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InstituteName: S.J.V.P.M,Polytechnic

Under Guidence:Tingre S.S.Presenter: More Rahul B.Branch:CO6C

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Overview of Virtual Hard Disk Image

Types:

Fixed hard disk imageDynamic hard disk image

Differencing hard disk image

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FixedHard Disk

ImageAfixed harddisk image is afile that isallocated to thesize of the

virtual disk.

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Dynamic HardDisk Image

Adynamic hard

disk image is afile that at anygiven time is as

large as theactual data

written to itplus

the size of theheader andfooter.

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Differencing

Hard DiskImage

Adifferencing harddisk imagerepresents the

current state of thevirtual hard disk as aset of modifiedblocks in

comparison to aparent image.

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Another Example

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All hard disk images share a basic footerformat. Each hard disk type extends thisformat according to its needs.

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F

or dynamic and differencing disk images, theData Offset field within the image footerpoints to a secondary structure that providesadditional information about the disk image.

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Cookie

Data Offset

Table Offset

Header Version

Max Table Entries

Block Size

Checksum

Parent Unique ID

Parent Time Stamp

Reserved

Parent Unicode Name

Platform Code. Parent Locator Entries

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Block Allocation

Table and DataBlocks

The Block

Allocation Table(BAT) is a table ofabsolute sectoroffsets into the file

backing the harddisk. It is pointed toby the Table Offsetfield of the DynamicDisk Header.

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Blocks are allocated on demand. When a dynamic disk iscreated, no blocks are allocated initially. Anewly created

image contains only the data structures described earlier(including the Dynamic Disk Header and the BAT).

When data is written to the image, the dynamic disk isexpanded to include a new block. The BAT is updated tocontain the offset for each new block allocated within theimage

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Versions prior to Microsoft Virtual Server 2005 supportedsplitting of disk images, if the disk image grew larger than the

maximum supported file size on the host file system.

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Adifferencing hard disk stores the file locator ofthe parent hard disk inside the differencing hard

disk itself. When a virtual machine tries to open adifferencing hard disk, both the differencing harddisk and the parent hard disk are opened. Theparent hard disk can also be a differencing hard

disk, in which case there could be a chain ofdifferencing hard disks which finally end in a non-differencing hard disk.

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For a write operation, all data is written tothe differencing hard disk image. The blockbitmap is marked dirty for all the sectors

written to the particular block.

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When a virtual machine reads sectors of ahard disk image, the differencing hard disk

subsystem checks the block bitmap in thedifferencing hard disk. The differencinghard disk subsystem reads the sectors

marked dirty from the differencing harddisk and the sectors marked clean from theparent hard disk.

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Every hard disk has a UUID stored in thehard disk footer. When a differencing hard

disk is created, it stores the UUID of theparent hard disk inside the differencinghard disk. The UUID and the parent disk

name is used to recognize the parent harddisk.

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THANK YOU