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Secondary Storage Unit 013: Systems Architecture Workbook: Secondary Storage 1G

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Notes Available in your SharePoint work space: This presentation Systems Architecture Secondary Storage 1G Workbook Take notes

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This topic covers: Disk Drives Different types of hard drive Fundamentals Characteristics RAID

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Secondary Storage characteristics Not directly accessible by the CPU Non Volatile Less expensive than primary storage Therefore much larger in capacity Used as part of virtual memory

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Main Memory Mass storage device Removable media drive Removable Medium Removable Medium Removable Medium Removable Medium Removable Medium Robotic access system Tertiary Storage Removable media drive CD-RW, DVD-RW Memory bus Removable Medium Offline Storage Primary Storage Secondary Storage

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Main Memory Mass storage device Removable media drive Removable Medium Removable Medium Removable Medium Removable Medium Removable Medium Robotic access system Tertiary Storage Removable media drive CD-RW, DVD-RW Memory bus Removable Medium Offline Storage Primary Storage Secondary Storage

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Hard Disk Drives Modern computers use disk drives as secondary storage Mechanical Based on rotating magnetic disks Inexpensive Order of magnitude slower access times than primary storage Typically in the order of thousands of a second compared to billionths of a second Mitigated by transferring data in large contiguous blocks

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HDD Operation Data is recorded by magnetising a thin film of ferromagnetic layer on a disk Data is encoded in a special code (zoned bit recording) Failures are divided into two main types: Predicable Degradation, wear and tear Unpredictable Sudden and without warning Mechanical failure accounts for 60% of HD drive failures SMART Self Monitoring, Analysis and Reporting Technology Used to Detects and reports errors indicating an impending failure Modern versions attempt to correct media errors

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Important HD Characteristics Seek time How long does it take between when the CPU requests a file and it receives the first byte Physical format Disks come in standard formats: 3 , 2 , 1.8 Data transfer rate How quickly can you get data in and out of the device Capacity How much data can it store Interface How does it connect to the computer

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HDD Layout

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Formatting Low level formatting The media is divided into tracks and sectors Performed by the manufacturer Zoned bit recording in which tracks on the outside contain more data High level formatting The file system structures are laid out This uses some of the space to organise files on the disk, files names and sequences of disk areas for those files Different OSs use different file systems

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Types of HDD (today) IDE Advanced Technology Attachment Also called EIDE, ATA, ATAPI, PATA SATA Serial ATA SCSI Small Computer Systems Interface SAS Serial Attached SCSI FLASH

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ATA (IDE, ATAPI. PATA, EIDE) Disadvantages Low transfer rate 5 to 133 MB per second Only one device on the ATA cable is able to read or write at any one time Limited cable length (18 inches) Advantages Low costs Large capacity

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Drive Arrangement Slave Master

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SATA Disadvantages Slower than SCSI Not supported in older systems Advantages Low cost Fast transfer rate compared to ATA 150MB to 600MB per second Smaller cables for better heat dissipation

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Drive Arrangement Point to point architecture

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SCSI Disadvantages Expensive Not widely supported Many different kinds of SCSI interface Higher rpm therefore more noise Advantages Fast transfer rate up to 640MB per second Better reliability More flexible in arrays (RAID) 16 devices per channel

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Drive Arrangment

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SAS Disadvantages SAS controllers cannot run SATA drives Disks are expensive - 236 for 300GB 15k rpm Advantages Point to point architecture SAS controllers can run SATA drives Support for over 65,000 devices Software transparent with SCSI Cable length of up to 10m

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Drive Arrangement SATA SAS

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Flash Disadvantages Expensive New Advantages Very, very fast Approaching RAM speed Can be in the same form factor as conventional disk drives Different form factors Different interfaces

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Practical 1 Dismantle a PC and report on the following: Type of HD installed Maximum data transfer rate HDD interfaces supported by your motherboard The maximum number of HDDs your system can support internally

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Issues with HDs Reliability Mechanical device Performance Far slower than Primary Storage The one part of a computer that acts as a bottleneck Key non-volatile device

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RAID Redundant Array of Inexpensive Disks Improves performance, data security or both Based on the premise that disks are cheap and fail often Involves additional administration and software Raid 0 = Partially writes to multiple disks Raid 1 = Writes to multiple disks at once Raid 5 = Uses a parity disk to protect against a single disk failure

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RAID 1 - Mirroring PC Controller DisksData = 200% 100%

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RAID 1 - Mirroring PC Controllers DisksData = 200% 100%

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RAID 0 - Striping PC Controller DisksData = 100% 50%

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RAID 0 - Striping PC Controllers DisksData = 100% 50%

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RAID 0+1 Mirroring plus Striping PC Controller DisksData = 200% 50%

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RAID 0+1 Mirroring plus Striping PC Controllers DisksData = 200% 50%

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RAID 5 PC Controller Disks Data = 100% 33.3% Parity

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Practical 2 A customer has a PC with two IDE drives configured. What is the most cost effective way to improve performance? What is the most cost effective way to improve data security? If money were no object what could you do to improve performance and data security for this customer?