Zswap vs. Zram: Choosing the Right Linux Memory Compression Tool
When your Linux system starts running low on physical memory (RAM), it turns to swap space—a dedicated area on your hard drive or SSD used as virtual memory. However, writing to a physical disk is significantly slower than accessing RAM, leading to a noticeable drop in performance. To solve this, the Linux kernel offers two powerful tools for memory compression: Zswap and Zram.
While they both use compression to improve performance under memory pressure, they operate in fundamentally different ways. Understanding this difference is crucial for properly configuring your system and avoiding common performance pitfalls. Many users mistakenly believe they are interchangeable, but choosing the wrong one can negate the benefits you’re trying to achieve.
What is Zswap? The Compressed Swap Cache
Think of Zswap as a fast, intelligent intermediary for your existing swap space. When the kernel needs to move a memory page to swap, Zswap steps in first. It compresses the page and attempts to store it in a dynamically allocated RAM pool.
If the compressed page fits, it stays in RAM, and no disk write occurs. This is incredibly fast. Only when this RAM pool is full, or the kernel decides it’s better to evict an older page, does Zswap decompress the least recently used page and write it to your actual swap device on disk.
The primary function of Zswap is to act as a compressed write-back cache.
- Key Advantage: Zswap significantly reduces the volume and frequency of disk I/O to your physical swap device. This is especially beneficial for systems with slow spinning hard drives (HDDs) or for extending the life of Solid-State Drives (SSDs) by minimizing write cycles.
- Crucial Point: Zswap is not a replacement for a traditional swap partition or swap file; it is an enhancement for it. You must have a regular swap device configured for Zswap to function. It works in front of your swap, not instead of it.
What is Zram? The RAM-Based Block Device
Zram takes a completely different approach. It allows you to create a compressed block device that resides entirely within your system’s RAM. You can then format this virtual device and designate it as your primary swap space.
When the system needs to swap, pages are compressed and written to this RAM-based block device. Since the entire operation happens within your system memory, it is orders of magnitude faster than writing to a physical disk. There is no underlying physical swap device involved unless you configure one as a lower-priority backup.
- Key Advantage: Zram allows you to create a high-speed swap space without needing a dedicated partition or file on a disk. This is ideal for embedded systems, single-board computers (like the Raspberry Pi), or any environment where you want to avoid disk-based swap entirely.
- Crucial Point: Zram acts as a standalone swap device made from a portion of your RAM. It is often used to completely replace a traditional swap partition, which is why many modern Linux distributions like Fedora and Pop!_OS now use Zram by default.
The Core Difference: Cache vs. Device
The most common mistake is failing to recognize the fundamental distinction between Zswap and Zram.
- Zswap is a cache. It filters and delays writes to a slow, physical swap device. Its goal is to avoid disk I/O whenever possible.
- Zram is a device. It creates a new, fast swap device from RAM. Its goal is to provide a swap solution when a physical one is slow, undesirable, or unavailable.
You should not enable both at the same time, as their functions are redundant and can lead to unpredictable performance. You must choose one based on your specific hardware and goals.
Which One Should You Use? A Practical Guide
Making the right choice depends entirely on your system’s configuration.
You should use Zswap if:
- You have a system with a traditional swap partition on a slow mechanical hard drive (HDD). Zswap will provide a massive performance boost by minimizing slow disk writes.
- You want to reduce wear and tear on an SSD but still want the safety net of a large swap partition for hibernation or handling extreme memory usage scenarios.
- You have a server or desktop with plenty of RAM where you want to optimize the performance of an existing, necessary swap file.
You should use Zram if:
- Your device has no dedicated swap partition (common in cloud instances or single-board computers).
- You want to completely avoid disk writes for swap to maximize the lifespan of an SSD or the responsiveness of your system.
- You have a fast system with enough RAM that you believe a compressed-in-RAM swap is sufficient for your workload. Most modern desktops and laptops fall into this category.
How to Check Your Current Configuration
Curious about what your system is using? You can use these simple commands in your terminal:
To check for Zram, list the active swap devices:
swapon --showIf you see a device listed like
/dev/zram0, then Zram is active.To check if the Zswap module is loaded and active, you can inspect the kernel boot messages:
bash
dmesg | grep zswap
If Zswap is enabled, you will see output like “zswap: loaded using…”
By understanding that Zswap enhances an existing swap device while Zram creates a new one, you can make an informed decision to properly optimize your Linux system’s memory management for superior performance and responsiveness.
Source: https://linuxblog.io/zswap-better-than-zram/
