Linux · Systems fluency
How does swap work and when should you use it?
Short answer
Swap moves less-used memory pages to disk so the kernel can use RAM for active work. It protects against sudden memory spikes but is not a substitute for enough memory, because swapping is orders of magnitude slower than RAM.
Swap can be a partition or a file. The kernel decides what to move based on pressure and swappiness. High swap-in and swap-out rates with slow response usually indicate memory pressure, while a small amount of swap usage can be normal.
swapon --show
fallocate -l 2G /swapfile
chmod 600 /swapfile
mkswap /swapfile
swapon /swapfileOperational considerations
- Add the swapfile to fstab to persist it across reboots
- vm.swappiness tunes the tendency to reclaim anonymous pages
- zram provides compressed RAM-backed swap with low latency
- Hibernation needs swap at least as large as memory and is often disabled in production
- SSD write endurance matters for heavy swap, though modern drives tolerate far more writes
Monitor si and so in vmstat. Sustained swapping means the workload does not fit in memory. The correct response is usually to reduce the working set, add memory, or fix a leak rather than to enlarge swap indefinitely.
Follow-ups you should expect
Interviewers use these to check whether the first answer was understanding or memorisation.
- 1Does swap usage always mean the system is unhealthy?
- 2What does vm.swappiness control?
- 3When would you use zram instead of disk swap?
- 4How do you tell whether a system is actively swapping?
Say it in the interview
Swap lets the kernel move less-used pages out of RAM. It can absorb spikes, but sustained swapping is a sign the working set does not fit. I monitor swap-in and swap-out rather than just usage. The fix for persistent pressure is usually less memory demand or more RAM, not an ever-larger swapfile.
Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.
Related questions
Reading is step one
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