A Low Tech Way To Warn Against Disk Full Conditions

An area where AD admins have to keep an eye on is disk full conditions on the volume that contains the AD database %windir%NTDSNTDS.DIT. It’s not a common condition, but if it happens it usually happens to ALL the DCs in a domain…and then you’re in a world of hurt.

If you’ve taken the default installation parameters, this volume is C: and so contains the OS, and also contains SYSVOL and the database transaction logs. There are several common ways the disk can fill on you.

When something like this happens, it’s a real “all hands on desk” alert because any of these problems will immediately replicate to all other DCs in the domain (or possibly forest if you’ve got GCs everywhere). Suddenly you’ve got authorization and authentication for an entire domain in danger.

Obviously the right solution is to monitor your DCs for such conditions. A quick and dirty way to buy yourself some extra time to get around a growing space problem is to pre-allocate some disk space with placeholder files. When you start getting disk full warnings, you can simply delete a placeholder file or two, and you’ve suddenly given yourself several hours (or days if it’s slow growth) to correct the condition.

Creating a placeholder file is simple:

So, I’d probably create a folder called “Placeholders” on the system partition and run this command a few times with different file names:

This will create a 5 GB placeholder file. NOTE: I do NOT recommend you create this file in a replicating directory, for example the NETLOGON share. It’s much better to manually create these on each DC than replicate 5 GB of sparse file around the domain!