Virtualization is becoming increasingly important in the datacenter as a way to respond quickly to the varying server demands. Depending on time of day and day of the week, as well as events in progress and many other factors, loads on any given machine may vary by factors of 100 or 1,000 or more. Giving a server more or less computing power, running multiple instances of the same server for load balancing purposes, or allowing failover from one virtual instance to another are increasingly important capabilities.
That's where HADR (high availability disaster recovery) tools come into play. High availability tools ensure that applications running in a virtual server session remain available even during failures of hardware, server OS, or application software. Disaster recovery tools enable the quick recovery of functionality after loss of hardware and are oriented toward restarting services at new locations.
HADR is complex enough when working with one OS per server. When you add in the multiple virtualization platforms out there, the numerous OSes, as well as all the storage and network settings and the additional complexities of boot images used by the hypervisors, HADR for virtual servers becomes even more complex. Just as there is no single solution for HADR in general, there is no single solution within virtualization: Different products address different areas, including backups, failover, deployment, and storage virtualization.
These HADR products range from software that is installed on either the virtualization server or on a separate server to hardware-software combinations that are installed separately. There are even specialized platforms that enable just one aspect of HADR for virtualization, such as DataCore's SANmelody, which enables storage systems to respond to the changing requirements of virtual servers as they are moved from one instance to another.
HADR Can Be H-A-R-D
HADR is fraught with complexity, due to the intricacies of virtualization itself. For example, moving a server instance from one physical server to another can be complicated by differing subnets, differing hardware from system to system, differing access to storage (the logical units or LUNs on a storage system are typically mapped to a specific piece of hardware), and other factors. Because there is generally no single overarching tool for this, management of the overall system is exceedingly complex.
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.