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Question

Complete the table shown below by inserting the time taken for each of the following RAID

systems and file

access requests. Each RAID system is to contain four disks worth of data (that is

data relating to the database). This means that each system will consist of at least four disks and,

in some cases, one or more additional disks containing redundant data. Note that this question

contains a fair amount of information, please read it carefully.


RAID Systems

▪ RAID 0

▪ RAID 1+0

▪ RAID 4

▪ RAID 5


Striping

▪ For RAID 0, RAID 1+0 and RAID 4 blocks of the file are striped in a round robin fashion across

the data disks starting with disk 0

o disk 0 - block 0 contains block 0 of the file

o disk 1 - block 0 contains block 1 of the file

o disk 2 - block 0 contains block 2 of the file

o ...

▪ For RAID 5 blocks of the file are striped in a round robin fashion across the system except that,

for each set of same numbered blocks in the system's disks, one disk contains parity data.

The parity data is also distributed across the disks in a round robin fashion starting with disk

0

o disk 0 - block 0 contains parity data

o disk 1 - block 0 contains block 0 of the file

o disk 2 - block 0 contains block 1 of the file

o ...


Requests

Requests are specified as blocks of a file (whose first block is block 0) that is striped across the

RAID systems.

▪ Read blocks 0 to 15

▪ Read blocks 1, 4, 10 and 15

▪ Read blocks 0, 6, 10 and 18

▪ Write blocks 0 to 15

▪ Write blocks 1, 4, 10 and 15

▪ Write blocks 0, 6, 10 and 18


▪ All writes must read the data before writing it except for writes to redundant data which

should only read data when necessary (that is, necessary to allow for the greatest amount of

parallelism).

Metrics


▪ Your answer should specify time as a fraction of the time that a single disk would take to

perform the same request. For example, a four data disk RAID 0 system can read blocks 0 to

15 of the file four times as fast as a single disk so write ¼ in the appropriate place in the table.


▪ Assume that it takes the same time to read/write any block, regardless of its position on

the disk, such that time is to be measured in units of disk reads or writes. The components of

those reads or writes (i.e. seek time, rotational delay and transfer time) should not be

considered. To elaborate the previous example of reading blocks 0 to 15, a single disk would

require 16 reads. The equivalent RAID 0 system would require 4 parallel reads of a sequence

of 4 blocks each and would therefore require ¼ of the time as the single disk.


▪ The cost is to be specified as a multiple of the cost of a single disk, and only considers the

number of disks required for the RAID system (not the RAID controller). For example, a four

data disk RAID 0 requires four disks and is therefore four times the cost of a single disk so

write 4 in the appropriate place in the table.


▪ When assessing the time taken to perform writes your answer should comply with the

following rule: data disks may not be written to at the same time as check disks. This means

that for writes no check disk should be written to during the write of any data disk though

data disks may be written to in parallel with other data disks and check disks may be written

to in parallel with other check disks. This rule does not mention reads so any read may be

performed in parallel with any read or write. RAID systems with distributed parity do not have

dedicated check disks. For such systems if a disk is writing (DB) data it is counted as a data

disk, whereas if it is writing parity (check) data it is counted as a check disk.


Table

Submit a table with the following rows and columns.


COLUMNS (4):

RAID 0, RAID 1+0, RAID 4, RAID 5


ROWS (7):

Read 0 to 15, Read 1, 4, 10 and 15, Read 0, 6, 10 and 18, Write 0 to 15, Write 1, 4, 10 and 15, Write 0, 6, 10 and 18, Cost


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