Post on 30-May-2018
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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