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In a vSphere upgrade process, there are two different approach for the host upgrade: a fresh re-install or a in-line upgrade. In the VMware site there is an interesting post about this choice.

The differences between an upgraded host and a freshly installed host are:

  1. Boot disk partition format:  When upgrading, the boot disk retains the older MBR (Master Boot Record) partition format where on a new install the boot disk is formatted as GPT (GUID Partition Table).  The key here being that the new GPT format enables you to use LUNs larger than 2TB and up to 64TB.  The larger LUN size support is typically not an issue for ESXi boot disks.
  2. VMFS volume version:  When you upgrade, the existing boot disk VMFS-3 volume is preserved.  On a fresh ESXi 5.0 install a new VMFS-5 volume is created.   Keep in mind that you can always upgrade the VMFS-3 volume to VMFS-5 after the host upgrade.  To understand the implications of upgrading a VMFS-3 volume to VMFS-5 check out this blog.
  3. Location of the ESXi scratch partition:  When you do a fresh ESXi install a 4GB VFAT partition is created for the scratch partition (assuming you are installing to a local or SAN disk).  However, when you upgrade, instead of a dedicated disk partition a scratch directory is created on the VMFS datastore.  While the location of /scratch is different, there is no operational significance to using a VMFS directory compared to a disk partition.  Also, keep in mind that you can always change the location of /scratch at any time.
  4. Disk partitions layout: ESX and ESXi use different disk partitions layout and those are also different from 3.x to 4.x versions.

With these considerations in mind, usually there isn’t any big disadvantages to upgrading a host compared to doing a fresh install.

Upgrading has this advantages:

  • The upgrade preserves the host configuration eliminating the need to manually reconfigure each host (but you can use scripting or host profiles to minimize the effort of a new ESXi deployment and configuration).
  • The data on the VMFS datastore is preserved eliminating the need to manually migrate data off the boot disk or having to rely on backup and restore tools (but also during a fresh install you can choose to keep the VMFS partitions).
  • With VUM the upgrade can be completely automated and orchestrated in a simple way.

A fresh install may have advantages:

  • Could be faster (for example compared to a ESX upgrade).
  • The upgrade is not always possible (for example for a wrong disk partitions layout)
  • Is a way to start with a “clean” configuration.
  • Is the only way to implement an AutoDeploy environment.
  • Is the only (simple) way to change the installation location (for example from local storage to flash card).

Note that the upgrade can also be handle with the same installation media (from CD, USB, PXE boot) as the full re-installation. I found that in some cases (3rd part packages installed) the VUM approach may fail to remediate and an upgrade process from the installation media instead can work fine.

See also:

Andrea MauroAbout Andrea Mauro (2310 Posts)

Virtualization & Cloud Architect. VMUG IT Co-Founder and board member. VMware VMTN Moderator and vExpert (2010, 2011, 2012, 2013, 2014, 2015). PernixPro 2014. Dell TechCenter Rockstar 2014. MVP 2014. Several certifications including: VCDX-DCV, VCP-DCV/DT/Cloud, VCAP-DCA/DCD/CIA/CID/DTA/DTD, MCSA, MCSE, MCITP, CCA, NPP.