HotAdd is one of the available modes to backup a VM with Veeam, and one of the preferred once when the Veeam proxies are installed as virtual machines themselves. However, there are situations where HotAdd mode does not work, and since it appears to be a frequent question, I decided to write a post about it. There is even a KB article from Veeam to start with.
• LUN where Veeam Backup server resides on should be formatted with proper block size to be able to mount the largest virtual disk of the hot added VMs. 1MB block size – 256GB maximum file size 2MB block size – 512GB maximum file size 4MB block size – 1024GB maximum file size 8MB block size – 2048GB maximum file size To be more clear, this problem happens when you are dealing with VMFS3 filesystems, since on VMFS5 there is a fixed 1MB block size regardless the size of the datastore. So be sure to check the version of the filesystem you are installing Veeam onto. To be completely sure, Veeam should always be installed on VMFS5 or on a VMFS3 datastore formatted with 8MB block size.
• Starting with Veeam Backup & Replication version 6.5 and vSphere 5.1, the maximum supported vmdk size is 1.98 TB. I wrote about this problem in a previous article: The “Real” maximum size of a VMDK file. Once again, to be sure your backups would work, do not exceed that size for all your VMDK files.
• HotAdd mode doesn’t work for independent disks as they cannot be backed up since the snapshot cannot be created on these volumes. This is a known limitation of indipendent disks, design your infrastructure accordingly.
• With HotAdd transport, full backup of a virtual machine fails when multiple disks have the same name, as they could if they are on different datastores. This is a tricky one! Be sure to double check your vmdk file names, especially if you are using Datastore Clusters!
• A single SCSI controller can have a maximum of 16 disks attached. To run multiple concurrent jobs with more than 16 disks, you need to add more SCSI controllers to your Veeam Proxy server that is responsible for hot adding the disks. In a job, every VM is processed sequentialy. But if you have multiple cuncurrent jobs running on the same proxy, and some VM with several disks, it can happen you have too much disks to process than the available SCSI channels, and the job will failback to network mode. A little trick for this one: you can only add a new controller if there is a disk connected to it. So, to add from the beginning additional controller, create different small disks (1 Gb is enough) and configure them to use a different SCSI channel, thus a new SCSI controller will be added.
• In case of standalone host connection (no vCenter added to the console), you can only hot add disks of VMs which are located on the same host as the proxy server. This may require additional proxy servers to be configured on each host in the environment. • Hot Add may fail if the VM you are trying to back up and the proxy server are in different clusters. • Virtual Appliance backup job fails after you’ve cloned or restored your VBR server. The reason for that is incorrect UUID reference for Veeam Backup VM.
Last one, not even present in the article, IDE disks cannot be mounted using HotAdd, only SCSI ones. I wrote about this problem in another article: Convert a virtual disk from IDE to SCSI.