1

Recap(RAID and Storage

Architectures)

2

RAID• To increase the availability and the performance

(bandwidth) of a storage system, instead of a single disk, a set of disks (disk arrays) can be used.

• However, the reliability of the system drops (n devices have 1/n the reliability of a single device).

• Reliability of N disks = Reliability of 1 Disk ÷N

– 50,000 Hours ÷ 70 disks = 700 hours

• Disk system Mean Time To Failure (MTTF): Drops from 6 years to 1 month!

3

RAID-0

Strip 12Strip 8Strip 4Strip 0

Strip 13Strip 9Strip 5Strip 1

Strip 14Strip 10Strip 6Strip 2

Strip 15Strip 11Strip 7Strip 3

• Striped, non-redundantExcellent data transfer rate Excellent I/O request processing rate Not fault tolerant

• Typically used for applications requiring high performance for non-critical data

4

RAID 1 - Mirroring

• Called mirroring or shadowing, uses an extra disk for each disk in the array (most costly form of redundancy)

• Whenever data is written to one disk, that data is also written to a redundant disk: good for reads, fair for writes

• If a disk fails, the system just goes to the mirror and gets the desired data.

• Fast, but very expensive.• Typically used in system drives and critical files

– Banking, insurance data– Web (e-commerce) servers

Strip 3Strip 2Strip 1Strip 0

Strip 3Strip 2Strip 1Strip 0

5

RAID 2: Memory-Style ECC

f0(b)b2b1b0 b3f1(b) P(b)

Data Disks Multiple ECC Disks and a Parity Disk

• Multiple disks record the (error correcting code) ECC information to determine which disk is in fault• A parity disk is then used to reconstruct corrupted or lost data

Needs log2(number of disks) redundancy disks

•Least used since ECC is irrelevant because most new Hard drives support built-in error correction

6

RAID 3 - Bit-interleaved Parity

• Use 1 extra disk for each array of n disks.• Reads or writes go to all disks in the array, with the extra

disk to hold the parity information in case there is a failure.• Performance of RAID 3:

– Only one request can be serviced at a time– Poor I/O request rate– Excellent data transfer rate– Typically used in large I/O request size applications, such as imaging

or CAD

100100111100110110010011

. . .

Logical record

1 0 0 1 0 0 1 1 0 1 1 0 0 1 1 0 1 1 1 0 0 1 0 0 1 1 0

Striped physicalrecords

P

Physical record

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RAID 4: Block Interleaved Parity

block 0

block 4

block 8

block 12

block 1

block 5

block 9

block 13

block 2

block 6

block 10

block 14

block 3

block 7

block 11

block 15

P(0-3)

P(4-7)

P(8-11)

P(12-15)

• Allow for parallel access by multiple I/O requests

• Doing multiple small reads is now faster than before.

•A write, however, is a different story since we need to update the parity information for the block.

•In this case the parity disk is the bottleneck.

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RAID 5 - Block-interleaved Distributed Parity

• To address the write deficiency of RAID 4, RAID 5 distributes the parity blocks among all the disks.

• This allows some writes to proceed in parallel

– For example, writes to blocks 8 and 5 can occur simultaneously.

– However, writes to blocks 8 and

11 cannot proceed in parallel.

0

4

8

12

20

. . .

1

5

9

16

21

. . .

2

6

13

17

22

. . .

3

10

14

18

23

. . .

7

11

15

19

. . .

P4

P3

P2

P1

P0

P5

R A ID 5

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Performance of RAID 5 - Block-interleaved Distributed Parity

• Performance of RAID 5– I/O request rate: excellent for reads, good for writes– Data transfer rate: good for reads, good for writes– Typically used for high request rate, read-intensive

data lookup– File and Application servers, Database servers,

WWW, E-mail, and News servers, Intranet servers

• The most versatile and widely used RAID.

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Which Storage Architecture?

• DAS - Directly-Attached Storage

• NAS - Network Attached Storage

• SAN - Storage Area Network

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Storage Architectures(Direct Attached Storage (DAS))

Unix

NetWare

NT/W2K

NetWare Server

Storage

NetWare Server

Storage

NT Server

Storage

NT Server

Storage

Virtual Drive 3Virtual Drive 3Unix Server

Storage

Unix Server

Storage

12

DAS

CPUs

Bus

Memory

SCSI Adaptor

SCSI Adaptor

SCSI Disk Drive

NIC

SCSI protocol

MS Windows

Traditional Server

BlockI/O

13

Storage Architectures(Network Attached Storage (NAS))

Hosts

IP Network

NAS Controller Disk subsystem

SharedInformation

14

NAS

CPUs

Bus

MemoryNIC

MS Windows

“Diskless” App Server(or rather a “Less Disk” server)

IP networkFile protocol (CIFS, NFS)

CPUs

Bus

MemoryNIC

Optimised OS

NAS appliance

SCSI Adaptor

SCSI Adaptor

SCSI Disk Drive

SCSI Adaptor

SCSI Adaptor

SCSI Disk Drive

SCSI protocolBlockI/O

15

The NAS Network

IP network

App Server App Server App ServerNAS Appliance

NAS - truly an appliance

16

Storage Architectures(Storage Area Networks (SAN))

StorageNetwork

Hosts

IPNetwork

Clients

SharedStorage

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SAN- Fibre Channel (FC)

CPUs

Bus

Memory

SCSI Adaptor

SCSI Adaptor

SCSI Disk Drive

NIC

SCSI protocol

MS Windows

DAS

CPUs

Bus

Memory

FC HBA(Host Bus Adaptor)

NIC

MS Windows

Server with FC

FC Adaptor

SCSI Disk Drive

Remote-ishStorage Unit

(to 30 metres)

SCSI Adaptor

SCSI overFC ProtocolBlock I/O

(to 3 metres)

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FC-based SAN

FC SwitchFabric

IP network

App server App server App server App server

FC StorageSub-system

FC StorageSub-system

FC StorageSub-system

FCBackupSystem