Module 4: Intelligent Storage Systems (RAID)

Question 1

What is RAID?

Answer 1

Redundant Array of Independent Disks - combines multiple drives into RAID set for protection and performance

Question 2

How are RAID sets organized?

Answer 2

divided into multiple LUNs - each LUN connected to a host and appears as a single drive

Question 3

How does RAID help with protection and performance?

Answer 3

protects against drive failures

serves IOs from multiple drives simultaneously to help performance

Question 4

What are the techniques RAID uses to protect data loss during drive failure?

Answer 4

mirroring and parity

Question 5

How is RAID typically implemented?

Answer 5

using specialized controller called RAID controller - present either on the storage or compute system

Question 6

What is software RAID?

Answer 6

uses compute system based software to perform RAID at the OS level

Question 7

What are the different RAID techniques?

Answer 7

striping
mirroring
parity

Question 8

What is striping?

Answer 8

spreading LUN data across multiple drives to use the drives in parallel

all read/write heads work simultaneously - allows more data to be processed in shorter time

Question 9

What is the set of strips across drives in striping called?

Answer 9

stripe - stripe size is the number of blocks in a strip - every strip in stripe must have same number of blocks

For example: in a four-disk striped RAID set with a strip size of 64 KB, the stripe size is 256 KB (64 KB x 4)

Question 10

What is the reason for striping?

Answer 10

high performance - no protection

Question 11

What happens if a drive fails during striping only?

Answer 11

no way to get that data back so whole LUN is considered unusable

Question 12

What is mirroring?

Answer 12

where same data is stored on two or more disks resulting in multiple copies of the same data

Question 13

What happens during drive failure in mirroring?

Answer 13

data remains intact on the surviving disk drive - controller continues to service data requests from surviving disk

when failed disk is replaced controller copies data from surviving disk to new one

Question 14

What is a drawback of mirroring?

Answer 14

very expensive - preferred for mission critical apps where data loss can’t be afforded

write performance slows since it needs to write twice

Question 15

What is a benefit of mirorring?

Answer 15

full protection from failure
read performance improves since reads can be serviced by both disks in pair

Question 16

What is parity?

Answer 16

method to protect striped data from drive failure w/o cost of mirroring

parity = mathematical construct that allows recreation of missing data

Question 17

What is the advantage of parity?

Answer 17

ensures protection of data without maintaining full set of duplicate date

Question 18

What is a downfall of parity?

Answer 18

RAID controller needs to do more work than for mirroring - has to do drive rebuilds by making calculations which slows rebuild times

not as safe as full mirror due to longer rebuild times

Question 19

RAID 0?

Answer 19

striping

Question 20

RAID 1?

Answer 20

mirroring

Question 21

RAID 1/0?

Answer 21

mirroring + striping

high protection but expensive

Question 22

RAID 5?

Answer 22

striped set w/ independent disk access and distributed parity

Question 23

RAID 6?

Answer 23

striped set w/ independent disk access and dual distributed parity

Question 24

What are the factors in choosing RAID set?

Answer 24

app performance
data availability requirements
cost

Question 25

What is the minimum RAID drive number for RAID 1/0?

Answer 25

4 - must always be an even number

Question 26

What happens if both disks in the same mirrored pair fail in RAID 1/0?

Answer 26

data will be lost since there is no parity in this RAID level

Question 27

How does RAID 10 work?

Answer 27

data first mirrored over each pair and then both copies striped across all the drives in the set

Question 28

How do you replace a drive in RAID 1/0?

Answer 28

only the mirror is rebuilt - storage controller uses surviving disk in mirrored pair to recover

Question 29

What are the minimum number of disks in RAID 6?

Answer 29

4 - can take up to two disk failures instead of one like RAID 5

Question 30

What is the impact on performance of RAID 5?

Answer 30

every disk write manifests as 4 IOs - two reads two writes

Question 31

What is the impact on performance of RAID 6?

Answer 31

every disk write manifests as 6 IOs - 3 reads 3 writes

Question 32

What is the impact on performance of RAID 1 and RAID 1/0?

Answer 32

every disk write manifests as 2 IOs - 2 writes

Question 33

What is a write penalty in RAID?

Answer 33

every write operation translates into more IO overhead for the disks - happens in mirroring/parity RAID groups

Question 34

How is data written in a parity RAID 5 configuration?

Answer 34

new data comes in - parity computed by reading old parity and old data - manifests 2 reads after new parity computed controller completes write IO by writing the new data and new parity onto the disk - manifests 2 writes write penalty = 4

Question 35

How is data written in a parity RAID 6 configuration?

Answer 35

disk write requires 3 reads - 2 parity and 1 data calculated both new parities and controller performs 3 writes - 2 parity and 1 data

Question 36

What is a Nested RAID Level?

Answer 36

combining RAID Levels together - RAID 1/0

Question 37

What do most RAID configurations have?

Answer 37

Hot spares - can take over in case of drive failure in the RAID group

Question 38

What does a RAID controller do?

Answer 38

controls the RAID - handles all r/w operations between host and front end and r/w operations between back end and disk

Question 39

What is something always true about RAID controllers?

Answer 39

will always have two for sake of redundancy

Question 40

What helps w/ RAID controller performance?

Answer 40

has certain number of front end ports to connect and certain amount of cache to differ r/w requests to immediately handle requests to disk

Question 41

What is a LUN?

Answer 41

logical slice of a single disk host treats it as its own drive w/ specific space - has no copy on another drive so its a single point of failure

Question 42

How is LUN made?

Answer 42

LUN partioned goes through allocation process and assigns it to the front end ports via mapping LUN masking process take the LUN to the physical host

Question 43

What happens when the hosts writes to a LUN?

Answer 43

first writes the block to cache - once cache block is filled will write back to host saying operation has completed allows more blocks to be written to cache for fast performance - eventually once all cache blocks are filled RAID controller wil ldump to disk

Question 44

How does the RAID controller relate to LUNs?

Answer 44

aware LUN is unprotected on its own - will dump data from cache to unprotected LUN since in no RAID setup its the only place it can go

Question 45

Cons of no RAID setup?

Answer 45

if drive fails than its full data loss performance based off a single drive so can cause bottlenecking

Question 46

What is a RAID Group?

Answer 46

logical group of one or more drives you eventually want to slice into LUNs

Question 47

How does the hot spare process works?

Answer 47

RAID controller takes all data written onto alive drive and copies it back to cache drive sitting there doing nothing gets placed into RAID group - RAID controller copies data from cache to new drive

Question 48

What is RAID 5 best for?

Answer 48

random read and writes

Question 49

What is RAID 3 best for?

Answer 49

sequential read and writes