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ISM v5 > Module 4: Intelligent Storage Systems (RAID) > Flashcards

Module 4: Intelligent Storage Systems (RAID) Flashcards

1
Q

What is RAID?

A

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

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2
Q

How are RAID sets organized?

A

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

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3
Q

How does RAID help with protection and performance?

A

protects against drive failures

serves IOs from multiple drives simultaneously to help performance

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4
Q

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

A

mirroring and parity

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5
Q

How is RAID typically implemented?

A

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

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6
Q

What is software RAID?

A

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

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7
Q

What are the different RAID techniques?

A

striping
mirroring
parity

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8
Q

What is striping?

A

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

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9
Q

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

A

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)

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10
Q

What is the reason for striping?

A

high performance - no protection

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11
Q

What happens if a drive fails during striping only?

A

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

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12
Q

What is mirroring?

A

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

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13
Q

What happens during drive failure in mirroring?

A

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

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14
Q

What is a drawback of mirroring?

A

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

write performance slows since it needs to write twice

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15
Q

What is a benefit of mirorring?

A

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

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16
Q

What is parity?

A

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

parity = mathematical construct that allows recreation of missing data

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17
Q

What is the advantage of parity?

A

ensures protection of data without maintaining full set of duplicate date

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18
Q

What is a downfall of parity?

A

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

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19
Q

RAID 0?

A

striping

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20
Q

RAID 1?

A

mirroring

21
Q

RAID 1/0?

A

mirroring + striping

high protection but expensive

22
Q

RAID 5?

A

striped set w/ independent disk access and distributed parity

23
Q

RAID 6?

A

striped set w/ independent disk access and dual distributed parity

24
Q

What are the factors in choosing RAID set?

A

app performance
data availability requirements
cost

25
Q

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

A

4 - must always be an even number

26
Q

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

A

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

27
Q

How does RAID 10 work?

A

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

28
Q

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

A

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

29
Q

What are the minimum number of disks in RAID 6?

A

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

30
Q

What is the impact on performance of RAID 5?

A

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

31
Q

What is the impact on performance of RAID 6?

A

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

32
Q

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

A

every disk write manifests as 2 IOs - 2 writes

33
Q

What is a write penalty in RAID?

A

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

34
Q

How is data written in a parity RAID 5 configuration?

A

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

35
Q

How is data written in a parity RAID 6 configuration?

A

disk write requires 3 reads - 2 parity and 1 data

calculated both new parities and controller performs 3 writes - 2 parity and 1 data

36
Q

What is a Nested RAID Level?

A

combining RAID Levels together - RAID 1/0

37
Q

What do most RAID configurations have?

A

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

38
Q

What does a RAID controller do?

A

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

39
Q

What is something always true about RAID controllers?

A

will always have two for sake of redundancy

40
Q

What helps w/ RAID controller performance?

A

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

41
Q

What is a LUN?

A

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

42
Q

How is LUN made?

A

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

43
Q

What happens when the hosts writes to a LUN?

A

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

44
Q

How does the RAID controller relate to LUNs?

A

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

45
Q

Cons of no RAID setup?

A

if drive fails than its full data loss

performance based off a single drive so can cause bottlenecking

46
Q

What is a RAID Group?

A

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

47
Q

How does the hot spare process works?

A

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

48
Q

What is RAID 5 best for?

A

random read and writes

49
Q

What is RAID 3 best for?

A

sequential read and writes

ISM v5 (32 decks)

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