We examine five products that marry high availability and disaster recovery to your virtual server environment
There's an additional complexity: the boot image. This image is the file stored on disk that encompasses the file system, boot sectors, boot files, operating system files, application files, and so forth required by an operating system. VMware can save the entire thing as a single image file on the local disk, but moving the instance from the local disk to a SAN requires converting it to a block device. This conversion process is not a problem, but an image converted to a block device cannot always be converted back to a local image. There are similar issues with other virtualization products.
Notably, some of the virtualization vendors are already baking HADR functionality into their wares to address these types of issues. VMware, for example, has unveiled several such features. There's Virtual Machine File System (VMFS), which supports storing OS images on shared volumes. Additionally, there's VMotion, which supports moving instances from one VMware server to another without having to bring the instance down first. Moreover, Site Recovery Manager provides central management of instances across multiple ESX servers. VMware HA (High Availability) can restart instances that stop responding, or restart them on other ESX servers if necessary. Finally, VMware Distributed Resource Scheduler (DRS) allows dynamic reallocation of resources to servers when loads increase or decrease on a given instance.
In this round-up, however, we'll be looking at third-party offerings intended to supplement virtualization products such as VMware, Microsoft's Hyper-V, Virtual Iron, and Parallels, as well as XenSource and other products based on Xen, as well as KVM, VServer, and other open-source virtualization platforms. The products include DataCore's SANmelody, Marathon Technologies' everRun VM for Xen, Scalent Systems' Scalent software, Stratus Technology's Avance, and Vizioncore vRanger Pro. Each addresses a different aspect of HADR for virtualization.
Tale of the HADR tape
SANmelody virtualizes storage and works with emBoot to enable VMware server instances to boot from the SAN rather than the local hard drive, making it easier to move instances from one system to another. The boot instances stored on the SAN can also be backed up using the snapshot functionality of the system, as well as being replicated on a second system.
everRun VM for Xen enables failover modes for Citrix's XenServer so that instances that fail on one system continue to be available on the second. This provides true continuous availability, with both the primary and secondary instance having the same IP address and even the same MAC address. There is no detectable interruption in service if one of the two instances fails.
Scalent offers an infrastructure virtualization system that provides an integrated platform for deployment, migration, and failover of virtual instances, allowing instances to be converted from local to SAN-based. It also automates changes in network settings, SAN settings, and more as instances are moved. It offers a uniquely flexible, quick and easy system for deploying, moving, or re-deploying server instances.
Avance integrates with Citrix's XenServer and provides automatic failover from one instance to another if a server fails. It uses a dedicated, hardened version of XenServer modified to provide rapid failover and high security. Each instance is separate, and there can be slight delays in responses to client systems during the switchover.