Provide a way to disable swap files added by default to all deployed machines

[Dmitrii Shcherbakov]

Currently all ubuntu installations have swap enabled by default even if there was no swap partition configured in the node storage configuration. For bionic this is somewhat consistent with [1] and [2], however, MAAS uses curtin to perform the installation and not d-i or subiquity.

It would make sense to have a UI option and a way to disable swap via API for individual nodes, by tag or completely for all deployed nodes.

This is relevant, for example, on hypervisor nodes where guest (QEMU process) pages can be otherwise swapped out by the host or storage nodes where certain designs prohibit swap usage completely making a compromise between trashing vs OOM killer behavior. There are other applications where this can be relevant as well.

The curtin logic is pretty straightforward: setup a swap file if swap size is not provided (None) and it is set to 'None' if there is no swap config. And the default curtin config generated by MAAS (see the attachment) does not contain a swap section.

https://github.com/CanonicalLtd/curtin/blob/ubuntu/18.1-52-g5f0082d1-0ubuntu1/curtin/swap.py#L54-L63
def setup_swapfile(target, fstab=None, swapfile=None, size=None, maxsize=None):
    if size is None:
        size = suggested_swapsize(fsys=target, maxsize=maxsize)

    if size == 0:
        LOG.debug("Not creating swap: suggested size was 0")
        return

    if swapfile is None:
        swapfile = "/swap.img"

https://github.com/CanonicalLtd/curtin/blob/18.1-52-g5f0082d1-0ubuntu1/curtin/commands/curthooks.py#L517-L520
def add_swap(cfg, target, fstab):
    if 'swap' in cfg and not cfg.get('swap'):
        LOG.debug("disabling 'add_swap' due to config")
        return
    swapcfg = cfg.get('swap', {})
    fname = swapcfg.get('filename', None)
    size = swapcfg.get('size', None)
    maxsize = swapcfg.get('maxsize', None)
    # ...
    swap.setup_swapfile(target=target, fstab=fstab, swapfile=fname, size=size,
                        maxsize=maxsize)

https://github.com/CanonicalLtd/curtin/blob/ubuntu/18.1-52-g5f0082d1-0ubuntu1/curtin/commands/curthooks.py#L1163-L1167
def builtin_curthooks(cfg, target, state):
# ...
    with events.ReportEventStack(
            name=stack_prefix + '/setting-up-swap',
            reporting_enabled=True, level="INFO",
            description="setting up swap"):
        add_swap(cfg, target, state.get('fstab'))

https://curtin.readthedocs.io/en/latest/topics/config.html#swap
"size: <Size string> Configure the exact size of the swapfile. Setting size to 0 will disable swap."

maas-vhost1/2018-09-21/messages:Sep 21 19:06:03 maas-vhost1 cloud-init[1025]: Running command ['sh', '-c', 'rm -f "$1" && umask 0066 && { fallocate -l "${2}M" "$1" || dd if=/dev/zero "of=$1" bs=1M "count=$2"; } && mkswap "$1" || { r=$?; rm -f "$1"; exit $r; }', 'setup_swap', '/tmp/tmptm16lan8/target//swap.img', '2048'] with allowed return codes [0] (capture=False)
maas-vhost1/2018-09-21/messages:Sep 21 19:06:03 maas-vhost1 cloud-init[1025]: Setting up swapspace version 1, size = 2 GiB (2147479552 bytes)
maas-vhost1/2018-09-21/messages:Sep 21 19:06:03 maas-vhost1 cloud-init[1025]: creating swap file '/swap.img' of 2048MB took 0.122 seconds
maas-vhost1/2018-09-21/messages:Sep 21 19:06:03 maas-vhost1 cloud-init[1025]: finish: cmd-install/stage-curthooks/builtin/cmd-curthooks/setting-up-swap: SUCCESS: setting up swap

# swap is on, whether you need it or not
free -m
              total used free shared buff/cache available
Mem: 1993 90 1677 0 225 1756
Swap: 2047 0 2047

One could set vm.swappiness=0 [3], however, this is not ideal because it does not entirely disable swap [3]:
"A value of 0 instructs the kernel not to initiate swap until the amount of free and file-backed pages is less than the high water mark in a zone."

The default value for vm.swappiness is 60 [4] and has to be changed explicitly as well.

[1] https://wiki.ubuntu.com/ZestyZapus/ReleaseNotes#Swap
[2] http://blog.surgut.co.uk/2016/12/swapfiles-by-default-in-ubuntu.html
Starting from 17.04 Zesty Zapus release, instead of creating swap partitions, swapfiles will be used by default for non-lvm based installations.
[3] https://www.kernel.org/doc/Documentation/sysctl/vm.txt
[4] http://kernel.ubuntu.com/git/ubuntu/ubuntu-xenial.git/tree/mm/vmscan.c?h=Ubuntu-hwe-4.15.0-35.38_16.04.1#n152

[Blake Rouse (blake-rouse)]

Have you tried:

maas {session} machine update {system_id} swap_size=0

[Dmitrii Shcherbakov (dmitriis)]

Blake,

Yes, this works.

I'll change this to UI-only then.

[Dmitrii Shcherbakov (dmitriis)]

Would it make sense to have a global option for this?

[Jerzy Husakowski (jhusakowski)]

Option to be added to the UI, it is already supported in the API.

[Thorsten Merten (thorsten-merten)]

@ack does the websocket API already support this?