Chapter 12: Mass-Storage
Systems
Operating System Concepts – 8th Edition Silberschatz, Galvin and Gagne ©2009
Chapter 12: Mass-Storage Systems
Overview of Mass Storage Structure
Disk Structure
Disk Attachment
Disk Scheduling
Disk Management
Swap-Space Management
12.2
Objectives
Describe the physical structure of secondary and tertiary storage
devices and the resulting effects on the uses of the devices
Explain the performance characteristics of mass-storage devices
12.3
Overview of Mass Storage Structure
Magnetic disks provide bulk of secondary storage of modern computers
Drives rotate at 60 to 200 times per second
Transfer rate is rate at which data flow between drive and computer
Positioning time (random-access time) is time to move disk arm to
desired cylinder (seek time) and time for desired sector to rotate
under the disk head (rotational latency)
Head crash results from disk head making contact with the disk
surface
That’s bad
Disks can be removable
Drive attached to computer via I/O bus
Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel,
SCSI
Host controller in computer uses bus to talk to disk controller built
into drive or storage array
12.4
Moving-head Disk Mechanism
12.5
Overview of Mass Storage Structure (Cont.)
Magnetic tape
Was early secondary-storage medium
Relatively permanent and holds large quantities of data
Access time slow
Random access ~1000 times slower than disk
Mainly used for backup, storage of infrequently-used data, transfer
medium between systems
Kept in spool and wound or rewound past read-write head
Once data under head, transfer rates comparable to disk
20-200GB typical storage
Common technologies are 4mm, 8mm, 19mm, LTO-2 and SDLT
12.6
Disk Structure
Disk drives are addressed as large 1-dimensional arrays of logical
blocks, where the logical block is the smallest unit of transfer
The 1-dimensional array of logical blocks is mapped into the sectors of
the disk sequentially
Sector 0 is the first sector of the first track on the outermost
cylinder
Mapping proceeds in order through that track, then the rest of the
tracks in that cylinder, and then through the rest of the cylinders
from outermost to innermost
12.7
Disk Attachment
Host-attached storage accessed through I/O ports talking to I/O busses
SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator requests
operation and SCSI targets perform tasks
Each target can have up to 8 logical units (disks attached to device
controller
FC is high-speed serial architecture
Can be switched fabric with 24-bit address space – the basis of storage
area networks (SANs) in which many hosts attach to many storage
units
Can be arbitrated loop (FC-AL) of 126 devices
12.8
Network-Attached Storage
Network-attached storage (NAS) is storage made available over a
network rather than over a local connection (such as a bus)
NFS and CIFS are common protocols
Implemented via remote procedure calls (RPCs) between host and
storage
New iSCSI protocol uses IP network to carry the SCSI protocol
12.9
Storage Area Network
Common in large storage environments (and becoming more common)
Multiple hosts attached to multiple storage arrays - flexible
12.10
Disk Scheduling
The operating system is responsible for using hardware efficiently —
for the disk drives, this means having a fast access time and disk
bandwidth
Access time has two major components
Seek time is the time for the disk are to move the heads to the
cylinder containing the desired sector
Rotational latency is the additional time waiting for the disk to
rotate the desired sector to the disk head
Minimize seek time
Seek time seek distance
Disk bandwidth is the total number of bytes transferred, divided by the
total time between the first request for service and the completion of
the last transfer
12.11
Disk Scheduling (Cont.)
Several algorithms exist to schedule the servicing of disk I/O requests
We illustrate them with a request queue (0-199)
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
Total head movement 640 cylinders
12.12
FCFS
Illustration shows total head movement of 640 cylinders
12.13
SSTF
Selects the request with the minimum seek time from the current head
position
SSTF scheduling is a form of SJF scheduling; may cause starvation of
some requests
Illustration shows total head movement of 236 cylinders
12.14
SSTF (Cont.)
12.15
SCAN
The disk arm starts at one end of the disk, and moves toward the other
end, servicing requests until it gets to the other end of the disk, where
the head movement is reversed and servicing continues.
SCAN algorithm Sometimes called the elevator algorithm
Illustration shows total head movement of 208 cylinders
12.16
SCAN (Cont.)
12.17
C-SCAN
Provides a more uniform wait time than SCAN
The head moves from one end of the disk to the other, servicing
requests as it goes
When it reaches the other end, however, it immediately returns to
the beginning of the disk, without servicing any requests on the
return trip
Treats the cylinders as a circular list that wraps around from the last
cylinder to the first one
12.18
C-SCAN (Cont.)
12.19
C-LOOK
Version of C-SCAN
Arm only goes as far as the last request in each direction, then
reverses direction immediately, without first going all the way to the
end of the disk
12.20
C-LOOK (Cont.)
12.21
Selecting a Disk-Scheduling Algorithm
SSTF is common and has a natural appeal
SCAN and C-SCAN perform better for systems that place a heavy
load on the disk
Performance depends on the number and types of requests
Requests for disk service can be influenced by the file-allocation
method
The disk-scheduling algorithm should be written as a separate
module of the operating system, allowing it to be replaced with a
different algorithm if necessary
Either SSTF or LOOK is a reasonable choice for the default algorithm
12.22
Suppose that a disk drive has 5,000 cylinders, numbered 0 to 4,999. The drive is
currently serving a request at cylinder 150, and the previous request was at
cylinder 805. The queue of pending requests, in FIFO
order, is:
069, 212, 296, 800, 544, 618, 356, 523, 965, 3681
Starting from the current head position, what is the total distance (in cylinders) that
the disk arm moves to satisfy all the pending requests for each of the following
disk-scheduling algorithms?
a. FCFS
b. SSTF
c. SCAN
d. LOOK
e. C-SCAN
f. C-LOOK
12.23
