Drive Letters and Mount Points
Windows traditionally uses drive letters to enable access to local or network drives. For example, the "A:" and "B:" letters used to refer to floppy drives (until floppy drives disappeared from the everyday use, replaced by far more efficient USB memory sticks). "C:" is usually the partition of the 'main' hard drive disk (HDD) where the operating system is installed. If you install two copies of Windows to two different partitions of the same HDD, you will notice that each copy will set its partition's drive letter to "C:". Thus drive letters will be swapped each time you load the other copy of Windows.
Having a single hard drive disk inside your computer, you can still have multiple drive letters. Normally each drive letter refers to its own partition (though it is possible to assign several letters to the same partition, in most cases it has no sense). A partition in simple words is a part of a HDD. If you have a HDD with a certain capacity like 1 TB, you can create one partition that would use all available space or two partitions that would use 500 GB each. Nothing can stop you from creating 10 partitions 100 GB each, or 1 partition 100 GB big and another one 900 GB big.
Ready-made preconfigured computers (notebooks, PCs) usually come with 1 HDD split into 2 partitions, using the "C:" and "D:" drive letters. The "C:" drive is used for the operating system and installed programs, while on the "D:" drive you can save your own data (documents, music, videos). If you need to reinstall Windows at some point, you can use the automatic "recovery" utility that comes with such computers. Such utility would erase all data from your drive C, reinstalling the system and programs and thus restoring your computer to its initial state. Once again, this destroys all your data on the drive C! However, no information is typically deleted on the drive D, so saving documents and other personal files there would be a natural solution. Unfortunately, Windows tends to offer a folder on the drive C for saving documents and other files, which sometimes leads to losing important information when the computer is "reset" to its initial state.
So, using one partition for the operating system with all programs and another one for your own documents, music and videos may help you keep your data safe. But using drive letters is not the only way to handle partitions in NTFS. It is possible to have two partitions with a single logical drive.
NTFS allows mounting a partition to a folder on an existing drive. This feature is quite interesting, though typical users of Windows may find it strange.
Consider that you have a drive C used by your operating system. Additionally, there is a HDD with lots of movies. Instead of assigning the HDD its own drive letter, you can create a "Movies" folder on your drive C and connect the contents of your HDD to it. It will look like you have all your movies stored on your drive C, in the "Movies" folder.
You can create and manage NTFS mount poins in Disk Management of the "Computer management" window.
Windows requires the folder you select here as a mount point to exist and be empty.
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Based on the results of numerous internet searches I can't find any (post-SQL Server 2000) reason to not use mount points.
The main reason is someone had a bad experience with them (or, conversely, no experience with them) and has completely ridden them off... forever. This is otherwise known as personal preference.
Now, there are some reasons that you couldn't use them. The number one reason I can think of is that a 3rd party driver or application/tool (think filter driver, disk replication, etc.) does not support it. A quick example of this is a block level disk replication tool that did not support anything other than NTFS, with only specific cluster sizes and couldn't go above 2 TB for any specific volume.
Is anyone aware of Windows OS limitations regarding this topic?
No. you can make many, many mount points. In fact, you'll generally have an issue with your device interfaces before you hit any appreciable limit inside of Windows Server (Assuming you're not using a version of Windows Server that is over 17 years old...).
•I've been hearing the claim "the OS doesn't recognize mount points" a lot lately. (Untrue, based on my research into the versions of Windows Server we use).
If the OS didn't recognize mount points, then how would it even let you use a mount point? That just makes no sense.
If the OS doesn't recognize mount points, why would it track them and query their metadata? Also, please note that a mount point is a construct of the filesystem which an OS may or may not support. Not all filesystems you come upon may support mount points, however the most common filesystem in Windows Server is NTFS which in fact does support mount points and it has for a while.
Just to bring this untrue item home even further; Windows Clustering has something called Cluster Shared Volumes (CSVs) which actually use mount points for the volumes... that's a native item using technology. I have to say, whomever told you this needs to be educated in the issue.
Is there any evidence- or experience-based reason NOT to use mount points with SQL Server?
Yes, there is always that one server running Windows NT 4... don't use it there. You may also want to make sure you're running a supported version of Windows Server and staying current with updates.
However, as I described above, there may be 3rd party items that are not supported or do not work properly with them. I'd say drop that provider and find a new one.
It's my understanding that mount points are incredibly useful for segregating workloads.
Mount points are just incredibly useful. There are many ways to use them, the most common is to get around the drive letter limitations (as in, there are only so many) of Windows. The next most common use is to have smaller manageable sized drives (think LUNs, virtual disk[VMDK, VHDX]) to help get away from insanely large and rarely manageable monolith volumes (it's really becomes a problem to manage drives in the 10TB range as a single LUN, virtual disk, etc.) especially on older versions of NTFS where the implementation was less than the possible usage... for example, in older versions of Windows the maximum NTFS size was 2TB.
Workload segregation is another great use. You can definitely see, there are many uses and it depends on your individual use case. There are also improper ways to use it... such as making a blanket statement that everything needs to be a mount point. That's just crazy administrative overhead at that point.