Devil's in the Virtual Details

Providers of host-based data replication are claiming to support VMware's virtual environment. But it's important, sources say, to probe exactly what that means.

Vendors such as CA, Double-Take, Neverfail, SteelEye, and Symantec all boast support for VMware virtualization. (See Double-Take Protects VMware.) But not all products are created equal, and the kind of support a replication package offers can determine how efficiently your network operates, how quickly you can recover from a hardware failure, and how much you'll spend to reach your goals.

Some products replicate VMware virtual machines on a one-to-one basis, while others can replicate the entire ESX server that the virtual machines run on. The kind of approach you deploy will determine how much you spend.

Double-Take, for instance, says the latest Double-Take for VMware Infrastructure can back up an entire VMware server without the licensing fees associated with multiple virtual machines. Similarly, SteelEye plans to announce support in March for ESX Console replication, so that firms can replicate entire servers (and save licensing costs as in Double-Take's case) as well as replicating data within and among individual VMs.

But there are tradeoffs with these products. Double-Take's VMware product doesn't yet have the byte-level replication that Double-Take offers in its regular replication packages. And because SteelEye deploys SCSI commands in dealing with links between hosts and shared storage, it's important to ascertain that the software works with other vendors' host-based packages designed to control multi-path links between servers and shared storage. (Got that?)SteelEye is ready to certify that its wares work with Hitachi DLM, IBM Subsystem Device Driver, EMC Powerpath, and Linux DMMP. It supports Microsoft Windows multipathing as well, and the firm claims support of HP SANs, too.

In contrast, Double-Take says it does not need to certify anything, since its host-based replication and high availability software just interacts with volumes on a Windows server. "We don't see anything underneath the file system," says Bob Roudebush, director of solutions engineering at Double-Take.

It's still a safe bet to check how well a system works with your existing (and in some instances very expensive) multi-path host software. "Making sure that there is interoperability between replication software and multi-path manager or other host-based intercept and filter drivers is important. Don't assume a vendor co-exists. Ask explicit questions about what they work with and in particular pertaining to the software you are using or plan on using," says Greg Schulz, founder of the StorageIO consultancy.

Costs can also mount in a nearly Byzantine way. Double-Take replication for Windows, for instance, costs $2,870 per server. Add support for virtual systems and you'll spend another $7,995 to support up to five instances of Double-Take. If you want to add specific support for VMware Infrastructure, it will cost $2,495 to protect up to 10 virtual machines on any ESX servers in your environment.

Similarly, CA XOsoft WANSync starts at $2,100 per server (you need two servers to start). WANSync/HA for failover starts at $3,000 per server.We haven't even gotten to the vagaries of pricing individual VM licenses against whole-server replication.

Despite the quirks of VMware replication, it can be worth every bit of time spent to set it up. Eugene Barnes, IT manager at surplus broker S.H. Smith & Company, for instance, is happy with the Double-Take software that enables him to protect data on five servers (three Exchange servers, one SQL server, and a main file server). In the event of slowdown or failure, the servers fail over to virtual machines located on an HP SAN. Three of the servers are local; two are at a remote site linked to HQ with a T1 line.

"I wanted something that would work cheaply and quickly," says Barnes. "I've been really happy with Double-Take." He plans to use the vendor's Double-Take for VMware Infrastructure later this year.

Mary Jander, Site Editor, Byte and Switch