VM /ETC

I was helping a customer P2V a large development SQL server this week and ran into a VMFS configuration issue that failed the conversion. We were using the Converter Enterprise for VirtualCenter 2.5 plugin. Almost as soon as we kicked off the job it failed with an error starting with “file size is larger than the maximum size supported by datastore”. The VMFS LUN we were using as the target was an empty 1.5 TB volume, and the new VM consisted of 2 virtual disks that totaled roughly 450 GB. We had plenty of room, but the problem was not the available storage space. Instead, the issue was that we exceeded the maximum possible .vmdk size for the default VMFS 1MB block setting.

When you add new storage to an ESX host and you format the LUN with the VMFS file system you have to choose what block size setting you want to use. See the screenshot for the dropdown box used to make this choice. Notice the Maximum file size description supposedly provided to help you understand this setting. It’s hardly intuitive in my opinion, so let me try to translate – Choosing the block size determines what maximum possible .vmdk size can be created on this LUN.

If you do not change the default setting when you format a VMFS LUN

you will be using the 1MB setting. That means that you can not create a .vmdk larger than 256 GB. This was the issue in our case. VMware Converter was combining the C:, E:, and F: partitions of the server into the Disk 0 .vmdk, and the file size ended up being 280 GB. Our resolution was to shrink the E: drive from 39 GB to 10 GB since that partition only had 190 mb in use. That change made the first .vmdk 251 GB. The second .vmdk remained at 170 GB so the total space used by the new VM was still 421 GB but each .vmdk was now below the max size limit and the P2V job was able to run.

As you can see from this example, it helps to understand the virtual disk sizes you will be using when planning your LUNs for virtual infrastructure. Larger block sizes allow for larger .vmdks, but if you are not careful about your design you may not be using your storage efficiently.

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