zram

Zram is a Linux kernel feature that provides a compressed RAM-backed block device, typically used as a swap device. It creates one or more in-memory block devices that store data in RAM after compressing it, which can reduce the amount of physical memory needed to hold swapped-out pages and improve overall responsiveness on systems with limited or slow storage.

How it works: When the zram module is loaded, the system exposes one or more /dev/zramN devices. The size of each device is configured by writing to its disksize attribute. Pages that would normally be swapped to disk are instead written to the zram device and compressed using a chosen algorithm (such as LZ4, Zstd, or LZO). When those pages are accessed again, they are decompressed. Because the data remains in RAM, access times are generally faster than disk-based swap. Zram can be used as swap, or as a RAM-based cache or disk substitute in some scenarios.

Configuration and usage: After loading the module, administrators allocate space for the zram devices, initialize them as swap with standard tools (for example, mkswap followed by swapon), and adjust the number of devices or compression algorithm as needed. Some distributions provide helper utilities or scripts to automate this process and to adjust memory pressure dynamically.

Advantages and use cases: Zram can reduce I/O pressure on slow storage, improve performance on systems with limited RAM, and enhance responsiveness under memory pressure. It is commonly used on mobile devices, embedded systems, and some Linux desktops and servers where fast, in-memory swap is beneficial.

Limitations: Compression and decompression consume CPU resources, and the effectiveness depends on data compressibility. If memory is abundant, zram may be underutilized; if CPU is a bottleneck or data is incompressible, performance gains may be limited. It does not replace a traditional disk-based swap for long-term memory overflow.