A process called sdmd is using huge amounts of resources...!? There's a process called sdmd running which reads gigabyte after gigabyte from disk and is slowing my computer to a crawl. What is that, and how can I get rid of it?
This may affect OS X El-Capitan or later installations running Server.app with active file sharing. sdmd is a service which produces indices and previews for iOS devices. In the process it parses everything in the share, which, depending on how large that shared folder is and where it's located, can take quite some time and resources.

I've seen this causing an issue with a relatively slow external RAID array, which another process was trying to write to; sdmd was causing so much I/O on the drive that the process couldn't write data fast enough, causing data to pile up in its memory, which saturated RAM, caused lots of swapping, and eventually caused OS X to suspend the process outright to keep the system from keeling over. Nice chain reaction...

To get rid of sdmd, disable iOS Access (next to AFP, SMB etc.) in the details of each individual shared folder in Server.app (or at least in the folders where it causes a problem).

Tip of the hat to https://osxbytes.wordpress.com/2015/10/02/server-app-5-0-4-sdmd-and-ios/ for this solution.