Reddit User Tried to Compress System Drive with WinRAR—and Nearly Disaster Struck

Recently, a Reddit user named NoDifference412 shared their "genius" idea with the community: the hard drive is out of space, games won't download, deleting files is painful, and buying a new drive feels too expensive—so why not just compress the Windows folder? One click on "Add to archive," wait for it to finish, and boom—a big chunk of free space appears out of thin air.

The post quickly drew a crowd. Those in the know spotted the problem immediately, but what was even more surprising was that a tech blogger actually put it to the test—and it turned out WinRAR really does let you do this. It complains that certain system files are in use while continuing to pack up every file it can touch in the Windows directory, and if you pick the wrong option, it will even delete the originals after compression.

Fortunately, the blogger hit the brakes in time and pivoted to a more interesting question: is compressing an entire hard drive actually a crazy idea? The answer is no. But "not crazy" doesn't mean "worth doing." The technological evolution behind this is far more fascinating than a Reddit fail thread.

The Starting Point of Compression: Sector Waste and the Limits of tar

To understand why someone would want to compress an entire system drive, we first need to go back to the basic unit of data storage. Disks are divided into sectors—traditionally 512 bytes each, and commonly 4096 bytes today. A sector is the smallest writable unit on a disk, which means if the data you're writing doesn't fill a sector, the remaining space is wasted, usually zero-filled.

Back when hard drives were only 10MB or 20MB, this waste was fatal. So someone came up with an idea: pack many small files into one large container so they share sectors, reducing internal fragmentation. That's the core idea behind the tar file—it doesn't change the data size itself, it just packages things together neatly. The blogger used a vivid analogy: vacuuming the air out of a bag lets you fit more in the freezer, but the chicken itself doesn't get smaller.

What actually makes files "smaller" is a different technical approach: finding repeating patterns in data. For example, twelve consecutive "red"s could be recorded as "12 red"; or more generally, find repeated byte sequences in a file and store just one position marker. These algorithms are simple but effective—back in the day they could compress a 160KB shareware program down to 95KB, saving nearly $20 in hard drive costs in 1982.

Internal Fragmentation and Clusters: Sectors are physical units, while file systems typically allocate space in "clusters" containing one or more sectors. Even if a file occupies only part of a cluster, the remaining space cannot be used by other files—this is internal fragmentation. tar reduces waste by concatenating multiple small files into a continuous stream so they share clusters, but the cost is worse random access—reading one file may require sequentially scanning the entire archive.

The Golden Age of LZ Algorithms, Stacker, and Whole-Disk Compression

Nearly all of these compression formats are built on the algorithm proposed by Lempel and Ziv, which is why so many compression schemes start with "LZ." Where there's profit, patents and lawyers follow—commercial use of GIF went through a legally messy period, which ironically led to the creation of the PNG format.

In the early 1980s, a company called Stack (with a c) combined tar's container idea with LZ compression to create Stacker. Its approach: compress files first, then pack the compressed data into a virtual hard drive container—completely transparent to the user, effectively doubling hard drive capacity. Even better, hard drives back then only ran at about 5 megabits per second, so reading compressed data was actually faster—read 5 megabits, decompress into memory as 10 megabits. For a small fee you could even add a hardware compression card to speed things up further.

This idea spread rapidly. Within a few years, whole-disk compression became almost standard, bundled into various DOS systems and early Windows, typically delivering about a 2:1 storage ratio. Of course, a small number of drives were also destroyed in the process.

The Core Idea of LZ Algorithms: LZ77 represents repetitions as "length-distance" pairs—searching a sliding window for previously seen identical byte strings and pointing to them with (distance, length); LZ78 builds a dictionary and encodes repeated strings as indices. Both exploit local data redundancy to achieve lossless compression. Modern variants like LZ4 and Zstandard trade off compression ratio against speed differently and are widely used in file systems, network transmission, and memory compression.

Why No One Compresses the System Drive by Default Anymore

Today, compression is built into many file systems. Windows 10 and 11 even include a tool called Compact OS specifically for compressing operating system files, trading a bit of computing overhead for a few GB of space.

So why isn't it enabled by default? The blogger's explanation: over thirty years, the math has changed. We still face lots of small files, but the .txt of yesteryear has long been replaced by formats like .docx—which are themselves zip packages disguised as documents. Over 90% of websites use some form of compression, many of them modern LZ variants. More importantly, compression has limits—beyond a certain point you lose meaningful data; and today storage is absurdly cheap, so fussing over a few KB of document space isn't worth it.

What about photos, music, and movies? The answer is you've already compressed them. This is exactly the divide between lossless and lossy compression: lossless algorithms can perfectly restore the original data with relatively lower compression ratios, suitable for game assets or software where every bit matters; lossy algorithms discard data that human ears and eyes are unlikely to notice—JPEG, MP3, AAC, and MPEG all fall into this category. They're already compressed close to the perceptual limit, and further compression visibly degrades quality.

How Compact OS Works: Introduced in Windows 10, Compact OS uses a WIMBoot-like mechanism to store system files in compressed form in a separate data area, with reparse points making system files "appear" to still be in their original locations. It uses XPRESS or LZX algorithms, achieving roughly 1.5:1 to 2:1 compression, at the cost of some read performance. Users can manually enable it via compact.exe /compactos:always, and the system automatically decides whether to enable it by default based on disk type and capacity.

Back to That Reddit User

So the user's direction was right—he just chose the wrong tool. What he actually wanted was Compact OS, not WinRAR. And even with the right tool, the gains would be quite limited, because most of what's on his drive—photos, music, videos, documents—is already in compressed formats, so there's little space to squeeze out. At best, it might get the system back online long enough for him to order a bigger hard drive.

The interesting thing about this story is how it turns a seemingly foolish question into a tour through forty-plus years of compression technology evolution. Advice on Reddit isn't always reliable, but a good question is often more valuable than a standard answer.

One-line Summary

A Reddit user wanted to use WinRAR to compress the Windows folder to free up space, sparking a look at forty years of compression technology evolution.

Paragraph Summary

The article starts with a Reddit user's post about using WinRAR to compress the Windows folder, noting that while WinRAR allows it, the operation could delete original files and damage the system. It then traces the evolution of compression technology: early tar packaged small files to share space and reduce internal fragmentation; LZ algorithms genuinely shrank data by finding repeating patterns; in the 1980s Stacker combined both for whole-disk compression, which actually improved read performance due to slow drives. Today compression is built into file systems and Compact OS, but isn't enabled by default because many files are already compressed and storage is extremely cheap. The article concludes that the user really needed Compact OS, not WinRAR, and that the gains would be limited.