Always-on virtualization

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.

[ See the Test Center reviews of VMware Infrastructure 3 with ESX Server 3.5 and VirtualCenter 2.5 and Microsoft's Windows Server 2008 Hyper-V and Virtual Machine Manager. Track virtualization news and reviews through InfoWorld's Virtualization Topic Center and reading David Marshall's Virtualization Report. ]

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.

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.

vRangerPro is a backup utility for VMware that allows both full and incremental backups of server instances, allowing for greater flexibility than the simple creation of an image allowed with the core VMware functionality. It can also back up a physical server and then restore it to a virtual instance, allowing for a disaster recovery strategy that uses far fewer servers at an alternate datacenter.

None of these products directly competes with each other; rather, they all help fill in some part of the large puzzle that is HADR for virtualization.

DataCore SANmelody 2.0
SANmelody from DataCore is not simply virtualization HADR software. Rather, it's storage virtualization software that encompasses many types of storage features. A couple of those features, however, make the product extremely useful in an HADR environment for virtualization.

The SANmelody software, which installs on Windows Server 2000 or Windows Server 2003, essentially turns a commodity Windows server plus storage into a SAN storage platform, with high-end features such as thin provisioning, support for boot from SAN, snapshot, and replication functionality. It works with internal storage and direct attached storage, as well as iSCSI or Fibre Channel SAN storage -- anything that Windows supports.

The aforementioned boot from SAN features are of particular note in the context of HADR: Boot from SAN allows administrators to easily create a flexible and resilient virtualization environment. The feature makes SANmelody a nice complement to versions of VMware previous to ESX 3.5, which don't support booting directly from SAN volumes: It integrates with emBoot's netBoot/i to enable boot from SAN with iSCSI as well as FC.

Because it works with emBoot, it should also work well with any open-source hypervisors that support it. Moreover, Datacore has worked with other manufacturers to ensure that SANmelody works with XenServer, Microsoft Virtual Server, and Virtual Iron.

Additionally, SANmelody simplifies the deployment of multiple instances of the same OS. Admins can create one boot volume, install an OS instance to it, then create snapshots and copy them to additional volumes very quickly.

Installation of SANmelody is no more difficult than any other Windows application, and because hardware support is based on Windows support, any hardware that runs Windows will work. After the software is installed, management of the storage system can be accomplished locally on the Windows server or via browser. The interface is clean and easy to navigate, and creating boot volumes for VMware or XenServer is simple.