Being a thoroughbred developer, I am going  to cut to the chase here and dive right into the core mechanisms available behind backing up VMs in VMware. As this is being introduced in our next immediate release, I feel it will be quite useful to everyone evaluating the product !

This article will provide an overview information on the SAN and HotAdd transport mechanisms. SAN and HotAdd transport are usually faster than over-the-network backup and restore using NBD (network block device) or NBDSSL (secure NBD).

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HOTADD TRANSPORT:

HotAdd involves attaching a virtual disk to the backup proxy just like attaching the disk to a virtual machine.The term HotAdd refers to the way the backups are completed. In HotAdd mode, the data volumes containing the virtual machines to be backed up are automatically mounted to the proxy, so that they can be accessed by the proxy as a local disk. The ESX host, that the proxy is running on, must have access to all datastores for the virtual machine. If the virtual machine and the proxy are not on the same host, all datastores must be shared between the hosts. HotAdd mode can closely match the performance of SAN mode.

HotAdd involves attaching a virtual disk to the backup proxy just like attaching the disk to a virtual machine in VMware.

HotAdd transport

Here is the gist of the requirements for HotAdd:

  • HotAdd is a SCSI feature and does not work for IDE disks.
  • HotAdd Mode works only in virtual proxy(client) machine and only with SCSI disk.
  • HotAdd mode is best when the proxy(client) machine is a virtual machine and resides on the same datacenter as the VM to be backed up.
  • Proxy machine should have access to the VM to be backed up, if it is present in some other datacenter.
  • Vmware tools must be installed.
  • For a Windows Server 2008 or Windows Server 2012 proxy, set SAN Policy onlineAll for ALL the  paths.

 In vSphere 5.0, the SCSI HotAdd feature is enabled only for vSphere editions Enterprise and higher, which have Hot Add licensing enabled. No separate Hot Add license is available for purchase as an add-on. In vSphere 4.1, Hot Add capability was also allowed in the Advanced edition. Customers with vSphere Essentials or Standard editions are not able to perform proxy-based backup, which relies on SCSI HotAdd. Those customers must use alternate transport modes.

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SAN TRANSPORT:

SAN requires a physical proxy to share a LUN with the ESX/ESXi host where a datastore resides, enabling direct access to raw data and bypassing the ESX/ESXi host altogether for I/O operations.With automatic transport mode selection, if SAN storage is connected to the ESX host, then the SAN mode is selected. Data is read directly from the storage where virtual machines reside, without going through the ESX host or transferring data over the local area network (LAN). The ESX host is contacted only to coordinate access to the LUN.

SAN requires a physical proxy to share a LUN with the ESX/ESXi host where a datastore resides.

san transport

Here are the Requirements of SAN Transport:

  • SAN Mode works only in physical proxy(client) machine and only with SAN storage connected with  Fiber Channel or iSCSI SAN Storage.
  • A virtual machine on SAN Storage or iSCSI SAN storage in the same ESXi server.
  • For a Windows Server 2008 or Windows Server 2012 proxy, set SAN Policy onlineAll for ALL the  paths.
  • SAN transport is not always the best choice for restores.
  • When writing using SAN mode during restore, disk size should be a multiple of the underlying VMFS block size, otherwise writes to the last fraction of a disk will fail. For example, if a datastore has 1MB block size and the virtual disk is 16.3MB large, the last 0.3MB will not get written and will fail with the Invalid Argument error. Your software must add 0.7MB of zeroes to complete the block. This caveat does not apply to eager-zeroed thick disk.
  • Vmware tools must be installed.

NBD AND NBDSSL:

  • NBD (network block device) & NBDSSL (encrypted NBD) transmit data over the TCP/IP connection between the ESX server and the proxy computer.
  • NBD serves as a fallback when other transport modes are not available.  The local area network (LAN) can be the production network or a dedicated backup network.
  • NBDSSL is similar to NBD mode, but data transfer between the proxy computer and the ESX server is encrypted. Encryption should be used for sensitive information, even within a private network.
NBDSSL is similar to NBD mode, but data transfer between the proxy computer and the ESX server is encrypted.

nbd & nbdssl

Please do drop us a note at vembu-sales@vembu.com on your thoughts about the article and about the transport modes discussed in it. If you want us to provide technical coverage on any aspect of our product, please feel free to mention it as well.

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