Docker instead of multiple VMs

Question

So we have around 8 VMs running on a 32 GB RAM and 8 Physical core server. Six of them run a mail server each(Zimbra), two of them run multiple web applications. The load on the servers are very high primarily because of heavy load on each VMs.

We recently came across Docker. It seems to be a cool idea to create containers of applications. Do you think it's a viable idea to run applications of each of these VMs inside 8 Docker Containers. Currently the server is heavily utilized because multiple VMs have serious I/O issues.

Or can docker be utilized in cases where we are only running web applications, and not email or any other infra apps. Do advise...

Answer 1

Docker will certainly alleviate your server's CPU load, removing the overhead from the hypervisor's with that aspect.

Regarding I/O, my tests revealed that Docker has its own overhead on I/O, due to how AUFS (or lately device mapper) works. In that front you will still gain some benefits over the hypervisor's I/O overhead, but not bare-metal performance on I/O. My observations, for my own needs, pointed that Docker was not "bare-metal performance like" when dealing with intense I/O services.

Answer 2

Have you thought about adding more RAM. 64GB or more? For a large zimbra deployment 4GB per VM may not be enough. Zimbra like all messaging and collaboration systems, is an IO bound application.

Having zmdiaglog (/opt/zimbra/libexec/zmdiaglog) data to see if you are allocating memory correctly would help. as per here; http://wiki.zimbra.com/wiki/Performance_Tuning_Guidelines_for_Large_Deployments#Memory_Allocation