Is there a reason to keep Windows' primary partition / drive C: small?
In my jobs almost two decades ago, IT experts would keep the size of Windows' main partition (C drive) extremely small compared to the other partitions. They would argued this runs PC at optimum speed without slowing down. [...] My question is this practice still good?
In general: No.
In older Windows versions, there were performance problems with large drives (more accurately: with large filesystems), mainly because the FAT filesystem used by Windows did not support large filesystems well. However, all modern Windows installations use NTFS instead, which solved these problems. See for example Does NTFS performance degrade significantly in volumes larger than five or six TB?, which explains that even terabyte-sized partitions are not usually a problem.
Nowadays, there is generally no reason not to use a single, large C: partition. Microsoft's own installer defaults to creating a single, large C: drive. If there were good reasons to create a separate data partition, the installer would offer it - why should Microsoft let you install Windows in a way that creates problems?
The main reason against multiple drives is that it increases complexity - which is always bad in IT. It creates new problems, such as:
- you need to decide which files to put onto which drive (and change settings appropriately, click stuff in installers etc.)
- some (badly written) software may not like not being put onto a drive different than C:
- you can end up with too little free space on one partition, while the other still has free space, which can be difficult to fix
There are some special cases where multiple partitions make still make sense:
- If you want to dual-boot, you (usually) need separate partitions for each OS install (but still only one partition per install).
- If you have more than one drive (particularly drives with different characteristics, such as SSD & HD), you may want to pick and choose what goes where - in that case it can make sense to e.g. put drive C: on the SSD and D: on the HD.
To address some arguments often raised in favor of small/separate partitions:
- small partitions are easier to backup
You should really back up all your data anyway, to splitting it across partitions does not really help. Also, if you really need to do it, all backup software I know lets you selectively back up a part of a partition.
- if one partition is damaged, the other partition may still be ok
While this is theoretically true, there is no guarantee damage will nicely limit itself to one partition (and it's even harder to check to make sure of this in case of problems), so this provides only limited guarantee. Plus, if you have good, redundant backups, the added safety is usually to small to be worth the bother. And if you don't have backups, you have much bigger problems...
- if you put all user data on a data partition, you can wipe and reinstall / not backup the OS partition because there is no user data there
While this may be true in theory, in practice many programs will write settings and other important data to drive C: (because they are unfortunately hardcoded to do that, or because you accidentally forgot to change their settings). Therefore IMHO it is very risky to rely on this. Plus, you need good backups anyway (see above), so after reinstallation you can restore the backups, which will give you the same result (just more safely). Modern Windows versions already keep user data in a separate directory (user profile directory), so selectively restoring is possible.
See also Will you install software on the same partition as Windows system? for more information.
The historical reason for this practice is most likely rooted in the performance properties of rotating magnetic HDDs. The area on spinning disks with the highest sequential access speed are the outermost sectors (near the start of the drive).
If you use the whole drive for your operating system, sooner or later (through updates etc) your OS files would be spread out all over the disk surface. So, to make sure that the OS files physically stay in the fastest disk area, you would create a small system partition at the beginning of the drive, and spread the rest of the drive in as many data partitions as you like.
Seek latency also partly depends on how far the heads have to move, so keeping all the small files somewhat near each other also has an advantage on rotational drives.
This practice has lost all its reason with the advent of SSD storage.
Is there a reason to keep Windows' primary partition / drive C: small?
Here are a few reasons to do that:
- All system files and the OS itself are on the primary partition. It is better to keep those files seperated from other software, personal data and files, simply because constantly meddling in the bootable partition and mixing your files there might occasionally lead to mistakes, like deleting system files or folders by accident. Organization is important. This is why the size of the primary partition is low -- to discourage users from dumping all their data in there.
- Backups - it's a lot easier, faster, and effective to backup and recover a smaller partition than a bigger one, depending on the purpose of the system. As noted by @computercarguy in the comments, it is better to backup specific folders and files, than backing up a whole partition, unless needed.
- It could improve performance, however, in a hardly noticeable manner. On NTFS filesystems, there are the so-called Master File Tables on each partition, and it contains meta-data about all the files on the partition:
Describes all files on the volume, including file names, timestamps, stream names, and lists of cluster numbers where data streams reside, indexes, security identifiers, and file attributes like "read only", "compressed", "encrypted", etc.
This might introduce an advantage, though unnoticeable, thus this could be ignored, as it really doesn't make a difference. @WooShell's answer is more related to the performance issue, even though it still is neglectable.
Another thing to note, is that in case of having an SSD + HDD, it is way better to store your OS on the SSD and all your personal files/data on the HDD. You most likely wouldn't need the performance boost from having an SSD for most of your personal files and consumer-grade solid state drives usually do not have much space on them, so you'd rather not try to fill it up with personal files.
Can someone explain why this practice is done and is it still valid?
Described some of the reasons why it is done. And yes, it is still valid, though not a good practice anymore as it seems. The most notable downsides are that end-users will have to keep track on where applications suggest to install their files and change that location (possible during almost any software installation, especially if expert/advanced install is an option) so the bootable partition doesn't fill up, as the OS does need to update at times, and another downside is that when copying files from one partition to another, it actually needs to copy them, while if they were in the same partition, it just updates the MFT and the meta-data, does not need to write the whole files again.
Some of these unfortunately can introduce more problems:
- It does increase the complexity of the structure, which makes it harder and more time-consuming to manage.
- Some applications still write files/meta-data to the system partition (file associations, context menus, etc..), even if installed in another partition, thus this makes it harder to backup and might introduce failures in syncing between partitions. (thanks to @Bob's comment)
To avoid the problem you're having, you need to:
- Always try to install applications on the other partitions (change the default installation location).
- Make sure to install only important software in your bootable partition. Other not-so-needed and unimportant software should be kept outside of it.
I am also not saying that having multiple partitions with a small primary one is the best idea. It all depends on the purpose of the system, and although it introduce a better way to organize your files, it comes with its downsides, which on Windows systems in the current days, are more than the pros.
Note: And as you've mentioned yourself, it does keep the data that is in separate partitions safe in case of a failure of the bootable partition occurs.