CDP Replication Chain
A replication chain is a sequence of files that allows you to roll back a VM replica to a specific point in time during failover. Veeam Backup & Replication creates replication chains for each VM added to a CDP policy.
The replication chain contains short-term and long-term restore points. Short-term restore points allow you to roll back VM replica data to a state created seconds or minutes ago, while long-term restore points — to a state created hours or days ago. Short-term restore points are always crash-consistent, long-term restore points can be crash-consistent or application-consistent depending on how long-term restore points are configured in CDP policy settings.
The replication chain is stored on the target datastore in the <replica_VM_name> folder. The replication chain consists of the files of the following types (only the key file types are listed):
- VMX — the configuration file of the VM replica.
The replication chain contains one .vmx file, other files from the list are created per virtual disk.
- VMDK — virtual disk files that store contents of replica hard disk drives.
On the datastore, you can see files under the following names:
- <disk_name>-flat.vmdk — files that store full copies of virtual disks, that is, store base disk data. These files are created during the initial synchronization and relate to the very first long-term restore point. This restore point is crash-consistent.
- <disk_name>-<index>.vmdk — files that store incremental changes made to virtual disks, that is, store delta disk data. These files relate to long-term restore points and are created according to the schedule configured in CDP policy settings.
- [For VSAN and VVOL starting from Veeam Backup & Replication 11a (build 188.8.131.521)] <disk_name>-<index>.tlog.vmdk — see description in the TLOG section.
- <disk_name>-interim.vmdk — files for protective virtual disks. Changes made to virtual disks will be written in these files if VM replicas are powered on.
Although VMDK files look like VMware snapshot files, they are not real snapshots. These files are created by I/O filter installed on the target host.
- VMFD — files that contains metadata for disks.
- TLOG — transaction log files that store incremental changes made to virtual disks during RPO set in CDP policy settings. These files relate to short-term restore points that are created once in RPO set in CDP policy settings. One transaction log file stores data for multiple short-term restore points.
[For VSAN and VVOL starting from Veeam Backup & Replication 11a (build 184.108.40.2061)] Veeam Backup & Replication saves transaction log files as virtual disk files (<disk_name>-<index>.tlog.vmdk) to avoid the VSAN limit of 255 GB and the VVOL limit of 4 GB for a file.
New transaction log files are created in the following cases:
- [For Veeam Backup & Replication 11a (build 220.127.116.111) and later] When a transaction log file reaches a specific size: 2 TB on VMFS datastores, 512 GB on VSAN and VVOL.
- [For Veeam Backup & Replication prior to 11a (build 18.104.22.1681)] When a .tlog file reaches a specific size: 2 TB on VMFS datastores, 255 GB on VSAN or 100 MB on VVOL.
- When a long-term restore point is created.
- When one quarter of the short-term retention is reached. For example, if you set short-term retention to 1 hour, a new transaction log file will be created every 15 minutes.
- After failback commit.
- META — files that contain metadata for transaction log files.
To roll back a VM replica to a specific point in time, the chain of files created for the replica must contain files with data for the base disk (<disk_name>-flat.vmdk) and a set of files that contain incremental changes for disks (<disk-name>-<index>.vmdk + .tlog or .tlog.vmdk for VSAN and VVOL). If any file in the replication chain is missing, you will not be able to roll back to the necessary state. For this reason, you must not delete files from the datastore manually. Instead, you must specify retention policy settings that will let you maintain the desired number of files. For more information on retention policies and how to configure them, see Retention Policies.