Debunking Zswap and Zram Myths
6 hours ago
- #Linux
- #Performance
- #Memory Management
- Prefer zswap over zram unless you have a specific reason to use zram.
- zswap integrates with kernel memory management, compresses pages in RAM, and tiers cold data to disk.
- zram is a compressed RAM block device with a hard capacity limit, leading to potential OOMs or LRU inversion when full.
- Avoid running zram alongside disk swap as it can push active working sets to slow disk while keeping cold pages in fast RAM.
- If using zram, pair it with a userspace OOM manager like systemd-oomd or earlyoom.
- zram on servers has issues with memory usage segregation and cgroup isolation.
- zswap automatically tiers data between compressed RAM and disk, degrading gracefully under memory pressure.
- zram's block device architecture treats it like any other storage device, leading to inefficiencies like unnecessary readahead.
- zswap's integration with memory management allows it to dynamically balance LRU based on pressure.
- LRU inversion occurs when zram fills with cold data, forcing hot data to disk, worsening performance.
- zram writeback support exists but requires manual configuration and lacks automatic tiering.
- zswap handles incompressible data better by rejecting poorly compressed pages and sending them to disk.
- Using zram can increase SSD wear by shifting pressure to the page cache, leading to more disk I/O.
- zswap reduces disk writes by acting as a write-reduction filter, absorbing high-frequency page transients.
- Fedora defaults to zram-only setups for security reasons, avoiding persistent storage of sensitive data.
- zswap is recommended for most use cases due to better integration with kernel memory management and graceful degradation.