GoKawiilDebunking zswap and zram myths
tl;dr:
If in doubt, prefer to use zswap. Only use zram if you have a highly specific reason to.
In terms of architecture:
zswap sits in front of your disk swap, compresses pages in RAM, and automatically tiers cold data to disk. It integrates directly with the kernel's memory management and distributes pressure gracefully.
zram is a compressed RAM block device with a hard capacity limit. When you put swap on it and it fills up, there is no automatic eviction, and the kernel has very little leverage to do anything about the situation. The system either OOMs or falls back to lower-priority swap, causing LRU inversion (see below). It only really makes sense for extremely memory-constrained embedded systems, diskless setups, or cases with specific security requirements around keeping private data off persistent storage. Swap on zram is also increasingly unsupported upstream.
My advice is:
Do not run zram alongside disk swap wherever possible. In such setups, zram fills fast RAM with cold, stale pages while pushing your active working set onto slow disk, making things actively worse than if you had no compressed swap at all.
wherever possible. In such setups, zram fills fast RAM with cold, stale pages while pushing your active working set onto slow disk, making things actively worse than if you had no compressed swap at all. If you must use zram , pair it with a userspace OOM manager like systemd-oomd or earlyoom. Without one, the kernel's OOM killer can leave the system hung for minutes before acting, as it often only fires after exhausting many cycles of aggressive reclaim that often accomplish nothing (more on this below).
, pair it with a userspace OOM manager like systemd-oomd or earlyoom. Without one, the kernel's OOM killer can leave the system hung for minutes before acting, as it often only fires after exhausting many cycles of aggressive reclaim that often accomplish nothing (more on this below). On servers, zram has additional significant problems. One major one is that its memory usage is totally segregated from the rest of the system, and as such is not charged to any cgroup, breaking isolation semantics.
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