How to zero unused memory to reduce VM snapshot size

Question

In Linux, how can I zero-out freed memory pages to reduce the size of VM snapshots?

Non-zeroed memory is stored in the snapshot even if that memory has been freed and is no longer used. For example, memory pages freed from the filesystem cache with

 sync && echo 3 > /proc/sys/vm/drop_caches

are needlessly stored in the snapshot because they haven't been zeroed.

Update A really horrible solution that appears to work is to malloc+memset memory in a loop until the OOM killer kicks in. This reduces the size of one VM snapshot I tested from about 800MB to about 400MB. Is there a better solution?

Answer 1

Something like this will free up disk cache, then alloc&memset the amount of free memory (minus 32MB, as headroom to avoid hitting swap) using dd. Will temporarily make the VM un-CoW memory in case of for example KSM, but will leave (unallocated) memory blocks filled with zeroes, which can then be shared/deduped with other VMs (with KSM) or easily be compressed.

#!/bin/bash

echo 3 > /proc/sys/vm/drop_caches
memfree=$(free -m | awk '/^Mem/ {print $4-32}')
if [ $memfree -gt 0 ]; then
        dd if=/dev/zero of=/dev/null bs=${memfree}M count=1
fi