36125: Backup Algorithm in ESX Environment

Last update: 18-11-2022

Step-by-step description of what happens when you choose to back up a virtual machine using the Agent for VMware (Virtual Appliance) or Agent for VMware (Windows)

Step-by-step description of what happens when you choose to back up a virtual machine using Agent for VMware (Virtual Appliance) or Agent for VMware (Windows).

The snapshot is performed by VMware itself with the following options:

  • quiesced = on
  • memory = off

See an example screenshot:

  1. Virtual Appliance sends the request to create the snapshot of the chosen virtual machines on the host:
    • Custom pre-backup commands are executed by VMware Tools automatically when quiesced snapshot is triggered. Usually, custom pre/post commands are used to quiesce an application that does not have its own VSS writer to ensure a consistent state before backup. Such a script, named pre-freeze-script, should be placed to a dedicated directory on a Windows machine or to /usr/sbin on a Linux machine. These scripts require VMware tools to be installed on the machine. In Linux, the script must have 0755 permissions to be executable and must be owned by root.
    • VMware stops R/W operations to vmNAME.vmdk file(s);
    • All changes in the virtual machines will be forwarded to the newly created snapshot (additional .vmdk file);
      (!) Snapshot file(s) name(s) should be similar to the virtual hard drive + 0001: vmNAME-0001.vmdk
  2. Virtual Appliance reconfigures the virtual machines:
    • VMware attaches* vmNAME.vmdk file(s) to Virtual Appliance;

      Since Acronis Cyber Backup v12 (and Acronis Cyber Protect Cloud 5.0) the virtual disks are also attached to Agent for VMware (Windows) running on Windows VM which resides on the ESXi host which has access to the datastore where the backed up VM disks reside. 

    • Virtual hard drive link can be easily identified under datastore or the Virtual Appliance settings by the following attribute: “_” (underscore):

      (!) These links can be deleted from Acronis Virtual Appliance if necessary.

  3. This data is copied (backed up) from initial vmNAME.vmdk file.
  4. Custom post-backup commands (named post-thaw-script), if any, are executed. Rules for these scripts are the same as for the pre-backup scripts.

(!) All changes in the virtual machines since the beginning of the backup operation will not be included to the archive. 

* (!) Attaching disks happens only in an ESX(i) 5.0+ or 4.0/4.1 environment with hot-pluggable hardware feature enabled by vSphere license (vSphere Standard/Enterprise). Otherwise there is a NBD mode used (network mode for reading data). Also there can be real disks attached to appliance instead of DatastoreName_FolderName_vmNAME.vmdk links. The way the disks are attached depends on whether there is vCenter or ESX(i) host registered in Acronis product. Disks links are attached in case there is vCenter registered and if there are individual hosts registered than the real disks are attached to the virtual appliance.

(!) Starting with Acronis Backup v12 (and Acronis Cyber Protect Cloud 5.0) there are always real disks attached to the backup agent machine instead of symlinks (native VMware VDDK hotadd function is used instead of utilizing Acronis proprietary mechanisms which were used in older products).