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8/3/2019 Multimedia Storage and Retrieval
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MULTIMEDIA STORAGE AND
RETRIEVAL
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INTRODUCTION
High performanceCPU,Storage Medium
Max computation rateCPUPerformance,Memory BusBandwidth,Video Bandwidth etc
Isochronous delivery of audioand video objects
Broadly usedMagnetic disks
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MAGNETIC MEDIATECHNOLOGY
Reduction in price per megabyte of high
capacity storage Low cost,High capacity
Multimedia magnetic disk storage
systemsSLEDs(Single Large ExpensiveDisks) and RAIDs(Redundant Array ofInexpensive Disks)
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HARD DISK TECHNOLOGY
Faster mass storage medium
ST506 and MFM Hard drives ESDI Hard drive
IDE
New Enhanced IDE Interface
SCSI
SCSI-2
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ST506 AND MFM Hard Drives
Interfacedefines the signalsand operation of signals b/whard disk controller and hard
disk Simple,Controls platter speed
and movement of heads for adrive
It doesnt define the format ofdata stored on the platter
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How data stored on the platter??Encoding Schemes
MFM
Parallel Data-Series of encoded
pulsesMFM(Modified Frequency Modulation)/FM
Data Separator Circuitryseparates data from sector
info
Drive Capacity--10MB-100MB
RLL(Run Length Limited)Packs 50% more bits thanMFM
Drive Capacity20MB-200MB
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ESDI (Enhanced Small DeviceInterface)-Hard Drive
Improvement over ST506Faster drives
and larger storage capacity
Converts data into serial bit streams anduses RLL for encoding
Defect mapContains locations of bad ordefective sectors on the drive
Supplies cylinder and sector information tocontroller
Data separator circuitry is a part of drive
Drive Capacity80MB-2GB
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IDE(Integrated Device Electronics)
Integrated controller
Parallel data interface-16 bit
Driver Capacity40MB-528MB Supports 2 Drives1 Master and the other Slave
Disadvantage: Jumper settings::add drivephysically removing the drive to
change jumper setting Allows only two drives
Advantage: Both hardware and software interfaces are very simplelow cost
hard disk controller
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New Enhanced IDE Interface
Maximum capacity around 8GB
Supports upto 4 drives
Also supports CD ROM and Tape drives
SCSI(Small Computer SystemInterface)
Used for large volume storage appsinRAID as well as optical disks
Drive Capacity20MB-2GB
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SCSI--2
Its SCSI1 with faster data transfer rates,widerdata paths
Two connector approach
Advantage::
SCSI-2 is back compatible with SCSI-1
Disadvantage::
Wide SCSI-2 has not happened yet
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Why SCSI1 & SCSI2 standardsimportant to multimedia??
Uses single bus for connecting different multimediai/p,o/p devices
Rich commom command set to support all devices
Offers performance upto 10MB/sec with standard 8bittransfers and 40MB/sec in fast and wide mode
Allows definition of vendor-unique command sets tocontrol special devices or special features
The tagged command queuing can improve performance
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Storage Densities and Latencies
Seek latencies::
Overlapped seek:seek on one drive ad then onsecond drive and then reconnect to first drive whenthe seek is complete
Midtransfer seek:device controller can be set to seekduring data transfer via a separate port provided on
SCSI chip
Elevator seek:A track close to the head will be readfirst and then a more distant track although the distanttrack is requested first
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Storage Densities and Latencies
Rotational latencies::
Zero latency read/write:Zero latency readsallow transferring data as soon as the headsettles instead of waiting a disk revolution forthe proper sector
Interleave factor:Keep up with the data streamwithout skipping sectors
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Storage Densities and Latencies
Transfer rate and I/O per second:rate at which data is transferredfrom the drive buffer to the host adapter memory
Max throughput:number of bytes transferred per second
Max Throughput for I/O=Block Transfer Size/Total Latency
Where
Total Latency=T1+T2+T3+T4+T5
T1=seek latencyT2=rotational latency
T3=time required to transfer data from disk to CPUs system memory
T4=firmware latency to setup transfer and complete transaction
T5=Final action on data eg:Display
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Storage Densities and Latencies
I/O per second:
Its a measure of the number of input/outputtransactions performed in a second
I/O per second=Max or overall throughput/block size
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Storage Densities and Latencies
Data Management:
A number of activities are involved in the data
management as
Command queuing:Allows execution of multiple sequentialcommands with system CPU intervention.Its helps inminimising head switching and disk rotational latency
Scatter gather:Scatter is a process whereby data is set forbest fit in the available block of memory or disk.Gatherreassembles data into contiguous blocks on disk or inmemory
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Seek ,Rotational and Data transferRelationships
Data transfer
Rotational latency
Seek latency
I/O % Totalaccesstime
Data block size increasing
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Disk Spanning
Method of attaching multiple drives to a single hostadapter
All drives appear as a single contiguous logical unit
Order of data writingFirst drive first until its full then to
2nd and so on
Good way of increasing storage capacity by adding
incremental drives
Doesnt offer fault tolerance or reliability(goes down) dueto combined MTBF
MTBF=MTBF of single drive/Total no of drives
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RAID (redundant array ofindependent disks)
Redundant array of inexpensive disks
Multiple disk database design
Not a hierarchy
7 levels (6 levels in common use) Set of physical disk drives viewed by the OS as
a single logical drive
Data are distributed across the physical drives
of an array Redundant disk capacity is used to store parity
information => data recoverability
Improve access time and improve reliability
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RAID Level 0Disk striping
Not a true member of the RAID family - does not includeredundancy to improve performance.
User and system data distributed across all disks in thearray in strips.
Imagine a large logical disk containing ALL data. This is
divided into strips (physical blocks or sectors) that aremapped round robin to the strips in the array.
A set of logically consecutive strips that maps exactly onestrip to each array member is referred to as a stripe.
+ If two different I/O requests are pending for two different
blocks of data then there is a good chance that the datawill beon different disks and can be serviced in parallel.
+ If a single I/O request is for multiple logically continuousstrips up to n strips can be handled in parallel.
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RAID Level 0
This software may execute either in the disk sub system or in a hostcomputer
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RAID Level 1Disk mirroring
Redundancy achieved through duplicating all data.
Data stripping is similar to RAID level 0.
Each logic strip is mapped to two physical disks.
Read request can be serviced from either of available 2 disks,which ever involves the minimum seek time and rotational latency
Write request requires both disks to be updated but this can bedone in parallel. (Slower write dictates overall speed).
Recover from failure is simple! (data may still be accessed fromthe second drive
Disadvantage:
Cost requires twice the disk space
Configuration is limited, so used only for system software andother highly critical files.
Improvement occurs if the application can split each read request sothat both disk members participate
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RAID Level 1
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RAID Level 2Bit interleaving ofdata
Utilizes parallel access techniques - All disks participate in the executionof every I/O request.
Spindles of individual drives are synchronized so that each disk head is inthe same position on each disk at any given time.
Data striping very small strips (single byte or word).
Error correcting code is calculated across corresponding bits on eachdisk, and the code bits are stored in corresponding bit positions onmultiple parity disks.
For Hamming Code number of parity (redundant) disks is proportionateto the log of the number of data disks.
On a single read, all disks are simultaneously accessed. The requested
data and the associated error correcting code are delivered to the arraycontroller. Array controller can detect and fix single bit errors.
For write all disks must be accessed.
Good choice only for an environment in which many errors occurtherefore not used much (given high reliability if individual disks and diskdrives).
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RAID Level 2
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RAID Level 3Parity drives
Similar to RAID 2 parallel access with data distributed in small strips.Requires only a single redundant disk because it uses a single parity bit forthe set of individual bits in the same position on all of the data disks.
If drives X0-X3 contain data, and X4 contains parity bits.X4(i) = X3(i) X2(i) X1(i) X0(i)
Redundancy in the case of disk failure, the data can be reconstructed.
If drive X1 fails it can be reconstructed as:X1(i) = X4(i) X3(i) X2(i) X0(i)
Performance can achieve high transfer rates, but only one I/O request canbe executed at one time. (Better for large data transfers in non transaction-oriented environments).
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RAID Level 4sector interleaving withdedicated parity drives
Each disk operates independently - Separate I/O requests satisfied in parallel.Suitable for applications with high I/O request rates and NOT well suited for thoserequiring high data transfer rates.Data striping. (Strips are larger than in lower RAIDs).Bit-by-bit parity strip is calculated across corresponding strips on each data disk, andstored in corresponding strip on the parity disk.Performance write penalty when I/O request is small size. Write must update userdata + corresponding parity bits.
X4(i) = X3(i) X2(i) X1(i) X0(i)If X1(i) is changed to X1(i)
X4(i) = X3(i) X2(i) X1(i) X0(i) = X4(i) X1(i) X1(i)To calculate new parity, the old user data,new user data and old parity strips must beread. Then it can update these two strips with the new data and the newly calculatedparity. Thus each strip write involves two reads and two writes.
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RAID Level 5Block interleavingof data
Same as RAID 4 but parity strips are distributed acrossall disks.Typical allocation uses round-robin.For an n-disk array, the parity strip is on a different disk forthe first n strips and the pattern then repeats.
Avoids potential bottleneck found in RAID 4.
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RAID Level 6
Two different parity calculations are carried out and stored in separateblocks on different disks.Example: XOR and an independent data check algorithm =>makes it possible to regenerate data even if two disks containinguser data fail.
No. of disks required = N + 2 (where N = number of disks required fordata).
Provides HIGH data availability.Incurs substantial write penalty as each write affects two parity blocks.
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COMPARISON OF RAID LEVELS
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COMPARISON OF RAID LEVELS
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