Chapter 10 and 11
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 (slide 16)
The one-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.
Access time has two major components:
>seek time is the time for the disk arm 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
Disk Scheduling - Shortest-Seek-Time-First (SSTF) Algorithm
>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 (slide 14)
Consider a disk queue holding requests to the following cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87. Using the FCFS scheduling algorithm, what is the order that the requests are serviced, assuming the disk head is at cylinder 88 and moving upward through the cylinders? A) 116 - 22 - 3 - 11 - 75 - 185 - 100 - 87 B) 100 - 116 - 185 - 87 - 75 - 22 - 11 - 3 C) 87 - 75 - 100 - 116 - 185 - 22 - 11 - 3 D) 100 - 116 - 185 - 3 - 11 - 22 - 75 - 87
A) 116 - 22 - 3 - 11 - 75 - 185 - 100 - 87
Which of the following disk head scheduling algorithms does not take into account the current position of the disk head? A) FCFS B) SSTF C) SCAN D) LOOK
A) FCFS
Which of the following statements regarding solid state disks (SSDs) is false? A) They generally consume more power than traditional hard disks. B) They have the same characteristics as magnetic hard disks, but can be more reliable. C) They are generally more expensive per megabyte than traditional hard disks. D) They have no seek time or latency.
A) They generally consume more power than traditional hard disks.
Disk Scheduling - FCFS
>simplest form of disk scheduling >does not provide the fastest service Ex. A disk queue with requests for I/O to blocks on Cylinders shown in the diagram (slide 13), assuming the disk head is initially at cylinder 53 >total head movement of 640 cylinders >the wild swing from 122 to 14 and then back to 124 >If the requests for cylinder 37 and 14 could be serviced together, before or after the requests at 122 and 124, the total head movement could be decreased substantially and performance could be improved
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 >sometimes called the elevator algorithm >illustration shows total head movement of 208 cylinders (slide 15)
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 (slide 17)
Computers access disk storage in two ways:
>via I/O ports (or host-attached storage) as in small systems >via a remote host in a distributed file system known as network-attached storage
What are the properties of magnetic tape?
>was early secondary-storage medium >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 >20-200GB typical storage >Common technologies are 4mm, 8mm, 19mm
What are the two components of positioning time? A) seek time + rotational latency B) transfer time + transfer rate C) effective transfer rate - transfer rate D) cylinder positioning time + disk arm positioning time
A) seek time + rotational latency
The SSTF scheduling algorithm ____. A) services the request with the maximum seek time B) services the request with the minimum seek time C) chooses to service the request furthest from the current head position D) None of the above
B) services the request with the minimum seek time
Swap space management ____. A) is a high-level operating system task B) tries to provide the best throughput for the virtual memory system C) is primarily used to increase the reliability of data in a system D) None of the above
B) tries to provide the best throughput for the virtual memory system
Consider a disk queue holding requests to the following cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87. Using the SSTF scheduling algorithm, what is the order that the requests are serviced, assuming the disk head is at cylinder 88 and moving upward through the cylinders? A) 116 - 22 - 3 - 11 - 75 - 185 - 100 - 87 B) 100 - 116 - 185 - 87 - 75 - 22 - 11 - 3 C) 87 - 75 - 100 - 116 - 185 - 22 - 11 - 3 D) 100 - 116 - 185 - 3 - 11 - 22 - 75 - 87
C) 87 - 75 - 100 - 116 - 185 - 22 - 11 - 3
Solid state disks (SSDs) commonly use the ___________ disk scheduling policy. A) SSTF B) SCAN C) FCFS D) LOOK
C) FCFS
Which RAID level is best for storing large volumes of data? A) RAID levels 0 + 1 and 1 + 0 B) RAID level 3 C) RAID level 4 D) RAID level 5
D) RAID level 5
Which of the following statements is false? A) Swapping works in conjunction with virtual memory techniques. B) Some systems allow for multiple swap spaces (disks). C) Solaris only swaps pages of anonymous memory. D) Typically, entire processes are swapped into memory.
D) Typically, entire processes are swapped into memory.
On media that uses constant linear velocity, the ____. A) disk's rotation speed increases as the head moves towards the middle of the disk from either side B) disk's rotation speed remains constant C) density of bits decreases from the inner tracks to the outer tracks D) density of bits per track is uniform
D) density of bits per track is uniform
Low-level formatting ____. A) does not usually provide an error-correcting code B) is usually performed by the purchaser of the disk device C) is different from physical formatting D) divides a disk into sections that the disk controller can read and write
D) divides a disk into sections that the disk controller can read and write
A disk drive is attached to a computer by a set of wires called an...
I/O bus
New ______ protocol uses IP network to carry the SCSI protocol
iSCSI
Boot block:
initializes all aspects of the sytem >the bootstrap is stored in ROM >>changing the bootstrap code requires changing the ROM hardware chips >>for this reason, most systems store a tiny bootstrap loader program in the boot ROM whose only job is to bring in a full bootstrap program from disk
Storage Area Network (SAN)
is a private network use storage protocols rather than networking protocols >connect servers and storage units
Network-attached storage (NAS)
is storage made available over a network rather than over a local connection (such as a bus)
Disk drives are addressed as...
large one-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer >size of logical block is usually 512 bytes
Bad blocks:
methods such as sector sparing used to handle bad blocks
High-end workstations and servers use...
more sophisticated I/O architecture such as SCSI and fiber channel (FC)
RAID
multiple disk drives provides reliability via redundancy
RAID schemes improve...
performance and improve the reliability of the storage system by storing redundant data >mirroring or shadowing keeps duplicate of each disk >block interleaved parity uses much less redundancy
Magnetic Disks
provide the bulk of secondary storage of modern computers
Head crash
results from disk head making contact with the disk surface
Solid state disks (SSDs) commonly use the FCFS disk scheduling algorithm. TRUE FALSE
TRUE
Windows allows a hard disk to be divided into one or more partitions TRUE FALSE
TRUE
Most drives rotate at..
60 to 200 times per second
RAID structure
>RAID is arranged into six different levels >several improvements in disk-use techniques involve the use of multiple disks working cooperatively >disk striping uses a group of disks as one storage unit
When selecting a disk-scheduling algorithm, remember...
>SSTF is common and has a natural appeal because it increases performance over FCFS >SCAN and C-SCAN perform better for systems that place a heavy load on the disk >>b/c they are less likely to cause a starvation problem >performance depends on the number and types of requests. >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
Properties of SAN
>common in large storage environments (and becoming more common) >multiple hosts attached to multiple storage arrays - flexible >FC is the most common SAN interconnect
OS is responsible for several aspects of disk management
>disk initialization >booting from disk >bad-block recovery
Disk formatting:
>low-level formatting or physical formatting-dividing a disk into sectors that the disk controller can read and write >to use a disk to hold files, the operating system still needs to record its own data structures on the disk. It does so in two steps: >>Partition the disk into one or more groups of cylinders >>>One partition can hold a copy of OS's executable code, while another holds user files >>Logical formatting or "making a file system"
Disk Scheduling
>minimize seek time >seek time~ seek distance >disk bandwidth is the total number of bytes transfered
Consider a disk queue holding requests to the following cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87. Using the SCAN scheduling algorithm, what is the order that the requests are serviced, assuming the disk head is at cylinder 88 and moving upward through the cylinders? A) 116 - 22 - 3 - 11 - 75 - 185 - 100 - 87 B) 100 - 116 - 185 - 87 - 75 - 22 - 11 - 3 C) 87 - 75 - 100 - 116 - 185 - 22 - 11 - 3 D) 100 - 116 - 185 - 3 - 11 - 22 - 75 - 87
B) 100 - 116 - 185 - 87 - 75 - 22 - 11 - 3
_____ is a technique for managing bad blocks that maps a bad sector to a spare sector. A) Sector slipping B) Sector sparing C) Bad block mapping D) Hard error management
B) Sector sparing
A RAID structure ____. A) is primarily used for security reasons B) is primarily used to ensure higher data reliability C) stands for redundant arrays of inexpensive disks D) is primarily used to decrease the dependence on disk drives
B) is primarily used to ensure higher data reliability
The location where Windows places its boot code is the _____. A) boot block B) master boot record (MBR) C) boot partition D) boot disk
B) master boot record (MBR)
Host-attached storage is ____. A) a special purpose storage system that is accessed remotely over a data network B) not suitable for hard disks C) accessed via local I/O ports D) not suitable for use in raid arrays
C) accessed via local I/O ports
A _____ is a private network connecting servers and storage units. A) host-attached storage B) network-attached storage C) storage-area network D) private-area network
C) storage-area network
The surface of a magnetic disk platter is divided into ____. A) sectors B) arms C) tracks D) cylinders
C) tracks
Consider a disk queue holding requests to the following cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87. Using the C-SCAN scheduling algorithm, what is the order that the requests are serviced, assuming the disk head is at cylinder 88 and moving upward through the cylinders? A) 116 - 22 - 3 - 11 - 75 - 185 - 100 - 87 B) 100 - 116 - 185 - 87 - 75 - 22 - 11 - 3 C) 87 - 75 - 100 - 116 - 185 - 22 - 11 - 3 D) 100 - 116 - 185 - 3 - 11 - 22 - 75 - 87
D) 100 - 116 - 185 - 3 - 11 - 22 - 75 - 87
RAID level ____ is the most common parity RAID system. A) 0 B) 0+1 C) 4 D) 5
D) 5
Data striping provides reliability for RAID systems. TRUE FALSE
FALSE
Disk controllers do not usually have a built-in cache. TRUE FALSE
FALSE
In asynchronous replication, each block is written locally and remotely before the write is considered complete. TRUE FALSE
FALSE
LOOK disk head scheduling offers no practical benefit over SCAN disk head scheduling. TRUE FALSE
FALSE
The operating system is responsible for using hardware efficiently:
for the disk drives, this means having a fast access time and disk bandwidth.
In Solaris, swap space is only used as a backing store for pages of anonymous memory. TRUE FALSE
TRUE
In general, LOOK disk head scheduling will involve less movement of the disk heads than SCAN disk head scheduling. TRUE FALSE
TRUE
In most RAID implementations, a hot spare disk is not used for data, but is configured for replacement should any other disk fail. TRUE FALSE
TRUE
RAID level 0 provides no redundancy. TRUE FALSE
TRUE
Can disks be removable?
Yes, allowing different disks to be mounted as needed
Clients access network attached storage via...
a remote-procedure-call interface >Implemented via remote procedure calls (RPCs) between host and storage >Common protocols are NFS (for Unix Systems) and CIFS for Windows machines.
Host-attached storage
accessed through I/O ports talking to I/O busses
Desktop PC uses...
an I/O bus architecture called IDE
What kind of buses are available?
buses vary, including enhanced integrated drive electronics, universal serial bus (USB), fibre channel (FC), SCSI
Transfer rate
the rate at which data flow between drive and computer
Positioning time (random-access time)
the time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)
disk bandwidth
total number of bytes transferred
