VM /ETC

Archive for the ‘RDM’ Category

“Should I use VMFS or RDMs (Raw Device Mappings) for my VMware ESX virtual machines (VMs)?” It’s a popular discussion not just for new virtual infrastructure (VI) planning but for existing environments as well. In my experience, Many VMware ESX virtual machines using RDMs were created because of a perceived boost in VM and application performance. Somehow, the wrong impression became widespread: the proprietary VMware VMFS file system created too much I/O overhead and slowed down the VM performance. Although the VMFS reputation has been publicly repaired again, there are still regular challenges explaining this virtual disk myth to VMware administrators.

There are definite pros and cons for using both VMFS and RDMs. This post suggests the 3 most common practical reasons (in my opinion) to use RDMs. That is, reasons that benefit VI administrators, leverage a VM’s native operating system,  or take advantage of technologies and process designed for physical environments. I also provide some links to a few other blogs that have recently explored the same RDM vs VMFS topic as supporting examples.

First, if proof is still needed that performance should not be a factor in the VMFS versus RDM decision, check out VMware’s own PDF titled Performance Characteristics of VMFS and RDM.

So what are some practical reason for using RDMs? Read the rest of this entry »

If you are considering virtualizing SQL, check out a great post from vm0.blogspot.com. Running SQL Server on ESX makes some great suggestions for planning  CPU, RAM, and Disk I/O to allow for the highest performance possible of your database virtual machines (VM).

The section on disk I/O made me think – specifically about whether creating RDMs to raw disks was really a better choice than carefully planning VMFS LUNs. There is just too much convenience with using VMFS in my opinion, and I’ve never really been convinced that any report or testing has proven RDMs offer better performance than carefully planned VMFS. So, instead of a long comment on the vm0 post I decided to blog about it here at VM /ETC.

Be sure to read the entire post for the RAM and CPU recommendations, but here is the section that inspired me: Read the rest of this entry »