Sunday, September 23, 2012

VHDX Format Specification

Microsoft Downloads - VHDX Format Specification v1.00

This specification describes the VHDX virtual hard disk format that provides a disk-in-a-file abstraction. This specification assumes that you are familiar with hard disk technologies, including how hard disks interface with the operating system or a virtual machine and understand how data is accessed and laid out on the physical medium. This specification is released under the Microsoft Open Source Promise (OSP) initiative to help guide development of VHDX virtual hard disk format implementations that are compatible with those provided by Microsoft.

I'm just a spec junky.. sorry... :)

From the DocX;

1.1 Features and Benefits [of the VHDX Format]

VHDX format features provide features at the virtual hard disk as well as virtual hard disk file layers and is optimized to work well with modern storage hardware configurations and capabilities.

At the virtual hard disk layer, benefits include the ability to represent a large virtual disk size up to 64 TB, support larger logical sector sizes for a virtual disk up to 4 KB that facilitates the conversion of 4 KB sector physical disks to virtual disks, and support large block sizes for a virtual disk up to 256 MB that enables tuning block size to match the IO patterns of the application or system for optimal performance.

At the virtual hard disk file layer, the benefits include the use of a log to ensure resiliency of the VHDX file to corruptions from system power failure events and a mechanism that allows for small pieces of user generated data to be transported along with the VHDX file.

On modern storage platforms, the benefits include optimal performance on host disks that have physical sector sizes larger than 512 bytes through improved data alignment and capability to use the information from the UNMAP command, sent by the application or system using the virtual hard disk, to optimize the size of the VHDX file.

The format is designed so that additional features could be introduced in the future by Microsoft or extended by other parser implementations. The format provides parsers the ability to detect features in a VHDX file that a parser does not understand.


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