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6 Further System Fundamentals (HL)

6.2 Magnetic Disk Storage

6.2.1 Magnetic Disk Storage

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Basics

•Binary code is magnetised on a spinning disk.

•They are reusable (rewritable, rw).

•Non-volatile.

•Can store data either sequentially or by direct access.

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Platters

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Platters

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Tracks and sectors

•A = track

•B = sector

•C = block

•D = file

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Blocking

•Block size is effectively the smallest unit of storage as it is the number of bytes that are read or written in one R/W operation.

•They can cover more than one sector.

•In some OS’s, they are called clusters.

•4kB for Windows.

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Cylinders

•The same track on all the surfaces of all platters taken together.

•Why is this important?

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Capacity

•Capacity = number of surfaces x

number of tracks x number of sectors per track x

number of bytes per sector.

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Access

• A file may be fragmented across many sectors when written.

•Direct access is possible by indexing the track and sector of first part of the file, then following pointers to subsequent fragments.

•For sequential access, the sectors should be contiguous.

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Access time•Seek time = time taken for the read/write

head to move over the track.

•Latency = time waiting for the desired sector to spin into place.

•Transfer time = to get data to the CPU.

•Access time = seek time + latency + transfer time.

•Takes milliseconds (compare with CPU speed).