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With a vSphere 5 upgrade there is an important vDesign decision: if you already have some VMFS3 datastores could be better upgrade them to the new version of build new datastores directly with VMFS5? The upgrade procedure is quite fast and friendly and could be applied to a live datastore, so seems that there isn’t a big different between an upgrade or a clean format.

But usually the recommendation is to re-format each LUN to VMFS-5 rather than upgrade it. This will fix a number of issues, including:

  • Mismatched block size: VMFS-5 introduces the 1 MB unified block size. We would be wise to avoid VMFS-5 LUNs with 2, 4 or 8 MB block sizes looking forward. But note that there aren’t remarkable performance difference between the different block size.
  • Use the GPT table: an upgraded VMFS5 keep the existing MBR schema… But if you grow it to more than 2 TB then it will be converted to GPT schema.
  • Adjust LUN size: We now won’t be constrained to previous size provisioning. Further, we can resize LUNs on the storage processor to usher in new features such as storage pools.
  • Partition alignment: also VMFS3 is aligned (except the first VMFS datastore created during the installation), but partition is starting on sector 128; in newly created VMFS-5 partitions start at sector 2,048.
  • Internal filesystem structure: upgraded VMFS5 continues to use 64KB sub-blocks and not new 8K sub-blocks and continues to have a file limit of 30,720 rather than the new file limit of > 100,000 for newly created VMFS-5.
  • Check zoning: This also is the right time to ensure that all WWPN or IQN access that’s in place is still correct. Chances are that for old LUNs there may have been hosts removed from the cluster, yet they may still be zoned in the storage processor or switch fabric to the LUNs.

Reformatting each LUN to VMFS-5 will set the tone for a clean infrastructure from the storage perspective that can go along with the upgrade. This of course means that VMs will have to be evacuated from existing VMFS-3 volumes via a technology such as Storage vMotion to a (possibly) temporary LUN. Outside of the storage provisioning metrics above, this is also a great way to clean up miscellaneous unused data on the VMFS-3 datastore. This can be VMs out of inventory, a forgotten ISO library or use of the VMFS filesystem for something other than holding a virtual machine.

But there is at least one reason to use the upgrade procedure: if you plan and need to reclaim free space from thin vmdk… in this case you need datastore with different block size as described in previous post.

About the difference between VMFS3 and VMFS5 see also:


Virtualization, Cloud and Storage Architect. Tech Field delegate. VMUG IT Co-Founder and board member. VMware VMTN Moderator and vExpert 2010-24. Dell TechCenter Rockstar 2014-15. Microsoft MVP 2014-16. Veeam Vanguard 2015-23. Nutanix NTC 2014-20. Several certifications including: VCDX-DCV, VCP-DCV/DT/Cloud, VCAP-DCA/DCD/CIA/CID/DTA/DTD, MCSA, MCSE, MCITP, CCA, NPP.