May 26, 2017
Benefits and Technologies of LAN-free Backup
During a backup, large amounts of data are transferred through the networks and can cause a slowdown of business-critical operations. Network administrators use various approaches to minimize the impact of backups on the daily operations. Among those are scheduling backups off the business hours and traffic-reducing techniques such as data compression and forever incremental backups. However, one of the best ways to optimize backup traffic is a segregation of production and backup networks. One of the possible approaches to do this is a LAN-free backup. The most common techniques used in virtual environments are the direct SAN access and HotAdd.
LAN-Free Backup: Direct SAN Access
System administrators use storage area networks (SAN) to improve data storage efficiency, scalability, and redundancy. In a nutshell, this approach allows connecting many storage disk drives into a single entity, so different servers can have access to the storage capacities with no ties to a specific HDD or SSD.
Physically, devices inside a SAN can be connected with Fibre Channel or Ethernet cables. Data transfer inside the SAN can be implemented with iSCSI, Fibre Channel, or Fibre Channel over Ethernet protocols. Data inside the SAN flows without exiting into the LAN, and direct SAN access leverages this benefit.
Within SAN, each storage volume (either physical or logical) has its logical unit number (LUN) that is used to connect it to a server. This approach adds more flexibility in administration: it is possible to connect a volume to more than one server, which ensures business continuity. Even if the primary server goes down, the data will not be lost because it is possible to switch to another one.
The principle of direct SAN access is simple: a backup application reads data directly from storage devices without passing it through the hosts.
Here is how direct SAN access is implemented in VMware environments. The backup application uses the Virtual Disk Development Kit (VDDK) which allows getting information about LUN configuration. During the backup, the application reads data directly from the LUN where the virtual disk of the source VM is located. This way, the data does not pass through the ESXi host with the source VM or the LAN. This is the fastest way to backup VMware VMs deployed on ESXi hosts connected to the SAN.
The benefit of using the VMware VDDK library is that the backup can be completely agentless, so the backup process will not impact the consistency of data of the source VM or affect the operations running on it.
However, direct SAN access has some constraints and requirements. For example, the backup application must be installed on either the physical server connected to the SAN or the VM running on ESXi host physically connected to SAN.
LAN-Free Backup: HotAdd
The other approach of LAN-free backup is HotAdd. It is called so because a snapshot of a source VM is mounted (i.e. “hot added”) as a virtual disk to the VM with a backup application installed.
When the HotAdd mode is used, a backup product can read VM data directly from the VM datastores through the storage I/O stack, bypassing the host’s TCP/IP stack that would otherwise impact every VM hosted on the server, and slow down the data transfer.
This transport mode is usually used when a direct SAN access is not available. For example, if you simply do not use SAN. The other case, if for some reason you do not want to load the SAN network with backup tasks, but this is a rare situation.
HotAdd yields in almost the same performance as the direct SAN access, but it has limitations as well. First, the backup application must be installed on the virtual machine so a virtual disk can be mounted to it. Second, the VM with the backup application must have direct access to the storage where a source VM resides. This storage may be the same physical disk if the backup application and the source VM are on the same host. In another case, such storage might be a shared one, like NAS or even SAN. Third, the HotAdd transport is not supported for IDE disks.
As we could see, the LAN-free backup methods allow dividing production and backup data streams. This results in faster backups and less load on the production LAN and ESXi hosts. The default configuration of NAKIVO Backup & Replication is set to "Auto" mode which automatically tries to run a backup in direct SAN mode, then in HotAdd, and only in the case if the previous two are not applicable, starts backup over the LAN.