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Using Hyper-V with large-sector drives in Windows Server 2008 and Windows Server 2008 R2
Article ID: 2515143 - View products that this article applies to.
This article describes the issues and guidance that are associated with running Hyper-V in Windows Server 2008 and Windows Server 2008 R2.
Over the next few years, the data-storage industry will be transitioning the physical format of hard disk drives from 512-byte sectors to 4,096-byte sectors (also known as 4K sectors). This transition is driven by several factors. These include increases in storage density and reliability.
However, most of the software industry depends on disk sectors that are 512 bytes long. Therefore. a change in this sector size would introduce major compatibility issues in many applications. To minimize the effect on the ecosystem, hard disk vendors are introducing transitional 512-byte emulation drives (also known as 512e drives). These drives have some of the advantages of 4 kilobyte (KB) native drives, such as improved format efficiency and an improved ECC scheme. And, they have fewer compatibility issues than they would have if they exposed a 4 KB sector size at the disk interface.
Some basic disk terminology that is used in this document is as follows:
Physical disks. Disks that are exposed by the physical hardware.
Virtual disks. Disks that are created by the software (VHD) stack.
Logical sector size. Minimum required alignment of IO that is exposed by a disk. Any IOs that do not confirm to this rule will fail.
Physical sector size. Actual physical sector size of storage data on a disk. This is the quantum of data access from the disk.
512e disks. Disks that directly report a 512-byte logical sector size but have a physical sector size of 4 KB – Firmware translate 512 byte writes to 4k writes RMW (Read Modify Write). In today’s drives, this translation introduces a performance penalty.
Native 4K disks. Disks that directly report a 4 KB logical sector size and have a physical sector size of 4 KB – The disk can accept only 4 KB IOs to the disks. However, the software stack can provide 512-byte logical sector size support through RMW support.
Effect when hosting VHDs on 512e disks512e disks can perform writes only on a physical sector. That is, a 512e disk cannot directly write a 512-byte sector write that is issued to it. The internal process in the disk that makes this write possible includes the following three steps:
Effect when hosting VHDs on native 4K disksThe VHD driver today assumes that the size of the physical sector of the disk is 512 bytes and issues 512-bytes IOs. This makes the VHD driver incompatible with these disks. Therefore, the VHD stack does not open the VHD files on physical 4 KB sector disks.
Recommended actions when you use large-sector disks
(http://support.microsoft.com/kb/2510009/ )Microsoft support policy for 4K sector hard drives in Windows
(http://go.microsoft.com/fwlink/?LinkId=151500)for other considerations.