My SSD is running out of disk space, and I see that "Compress your OS drive" is suggested as a means to save some disk space.

My questions:

1. I'm not sure whether there are any performance penalty associating with compressing the OS drive.


2. And I'm not even sure whether this option is the same with "Compress this drive to save disk space" at the C drive property as per below. This option does seem to come with performance penalty.

Edit:

My PC information:

1. Windows 10


2. 12GB RAM


3. 4 Cores with Intel i7

Compressing data is not the best solution when the drive is full. NTFS will suffer badly from lack of space, because there are no more good places to allocate new files or move old files around and fragmentation will increase. Especially when it begins to use the reserved space for the MFT then the MFT might become fragmented and the worst things may happen

In that case first some other things need to be done

• 
  Check your data usage with some tools like TreeSize or WinDirStat. You'll be surprised by how big some files or folders are. Move or delete them if possible. See The Four Best Free Tools to Analyze Hard Drive Space on Your Windows PC
  


• 
  Reduce pagefile size. Your system will crawl to death if just 1-3GB of pagefile is used. But you can't disable it entirely (at least not a good idea), so a lower limit of 1GB and upper limit of 4GB might be a fine choice. That might save you a few GBs compared to the default setting


• 
  Reduce hiberfil.sys. Open cmd as admin and run powercfg /h /size 40 to set the size to 40% of your RAM. From Windows 7 onwards it defaults to 75% the amount of RAM, so setting it to 50% will save you 3GB of disk space. You can also turn it off with powercfg /h off
  


• If you have another HDD then don't store too many unnecessary things on the system drive. Leave it for apps and files that need fast access
  
  
  
  
  • Move the documents folders to another drive
  
  
  • Move the store app install folder and save folder
    
  
  
  • Some other folders can also be moved by changing some configurations, or by creating a symlink. But that's for another episode
  
  
  
  

Then if more space is needed, just compact the OS. The performance difference is often negligible

The CompactOS feature does indeed worth it on small SSDs compared to HDDs. My old laptop has a 1TB HDD with a 32GB SSD cache. I've tried installing Windows onto the 32GB SSD and it ran noticeably smoother than on the HDD+cache

Regarding your questions:

I'm not sure whether there are any performance penalty associating with compressing the OS drive.

Compressing files always have a CPU time penalty. That doesn't mean that it'll be slow though, since on slow storage devices like HDDs that penalty might be far less than the wasted time on accessing/reading the data from HDD. That's why sometimes compressing easily-compressed data will make it faster to access. One example is that hibernation in newer Windows compresses data before writing to disk, which makes shutdown much faster because the CPU can compress faster than the drive can write

The difference may be less obvious on SSDs, but there's no way to know that for sure except benchmarking for your specific situation. Each usecase is different, and not all disks are created equal. For example old SSDs may run at only ~200MB/s which is just about as fast as an HDD nowadays (but their obvious advantage is the fast access time), whereas newer algorithms like LZ4 (which is used in Linux's zram) can sustain ~3.9GB/s which is even faster than modern SSDs. See

• 
  With SSDs and Compact, Windows can speed up even more for an example where CompactOS boosts system performance


• I tested 25 games against the Windows Compact function: 51GB more free space, and all the games run with no performance issues.

There are often no need to worry, because Windows will do a benchmark while installing to assess whether CompactOS should be enabled or not. Although after installing apps and things that benchmark result may not be correct anymore and you may want to do a reassessment yourself

And I'm not even sure whether this option is the same with "Compress this drive to save disk space" at the C drive property as per below

Basically both use the compression feature of NTFS and can be configured by compact.exe, but there are many major differences

• 
  

  The "Compress your OS drive" (i.e. CompactOS) uses the newly introduced compression algorithms XPRESS* and LZX in Windows 10's NTFS. They're designed for efficient storage for "static" files that don't change much (like executables)

  
  
  

  OTOH the "Compress this drive to save disk space" feature uses LZNT1 algorithm and is designed for compressing frequently changed data files for optimum performance. That means it'll be faster but the compression ratio will not be as high. It's like checking the "Compress contents to save disk space" option for all files and folders on the drive

  
  
  

  

  
  



• Files compacted by CompactOS will not be marked as compressed (either shown in blue or with double arrows) if that feature is enabled



• CompactOS compresses selected system files, while NTFS whole-disk-compression compresses every file on the disk

Further reading:

• File Compression versus Compact OS in Win 10?


• How can Windows 10 function on as little as 32 GB of disk space?

The link given by Xen2050 is also good. It shows that even with the drive compression feature the difference is not that much, and in some cases it'll be faster, as I mentioned above