.VHDX

Introduction

VHDX is the virtual hard disk format of Microsoft’s Hyper-V, introduced with Windows Server 2012 and Windows 8 as the successor to VHD. A VHDX disk can be up to 64 TB, against about 2 TB for VHD, and it keeps a log of changes to its own tables so that a power failure does not corrupt it.

Windows can attach a VHDX file as an ordinary drive, and can even start from one. Microsoft publishes the specification as MS-VHDX.

Structure

The file is divided into regions aligned to 1 MB. It starts with a 64 KB file type identifier, which begins with the signature vhdxfile and the name of the program that made it. Two copies of the header follow, at 64 KB and 128 KB, each starting head and carrying a sequence number, so that the newer valid copy wins. Two copies of the region table, at 192 KB and 256 KB, start regi and give the position of the block allocation table and of the metadata region.

The metadata region holds the disk’s settings: its virtual size, block size, sector sizes and, for a differencing disk, the location of the parent. The block allocation table (BAT) maps each block of the virtual disk, between 1 MB and 256 MB in size, to its place in the file. The log records changes to these structures before they are made.

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