RAID

1
RAID

• COP 5611
• Advanced Operating Systems
• Adapted from Andy Wangs slides at FSU

2
Parallel Disk Access and RAID

• One disk can only deliver data at its maximum
  rate
• So to get more data faster, get it from multiple
  disks simultaneously
• Saving on rotational latency and seek time

3
Utilizing Disk Access Parallelism

• Some parallelism available just from having
  several disks
• But not much
• Instead of satisfying each access from one disk,
  use multiple disks for each access
• Store part of each data block on several disks

4
Disk Parallelism Example

open(foo)
read(bar)
write(zoo)
File System
5
Data Striping

• Transparently distributing data over multiple
  disks
• Benefits
• Increases disk parallelism
• Faster response for big requests
• Major parameters are number of disks and size of
  data interleaf

6
Fine-Grained Vs. Coarse-Grained Data Interleaving

• Fine grain data interleaving
• High data rate for all requests
• But only one request per disk array
• Lots of time spent positioning
• Coarse-grain data interleaving
• Large requests access many disks
• Many small requests handled at once
• Small I/O requests access few disks

7
Reliability of Disk Arrays

• Without disk arrays, failure of one disk among N
  loses 1/Nth of the data
• With disk arrays (fine grained across all N
  disks), failure of one disk loses all data
• N disks 1/Nth as reliable as one disk

8
Adding Reliability to Disk Arrays

• Buy more reliable disks
• Build redundancy into the disk array
• Multiple levels of disk array redundancy possible
• Most organizations can prevent any data loss from
  single disk failure

9
Basic Reliability Mechanisms

• Duplicate data
• Parity for error detection
• Error Correcting Code for detection and correction

10
Parity Methods

• Can use parity to detect multiple errors
• But typically used to detect single error
• If hardware errors are self-identifying, parity
  can also correct errors
• When data is written, parity must be written, too

11
Error-Correcting Code

• Based on Hamming codes, mostly
• Not only detect error, but identify which bit is
  wrong

12
RAID Architectures

• Redundant Arrays of Independent Disks
• Basic architectures for organizing disks into
  arrays
• Assuming independent control of each disk
• Standard classification scheme divides
  architectures into levels

13
Non-Redundant Disk Arrays (RAID Level 0)

• No redundancy at all
• So, what we just talked about
• Any failure causes data loss

14
Non-Redundant Disk Array Diagram (RAID Level 0)

open(foo)
read(bar)
write(zoo)
File System
15
Mirrored Disks (RAID Level 1)

• Each disk has second disk that mirrors its
  contents
• Writes go to both disks
• No data striping
• Reliability is doubled
• Read access faster
• - Write access slower
• - Expensive and inefficient

16
Mirrored Disk Diagram (RAID Level 1)
open(foo)
read(bar)
write(zoo)
File System
17
Memory-Style ECC (RAID Level 2)

• Some disks in array are used to hold ECC
• E.g., 4 data disks require 3 ECC disks
• More efficient than mirroring
• Can correct, not just detect, errors
• - Still fairly inefficient

18
Memory-Style ECC Diagram (RAID Level 2)
open(foo)
read(bar)
write(zoo)
File System
19
Bit-Interleaved Parity (RAID Level 3)

• Each disk stores one bit of each data block
• One disk in array stores parity for other disks
• More efficient that Levels 1 and 2
• - Parity disk doesnt add bandwidth
• - Cant correct errors

20
Bit-Interleaved RAID Diagram (Level 3)
open(foo)
read(bar)
write(zoo)
File System
21
Block-Interleaved Parity (RAID Level 4)

• Like bit-interleaved, but data is interleaved in
  blocks of arbitrary size
• Size is called striping unit
• Small read requests use 1 disk
• More efficient data access than level 3
• Satisfies many small requests at once
• - Parity disk can be a bottleneck
• - Small writes require 4 I/Os

22
Block-Interleaved Parity Diagram (RAID Level 4)
open(foo)
read(bar)
write(zoo)
File System
23
Block-Interleaved Distributed-Parity (RAID Level
5)

• Sort of the most general level of RAID
• Spread the parity out over all disks
• No parity disk bottleneck
• All disks contribute read bandwidth
• Requires 4 I/Os for small writes

24
Block-Interleaved Distributed-Parity Diagram
(RAID Level 5)
open(foo)
read(bar)
write(zoo)
File System
25
Where Did RAID Look For Performance Improvements?

• Parallel use of disks
• Improve overall delivered bandwidth by getting
  data from multiple disks
• Biggest problem is small write performance
• But we know how to deal with small writes . . .