Using Hyper-V with large sector drives on Windows Server 2008 and Windows Server 2008 R2

This article details the issues and guidance associated with running Hyper-V on 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 4096-byte sectors (also known as 4k sectors). This transition is driven by several factors, including increases in storage density and reliability. 

However, most of the software industry has depended on disk sectors being 512 bytes in length, and thus a change in this sector size would introduce major compatibility issues in many applications. In order to minimize the impact on the ecosystem, hard drive vendors are introducing transitional “512 byte emulation drives”, or named “512e” for short. These drives have some of the advantages which 4kB native drives have, such as improved format efficiency and an improved ECC scheme, but with fewer compatibility issues than by exposing a 4kB sector size at the disk interface. 

The table below introduces some basic disk terminologies that will be referred to in the rest of the document:

Impact When Hosting VHDs On 512e Disks

512e disks can only perform writes in terms of a physical sector i.e. it cannot directly write a 512 byte sector write issued to it. The internal process in the disk that makes this write possible contains three steps

• The disk first has to read the 4KB physical sector into its internal cache which contains the 512 byte logical sector referred to in the write
• Modify the data in the 4K buffer to include the updated 512 byte
• Finally, the disk must perform a write of the updated 4kB buffer back to its physical sector on the disk. 

This process is called a “Read-Modify-Write” or RMW. In today’s drives, this RMW process is causes performance degradation in VHDs due to the following reasons

1. Dynamic and differencing VHDs have a 512 byte sector bitmap in front of their data payload. In addition, footer/header/Parent Locators all align to 512 byte sector. So it is common for the VHD driver to issue 512 byte writes to update these structures, resulting in the RMW behavior described above.
2. It is common for applications to issue reads and writes in multiples of 4kB sizes (the default cluster size of NTFS). Due to the fact that there is a 512 byte sector bitmap in front of the data payload block of dynamic and differencing VHDs, the 4kB blocks are not aligned to the physical 4kB boundary. Each 4k write issued by the current parser to update the payload data will then translate into two reads for two blocks on the disk, which are then updated and subsequently written back to the two disk blocks. 
   
   In addition, drives that perform RMW operations in volatile storage introduce potential reliability concerns in the event a RMW operation is interrupted. Such reliability concerns may extend beyond the modified 512-bytes to the entire 4k sector of data.
3. Since VHDs can only expose a virtual disk with a logical sector size of 512 bytes, applications will continue to issue 512 byte aligned IOs even if they are capable of issuing 4k aligned IOs optimize performance on these disks.

Impact When Hosting VHDs On Native 4K Disks

The VHD driver today assumes that the physical sector size of the disk to be 512 bytes and issues 512 bytes IOs which makes it incompatible with these disks. The VHD stack fails to open the VHD files on physical 4kB sector disks for this reason.

Recommended Actions When Using Large Sector Disks:

1. Internal testing has shown that the performance degradation of around 30% for most workloads on specific 512e drives. The performance degradation could also be affected by
   
   • Hardware specifics like the amount of cache and optimizations implemented in the firmware of the disk.
   • If the workload triggers a larger number of unaligned IOs incurring the RMW penalty.
   
   If the workload is performance critical, then
   
   • Use fixed VHDs as they that don’t suffer from the performance issues associated with dynamic and differencing disks (detailed in the “Impact When Hosting VHDs On 512e Disks” section)
   • If dynamic or differencing disks are needed, then use regular 512 byte drives for hosting the VHDs associated the workload.
   
   Please work with your storage hardware and RAID vendors to validate impact to your storage setup.
   
2. Windows 2008 and Windows 2008 R2 support using 4k drives, but you cannot use native 4kB disks for creating VHDs with Hyper-V on Windows server 2008 and Windows server 2008 R2. 

For additional information, please see Microsoft Windows Support Policy for Large Sector Drives
 http://support.microsoft.com/kb/2510009 (http://support.microsoft.com/kb/2510009) .

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