Secondary Storage - General Knowledge

Secondary Storage A BRIEF REVIEW OF HARD DISK DRIVES AND STORAGE TECHNOLOGIES

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Internal Structure

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Internal Structure (Cont.)

• Head flies above platters

• Platter are divided into circular tracks and tracks which are subdivided into sectors. The set of tracks at one arm position make cylinder.

• Logical blocks, the smallest

unit of transfer (512 bytes)

that maps to the sectors.

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Internal Structure (Speed)

• Disk speed has two parts:• Transfer Rate, the rate at each data flow between the drive and computer.

• Efficient Transfer Rate• Position Time (Random-Access Time),

• Seek time, the time necessary to move disk arm to the desired cylinder.• Rotational Latency, time necessary for the desired sector to rotate to disk head.

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Internal Structure (Head Crash - Connection)

• Head Crash, head may contact the surface

• I/O bus connects Disk Drive to the computer.• EIDE• ATA, PATA, SATA• USB• FC• SCSI• FireWire! (Developed by Apple, IEEE 1934 standard) (400 Mbps)

• A disk controller is built into each disk drive that has a cache ...

SATA

SCSI

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Magnetic Tapes

• Early secondary storages with very slow access time (1000 times slower than HDD)

• Can be used for back up or non-frequently used data.

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Disk Attachment

• Host-Attachment Storage (via I/O) common for small systems.

• Network-Attach Storage, remote host in a distributed file system• Remote-procedure call interface (NFS for UNIX, CIFS for windows)• NAS is implemented as a RAID (redundant array of independent disks) array

with software that implements RPC interface.

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NAS

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Server

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Storage Area Network

• Drawback in NAS: Storage I/O operations consume Bandwith

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SAN

• Oracle

• Google

• Hp

• …

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RAID

• Disks get smaller and cheaper

• Redundant Arrays of Independent (not completely) Disks

• Inexpensive -> Independent

• Increasing read and write rate (Parallel) - increase the throughput (load balancing) / reduce response time of large accesses

• Increasing Reliability (Redundant) – Mirroring

• RAID levels (Redundancy to Striping)

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RAID Levels – 0

• Nothing

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RAID Levels – 1

• Mirroring

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RAID Levels – 2

• Parity bits - ECC

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RAID Levels – 3

• Parity bits

• Less disks

• Much Speed

• fewer I/O

• Dedicated Parity Hardware

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RAID Levels – 4

• Like 3 but blocks in each disk

• Higher level of I/O rate

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RAID Levels – 5

• Spreading the parity blocks

• Safer - Most Common

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RAID Levels – 6

• Like 5, Parity + Reed Solomon ECC Code

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RAID Levels – 0+1 , 1+0

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Choosing a RAID Level

• Continues supply of data is needed• Rebuilding is easiest in RAID level 1

• Level 0 for high performance where data loss is not so important

• Level 1+0 and 0+1 for both Performance and reliability (ex. Small Databases)

• Level 5 can be used instead of 1

• Level 6 is not supported commonly, but it should be more reliable that level 5

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WHAT SHOULD WE THINK ABOUT?

• How many disks should be in a given RAID set? • More disks, More Data-Transfer Rate,

More Expensive

• How many bits should be protected by each parity bit?• Less bits each parity, More Chance to

Modify the Failure, More Overhead

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SSD VS. HDDSolid State Drive

Hard Disk Drive

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THE END