As well as backup, replication is a job-driven process; in many ways, it works similarly to forward incremental backup:
•During the first run of a replication job, Veeam Backup & Replication copies the whole VM image and registers a replicated VM on the target ESX host.
•During subsequent runs of a job, Veeam Backup & Replication copies only incremental changes, and creates restore points for a VM replica — so you can recover your VM to the necessary state. Every restore point is in fact a usual VMware snapshot.
•When you perform incremental replication, data blocks that have changed since the last replication cycle are written to the snapshot delta file next to a full VM replica. The number of restore points in the chain depends on your retention policy settings.
Replication infrastructure and process is very similar to those used for backup. It includes a source host, a target host with associated datastores, one or two proxy servers and a repository. The source host and the target host produce two terminal points between which replicated data is moved. Replicated data is collected, transformed and transferred with the help of Veeam transport services. In addition to source-side transport service and transport service hosted on a repository, replication process involves a target-side transport service that interacts with the target host.
The transport service hosted on a repository works with replica metadata files.
Although the replica data is written to the target datastore, certain replica metadata must be located on a backup repository. This metadata is used by the source proxy and thus should be deployed close to the source host.
1.When a new replication session is started, the source-side transport service operates in the same way as in backup process. In addition, in all cases when use of VMware CBT is not possible, the source-side transport service interacts with the transport service hosted on the repository to obtain replica metadata — in order to detect what blocks have changed since the previous job run.
2.Copied blocks of data are compressed and moved from the source-side transport service to the target-side transport service.
In on-site replication scenarios, the source-side transport service and the target-side transport service may run on the same backup proxy server.
3.The target-side transport service then decompresses replica data and writes the result to the destination datastore.
Veeam Backup & Replication supports a number of replication scenarios that depend on the location of the target host and will be discussed later in this guide.
During replication cycles, Veeam Backup & Replication creates the following files for a VM replica:
•A full VM replica (a set of VM configuration files and virtual disks). During the first replication cycle, Veeam Backup & Replication puts these files to the selected datastore to the ReplicaName folder, and registers a VM replica on the target host.
•Replica restore points (snapshot delta files). During incremental replication, Veeam Backup & Replication creates a snapshot delta file in the same folder, next to a full VM replica.
•Replica metadata used to store replica checksums. Veeam Backup & Replication uses this file to quickly detect changed blocks of data between two replica states. A metadata file is written to the backup repository.
During the first run of a replication job, Veeam Backup & Replication creates a replica with empty virtual disks on the target datastore. If the Virtual Appliance mode is applicable, replica virtual disks are mounted to the backup proxy and populated through ESX I/O stack. This results in increased writing speed and fail-safe replication to ESXi targets.
To streamline the replication process, you can deploy the backup proxy on a virtual machine. The virtual backup proxy must be registered on an ESX(i) host that has a direct connection to the target datastore. In this case, the backup proxy will be able to use the Virtual Appliance transport mode for writing replica data to target.
If the backup proxy is deployed on a physical server, or use of the Virtual Appliance mode is not possible for other reasons, Veeam Backup & Replication will use the Network transport mode to populate replica disk files.