Virtual machine replication works at the server virtualization layer and relies on replication software that can...
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copy all changes made to a virtual machine disk file (VMDK) to another host. It requires a warm or hot disaster recovery (DR) site with dedicated network connectivity linking the production and recovery sites. A snapshot is taken of the virtual machine (VM) at the virtualization layer, which deflects writes to the virtual disk to a separate delta file. The virtual disk is then mounted by the replication software and any updates since the last replication cycle are copied to another identical virtual disk on a virtual host at the disaster recovery site.
VMware vSphere's new vStorage APIs enhance this process because of the new Changed Block Tracking (CBT) feature. CBT provides much quicker incremental backups and replications because the VMkernel tracks which disk blocks have changed since the last replication. This allows shorter intervals between replication operations, resulting in near-continuous data protection (CDP). A big advantage of this method is that any type of storage can be used on both the source and target virtual hosts. When it's necessary to cut over to the DR site due to an outage at the main site, you can power on the replicated VM at the DR site and begin using it; changed blocks are then tracked on the remote site VM so they can be replicated to the main site for failback. Applications that support this method include:
- Double-Take Software Inc.'s Double-Take Availability can replicate both physical servers and VMs to a virtual host at a DR site. Replication can occur either inside the guest OS or at the virtual host level.
- PHD Virtual Technologies' esXpress combines disk-to-disk backup with replication; it can do a simple full-VM copy to another site or incremental block-level updates.
- Veeam Software's Veeam Backup & Replication combines disk-to-disk (D2D) backup and replication in one product. It has built-in data deduplication and uses CBT to achieve near CDP; changed blocks are injected into the target VMDK during each replication cycle.
- VizionCore Inc.'s vReplicator is a dedicated replication product for virtual machines (vRanger Pro is their backup product). It supports CBT and Active Block Mapping (ABM) to detect white space in a VM so it can be ignored.
The importance of proper quiescing
Whatever data protection method you use -- storage replication, backups or virtual machine replication -- it's very important that you properly quiesce your virtual machines to ensure data integrity. Quiescing is the process of pausing the operating system and applications, and forcing all pending data in memory to be written to disk. There are two ways to quiesce. The first way is done at the operating system level and it tells the OS to write all pending data in memory to disk; however, because the OS isn't aware of what applications are doing, this could cause corrupt or incomplete application data. The second method is done at the application level where applications like Microsoft Exchange or SQL Server are notified so that they can complete any pending transactions before writing the pending data to disk. The latter is called "application consistent" quiescing, and it ensures that all application data is properly backed up without any loss of data.
Without any quiescing, a virtual machine is considered to be in a "crash consistent" state, meaning the backup that's made is of a VM that's been powered off with any held-in memory data not accounted for. Microsoft Corp. VM's have a special Volume Shadow Copy Service (VSS) driver built into the OS to quiesce the operating system, but it often won't provide application quiescing. To achieve application consistent backups, you may have to install a special driver inside the guest OS. When choosing any backup or replication application make sure it includes the proper quiescing for the critical application data you're trying to protect.
Part of this guide was originally published in Storage magazine.
Virtualization and disaster recovery guide