Where Did My Disk Space Go?
Where Did My Disk Space Go? – Troubleshooting Virtual Machine Storage Issues in Hyper-V
With Hyper-V, it is very easy to over commit your storage space by creating dynamic VHD files. It is also easy to consume all of your available storage by creating multiple, parallel snapshots, creating large fixed VHD files, and letting your dynamic VHD files expand as the guest operating systems demand more space.
What happens when you hit the limit on your storage devices? Your virtual machines are placed into the dreaded “Paused Critical” or “Major Failure” states.
If we were to run out of space for expanding a dynamic or differencing virtual hard disk – we would start failing disk write operations inside the virtual machine. The result would most likely be that the guest operating system would crash. To avoid this problem we periodically poll the storage that is being used for the virtual hard disks. If the free space falls under 200 megabyte we will pause the virtual machine and mark it as “Paused-Critical”.
This raises the unfortunate possibility of a mission-critical production application suddenly becoming unavailable because an unrelated virtual machine decided it needed more space.
Just what files do virtual machines need and where are they located? Again, Virtual PC Guy is a good source of information. In his articles “Understanding where your virtual machine files are in Hyper-V” and “Where are my snapshot files” he discusses the types of files virtual machines use, where Hyper-V stores them, and outlines some perils:
One thing that often trips people up is if they have their virtual hard disks configured on one drive – but have left their snapshot files stored on the system drive.
Savison’s Vital Signs for Hyper-V can help guard against virtual machines running out of storage space, can help you proactively manage your VHD placement, and can show you where you can free up space if you have, unfortunately, run out.
The KPI tab within our dashboard will allow you to monitor both physical storage space as well as disk errors reported by the virtual machines – a sign that their VHDs are not able to expand.
The storage tab graphically shows the occupancy of all drives broken down by virtual machine, shows all files used by your virtual machines, and sums up the total physical storage required by each virtual machine.
With this information in hand, you can make intelligent decisions about where to place new VHD files and to determine where you can free up space – whether by removing snapshots or moving virtual machines – for your mission-critical VMs.
For more information about this Blog article or Vital Signs contact us! You can read more blogs articles at blog.savision.com.
