accesses simply do not
apply.
● However, SSDs also have their weaknesses: They are more expensive than hard
drives, generally not as large, and may have shorter life spans.
● SSDs are especially useful as a high-speed cache of hard-disk information that
must be accessed quickly. One example is to store file system meta-data, e.g.
directory and anode information that must be accessed quickly and often. Another
variation is a boot disk containing the OS and some application executables, but
no vital user data. SSDs are also used in laptops to make them smaller, faster, and
lighter.
● Because SSDs are so much faster than traditional hard disks, the throughput of
the bus can become a limiting factor, causing some SSDs to be connected directly
to the system bus (for example PCI)
Magnetic Tapes:
● It was used as an early secondary-storage medium. Although it is relatively
permanent and can hold large quantities of data, its access time is slow compared
with that of main memory and magnetic disk.
● random access to magnetic tape is about a thousand times slower than random
2
, access to magnetic disk, so tapes are not very useful for secondary storage.
● Tapes are used mainly for backup, for storage of infrequently used information,
and as a medium for transferring information from one system to another.
● Accessing a particular spot on a magnetic tape can be slow, but once reading or
writing commences, access speeds are comparable to disk drives.
● Capacities of tape drives can range from 20 to 200 GB and compression can
double that capacity.
Disk Structure:
● The one-dimensional array of logical blocks is mapped onto the sectors of the
disk sequentially.
● Sector 0 is the first sector of the first track on the outermost cylinder. The
mapping proceeds in order through that track, then through the rest of the tracks
in that cylinder, and then through the rest of the cylinders from outermost to
innermost.
● By using this mapping, convert a logical block number into an old-style disk
address that consists of a cylinder number, a track number within that cylinder,
and a sector number within that track.
● In practice, it is difficult to perform this translation, for two reasons.
● First, most disks have some defective sectors, but the mapping hides this by
substituting spare sectors from elsewhere on the disk.
● Second, the number of sectors per track is not a constant on some drives.
Modern disks pack many more sectors into outer cylinders than inner ones, using one
of two approaches:
⮚ With Constant Linear Velocity, CLV: the density of bits is uniform from
cylinder to cylinder. Because there are more sectors in outer cylinders, the disk
spins slower when reading those cylinders, causing the rate of bits passing under
the read-write head to remain constant. This is the approach used by modern CDs
and DVDs.
⮚ With Constant Angular Velocity, CAV: the disk rotates at a constant
angular
speed, with the bit density decreasing on outer cylinders. (These disks would have
a constant number of sectors per track on all cylinders.)
Disk Attachment:
Disk drives can be attached either directly to a particular host (a local disk) or to a
network.
Host-Attached Storage:
● Local disks are accessed through I/O Ports .
● The typical desktop PC uses an I/O bus architecture called IDE or ATA.
● This architecture supports a maximum of two drives per I/O bus.
● A newer, similar protocol that has simplified cabling is SATA.
High-end workstations and servers generally use more sophisticated I/O architectures such
as fibre channel (FC), a high-speed serial architecture that can operate over optical fiber
or over a four-conductor copper cable.
● It has two variants.
3
, ⮚ One is a large switched fabric having a 24-bit address space. This variant is
expected to dominate in the future and is the basis of storage-area networks
(SANs).
Because of the large address space and the switched nature of the
communication, multiple hosts and storage devices can attach to the fabric,
allowing great flexibility in I/O communication.
⮚ The other FC variant is an arbitrated loop (FC-AL) that can address 126
devices (drives and controllers).
A wide variety of storage devices are suitable for use as host-attached
storage. Among these are hard disk drives, RAID arrays, and CD, DVD, and tape drives. The
I/O commands that initiate data transfers to a host-attached storage device are reads and
writes of logical data blocks directed to specifically identified storage units (such as bus
ID or target logical unit).
Network-Attached Storage:
A network-attached storage (NAS) device is a special-purpose storage system that is
accessed remotely over a data network.
Clients access network-attached storage via a remote-procedure-call interface such as NFS
for UNIX systems or CIFS for Windows machines.
The remote procedure calls (RPCs) are carried via TCP or UDP over an IP network — usually
the same local- area network (LAN) that carries all data traffic to the clients.
Thus, it may be easiest to think of NAS as simply another storage-access protocol. The
network- attached storage unit is usually implemented as a RAID array with software that
implements the RPC interface.
Network-attached storage provides a convenient way for all the computers on a LAN to
share a pool of storage with the same ease of naming and access enjoyed with local
host-attached storage.
However, it tends to be less efficient and have lower performance than some direct-
attached storage options.
ISCSI is the latest network-attached storage protocol. In essence, it uses the IP network
protocol to carry the SCSI protocol. Thus, networks — rather than SCSI cables— can be
used as the interconnects between hosts and their storage. As a result, hosts can treat
their storage as if it were directly attached, even if the storage is distant from the host.
Storage-Area Network:
4
MASS STORAGE STRUCTURE
Mass Storage Structure:
Overview of Mass-Storage Structure: we present a general overview of the physical
structure of secondary and tertiary storage devices.
Magnetic Disks: It provide the bulk of secondary storage for modern computer systems.
Conceptually, disks are relatively simple in given Figure.
Each disk platter has a flat circular shape, like a CD. Common platter diameters range
from 1.8 to 3.5 inches.
The two surfaces of a platter are covered with a magnetic material. We store information
by recording it magnetically on the platters.
A read – write head “flies” just above each surface of every platter. The heads are
attached to a disk arm that moves all the heads as a unit. The surface of a platter is
logically divided into circular tracks, which are subdivided into sectors.
The set of tracks that are at one arm position makes up a cylinder. There may be
thousands of concentric cylinders in a disk drive, and each track may contain hundreds
of sectors. The storage capacity of common disk drives is measured in gigabytes.
When the disk is in use, a drive motor spins it at high speed. Most drives rotate 60 to 250
times per second, specified in terms of rotations per minute (RPM).
The transfer rate is the rate at which data flow between the drive and the computer. The
positioning time, or random-access time, consists of two parts: the time necessary to
move the disk arm to the desired cylinder, called the seek time, and the time necessary
for the desired sector to rotate to the disk head, called the rotational latency.
1
,Because the disk head flies on an extremely thin cushion of air (measured in microns),
there is a danger that the head will make contact with the disk surface. Although the
disk platters are coated with a thin protective layer, the head will sometimes damage
the magnetic surface. This accident is called a head crash.
A head crash normally cannot be repaired; the entire disk must be replaced.
A disk can be removable, allowing different disks to be mounted as needed.
Removable magnetic disks generally consist of one platter, held in a plastic case to
prevent damage while not in the disk drive.
Other forms of removable disks include CDs, DVDs, and Blu-ray discs as well as removable
flash-memory devices known as flash drives.
A disk drive is attached to a computer by a set of wires called an I/O bus.
Several kinds of buses are available, including advanced technology attachment
(ATA), serial ATA (SATA), eSATA, universal serial bus (USB), and fibre channel
(FC).
The data transfers on a bus are carried out by special electronic processors called
controllers.
⮚ The host controller is the controller at the computer end of the bus.
⮚ A disk controller is built into each disk drive.
Solid-State Disks – New:
● As technologies improve and economics change, old technologies are often used
in different ways. One example of this is the increasing use of solid-state disks,
or SSDs.
● SSD is nonvolatile memory that is used like a hard drive.
● SSDs use memory technology as a small fast hard disk. Specific implementations
may use either flash memory or DRAM chips protected by a battery to sustain
the information through power cycles.
● Because SSDs have no moving parts, they are much faster than traditional hard
drives, and certain problems such as the scheduling of disk accesses simply do not
apply.
● However, SSDs also have their weaknesses: They are more expensive than hard
drives, generally not as large, and may have shorter life spans.
● SSDs are especially useful as a high-speed cache of hard-disk information that
must be accessed quickly. One example is to store file system meta-data, e.g.
directory and anode information that must be accessed quickly and often. Another
variation is a boot disk containing the OS and some application executables, but
no vital user data. SSDs are also used in laptops to make them smaller, faster, and
lighter.
● Because SSDs are so much faster than traditional hard disks, the throughput of
the bus can become a limiting factor, causing some SSDs to be connected directly
to the system bus (for example PCI)
Magnetic Tapes:
● It was used as an early secondary-storage medium. Although it is relatively
permanent and can hold large quantities of data, its access time is slow compared
with that of main memory and magnetic disk.
● random access to magnetic tape is about a thousand times slower than random
2
, access to magnetic disk, so tapes are not very useful for secondary storage.
● Tapes are used mainly for backup, for storage of infrequently used information,
and as a medium for transferring information from one system to another.
● Accessing a particular spot on a magnetic tape can be slow, but once reading or
writing commences, access speeds are comparable to disk drives.
● Capacities of tape drives can range from 20 to 200 GB and compression can
double that capacity.
Disk Structure:
● The one-dimensional array of logical blocks is mapped onto the sectors of the
disk sequentially.
● Sector 0 is the first sector of the first track on the outermost cylinder. The
mapping proceeds in order through that track, then through the rest of the tracks
in that cylinder, and then through the rest of the cylinders from outermost to
innermost.
● By using this mapping, convert a logical block number into an old-style disk
address that consists of a cylinder number, a track number within that cylinder,
and a sector number within that track.
● In practice, it is difficult to perform this translation, for two reasons.
● First, most disks have some defective sectors, but the mapping hides this by
substituting spare sectors from elsewhere on the disk.
● Second, the number of sectors per track is not a constant on some drives.
Modern disks pack many more sectors into outer cylinders than inner ones, using one
of two approaches:
⮚ With Constant Linear Velocity, CLV: the density of bits is uniform from
cylinder to cylinder. Because there are more sectors in outer cylinders, the disk
spins slower when reading those cylinders, causing the rate of bits passing under
the read-write head to remain constant. This is the approach used by modern CDs
and DVDs.
⮚ With Constant Angular Velocity, CAV: the disk rotates at a constant
angular
speed, with the bit density decreasing on outer cylinders. (These disks would have
a constant number of sectors per track on all cylinders.)
Disk Attachment:
Disk drives can be attached either directly to a particular host (a local disk) or to a
network.
Host-Attached Storage:
● Local disks are accessed through I/O Ports .
● The typical desktop PC uses an I/O bus architecture called IDE or ATA.
● This architecture supports a maximum of two drives per I/O bus.
● A newer, similar protocol that has simplified cabling is SATA.
High-end workstations and servers generally use more sophisticated I/O architectures such
as fibre channel (FC), a high-speed serial architecture that can operate over optical fiber
or over a four-conductor copper cable.
● It has two variants.
3
, ⮚ One is a large switched fabric having a 24-bit address space. This variant is
expected to dominate in the future and is the basis of storage-area networks
(SANs).
Because of the large address space and the switched nature of the
communication, multiple hosts and storage devices can attach to the fabric,
allowing great flexibility in I/O communication.
⮚ The other FC variant is an arbitrated loop (FC-AL) that can address 126
devices (drives and controllers).
A wide variety of storage devices are suitable for use as host-attached
storage. Among these are hard disk drives, RAID arrays, and CD, DVD, and tape drives. The
I/O commands that initiate data transfers to a host-attached storage device are reads and
writes of logical data blocks directed to specifically identified storage units (such as bus
ID or target logical unit).
Network-Attached Storage:
A network-attached storage (NAS) device is a special-purpose storage system that is
accessed remotely over a data network.
Clients access network-attached storage via a remote-procedure-call interface such as NFS
for UNIX systems or CIFS for Windows machines.
The remote procedure calls (RPCs) are carried via TCP or UDP over an IP network — usually
the same local- area network (LAN) that carries all data traffic to the clients.
Thus, it may be easiest to think of NAS as simply another storage-access protocol. The
network- attached storage unit is usually implemented as a RAID array with software that
implements the RPC interface.
Network-attached storage provides a convenient way for all the computers on a LAN to
share a pool of storage with the same ease of naming and access enjoyed with local
host-attached storage.
However, it tends to be less efficient and have lower performance than some direct-
attached storage options.
ISCSI is the latest network-attached storage protocol. In essence, it uses the IP network
protocol to carry the SCSI protocol. Thus, networks — rather than SCSI cables— can be
used as the interconnects between hosts and their storage. As a result, hosts can treat
their storage as if it were directly attached, even if the storage is distant from the host.
Storage-Area Network:
4
