How to zero unused memory to reduce VM snapshot size

 

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?

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

To reduce the size of a virtual machine:

  • Log into the virtual machine; open a terminal and type 

    cd /home/

sudo dd if=/dev/zero of=foo bs=4096

sudo rm foo

  • Shut down the virtual machine, enter the VmWare/VmPlayer menu and go to

    vmware -> virtual machine settings -> HD -> utilities -> compact

    and compact the virtual machine

    • If you want to further reduce the size, use 7-zip. For example, on Linux, you can type: 

    7z a -t7z -m0=lzma -mx=9 -mfb=64 -md=32m -ms=on archive.7z directory
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