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22 Cards in this Set

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  • Back

What is transfer rate?

rate at which data flows between drive and computer

What is positioning time?

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).

What is a head crash?

results from the disk head making contact with the disk surface.

How are disk drives addressed?

as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer.

Sector 0 is the first sector of the first track on the outermost cylinder.

Why is access to the disk scheduled?

Since the OS is responsible for using hardware efficiently, we need to have fast access time and disk bandwidth to the disk.

What are the two major components of access time?

Seek time - the time for the disk arm to move the heads to the cylinder containing the desired sector.

Rotational latency - the additional time waiting for the disk to rotate the desired sector to the disk head.

What is FCFS disk scheduling and what are its pros/cons?

The head moves to the first disk request on the queue.

Easy to implement, but doesn't have good performance

What is SSTF and what are its pros and cons?

Shortest Seek Time First selects the request with the minimum seek time from the current head position.

Has good performance in terms of # of tracks traversed.

Like SJF, SSTF may cause starvation of some request (sticky head).

What is SCAN and what are its pros/cons?

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 (elevator algorithm).

Uneven waiting time.

If a request arrives in the queue just in front of the head, it will be serviced quickly. A request arriving just behind the head has to wait for the arm to come back after hitting one of the ends of the cylinder.

What is C-SCAN and what are its pros/cons?

Provides a more uniform wait time than SCAN because the head moves from one end of the disk to the other, servicing request as it goes and when it reaches the other end, it immediately returns to the beginning of the disk without servicing any requests on the return trip.

The heaviest density of requests is at the opposite end of the disk.

What is C-LOOK?

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.

When do you use each disk-scheduling algorithm?

SSTF is common and has 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

Why do we use a RAID (Redundant Array of Inexpensive Disks)?

Uses multiple disk drives to provide high availability via redundancy, performance or a combination of both.

Increases the mean time to failure.

Protects against data loss.

Multiple drives working in parallel increasing system performance.

What is Raid 0?

RAID 0 stripes disks which improves r/w performance but provides no redundancy / fault tolerance. Drive failure will result in data loss for all drives.

It is cost effective for the r/w performance and utilizes full disk capacity.

What is RAID 1?

RAID 1 mirrors disk which increases reliability.

It is easy to implement and is fault tolerant since we have multiple copies of the same data.

It is inefficient use of disk space and doubles the number of writes.

What is the differences between RAID 0-1 vs. RAID 1-0?

0-1 - Stripe first, then mirror

1-0 - Mirror first and then stripe

What is RAID2?

Raid 2 uses striping at the bit level (as opposed to at the block level) and also provides ECC (memory style error-correcting code) by using a parity bit.

There is significant overhead in using RAID2 to implement the ECC for every disk drive.

How is the parity bit calculated and checked?

Calculating the parity bit for an even number of data bits. If the total number of bits is even, the parity bit is set to 1. Therefore the total number of bits sent (including the parity) is odd.

When validating the set of bits on the receiving end, the group of bits is checked to see if the group totals to an odd number. If the total is even, a transmission error occurred.

What is RAID3?

Improves on RAID2, using bit interleaving parity by using disk controllers which can detect whether a sector has been read correctly. So a single parity bit can be used for error correction as well as detection. If one of the sectors is damaged, we know exactly which sector it is. RAID 3 uses a signel parity disk.

Advantages over RAID1: only one parity disk needed for several regular disk, r/w is improved since they are spread out over multiple disks.

Disadvantages: fewer I/Os per second since every disk has to participate in every I/O request. Slower writes than with non-parity RAID arrays.

What is RAID4?

Uses block level striping and block level interleaving parity.

Writing to a RAID4 has a bottleneck since writing to disk means also writing to the parity disk.

What is RAID5?

Uses block-interleaved distribution parity meaning it spreads data and parity among N+1 disks rather than storing parity in one disk.

By spreading the parity across all the disks, RAID 5 avoids potential overuse of a single parity disk (which can occur in RAID 4).

What are the problems with RAID?

RAID protects against physical media errors, but not other hardware and software errors.

RAID does not always assure that data are available for OS and its users because a pointer to a file could be wrong.

Incomplete writes could result in corrupt data.