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 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: It 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.
For the purposes of this review, I did not test all the functionality of SANmelody. Instead, I looked at creating boot images for both VMware ESX 3.5 and XenServer on the storage attached to the SANmelody system. I easily created boot images and then booted the instances from the SAN storage rather than local storage. I also used VMware's VMotion easily to migrate an instance from one VMware server to another without copying files or doing anything other than changing which LUN was presented to which server.
The capabilities you get with SANmelody are not unique. Many storage systems from companies such as Compellent, Xiotech, NetApp, EMC, 3Par, and EqualLogic support boot from SAN. Moreover, dedicated (and much more expensive) storage platforms from the likes of Compellent and EMC are custom tuned and optimized; thus, you shouldn't expect to get similar levels of performance from a SANmelody system. SANmelody's strength, however, lies in its ease of use and its unique capability of adding high-level storage features -- such as boot to SAN -- to inexpensive commodity storage.