[pve-devel] [PATCH qemu-server] vzdump: use minimal VM config for offline backup

[Dominik Csapak]

does that not break the feature that
we can start a vm that started a backup while stopped?

atm we can start a backup on a stopped vm, and then simply start
it, without aborting the backup. if i read the patch
correctly, the vm now has just a minimal config and not
what the user configured

On 8/24/20 11:21 AM, Stefan Reiter wrote:
> In case we backup a stopped VM, we start an instance of QEMU to run the
> backup job. This instance will be killed afterwards without ever running
> the actual VM, so there's no need to potentially allocate or use host
> system resources for features never used.
> 
> The minimal_trim_opts array contains elements that will be cleaned from
> the config before starting.
> 
> We only write back the config in case of resume, which is never set together
> with the new "minimal" option.
> 
> Reported in the forum:
> https://forum.proxmox.com/threads/pbs-tries-to-start-vms-during-backup-not-enough-ram.74773/
> 
> Signed-off-by: Stefan Reiter <s.reiter at proxmox.com>
> ---
> 
> It is weird to me that this even happens, AFAIU QEMU only allocates the guest
> RAM as virtual as long as the guest doesn't write to it - but the forum post
> proves that it does, and it's probably also a good idea to not assign hostpci
> and usb devices for offline backups.
> 
>   PVE/QemuServer.pm        | 34 ++++++++++++++++++++++++++++++++++
>   PVE/VZDump/QemuServer.pm |  1 +
>   2 files changed, 35 insertions(+)
> 
> diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
> index a5ee8e2..73eb561 100644
> --- a/PVE/QemuServer.pm
> +++ b/PVE/QemuServer.pm
> @@ -4778,6 +4778,37 @@ sub vm_migrate_alloc_nbd_disks {
>       return $nbd;
>   }
>   
> +my @minimal_trim_opts = (
> +    qr/^hostpci\d+/,
> +    qr/^serial\d+/,
> +    qr/^usb\d+/,
> +    qr/^net\d+/,
> +    qr/^rng\d+/,
> +    qr/^ivshmem/,
> +    qr/^hugepages/,
> +    qr/^smp/,
> +    qr/^cores/,
> +    qr/^sockets/,
> +    qr/^vcpus/,
> +    qr/^cpu/,
> +    qr/^agent/,
> +    qr/^numa(?:\d+)?/,
> +);
> +
> +my sub vm_trim_conf_minimal {
> +    my ($conf) = @_;
> +
> +    $conf->{memory} = 1;
> +    $conf->{balloon} = 0;
> +
> +    # remove anything that does not affect backup but can claim host resources
> +    foreach my $r (@minimal_trim_opts) {
> +	foreach my $key (keys %{$conf}) {
> +	    delete $conf->{$key} if $key =~ $r;
> +	}
> +    }
> +}
> +
>   # see vm_start_nolock for parameters, additionally:
>   # migrate_opts:
>   #   storagemap = parsed storage map for allocating NBD disks
> @@ -4823,6 +4854,7 @@ sub vm_start {
>   #   timeout => in seconds
>   #   paused => start VM in paused state (backup)
>   #   resume => resume from hibernation
> +#   minimal => only use necessary resources (backup)
>   # migrate_opts:
>   #   nbd => volumes for NBD exports (vm_migrate_alloc_nbd_disks)
>   #   migratedfrom => source node
> @@ -4851,6 +4883,8 @@ sub vm_start_nolock {
>   	$conf = PVE::QemuConfig->load_config($vmid); # update/reload
>       }
>   
> +    vm_trim_conf_minimal($conf) if $params->{minimal};
> +
>       PVE::QemuServer::Cloudinit::generate_cloudinitconfig($conf, $vmid);
>   
>       my $defaults = load_defaults();
> diff --git a/PVE/VZDump/QemuServer.pm b/PVE/VZDump/QemuServer.pm
> index 7297795..ea8faa3 100644
> --- a/PVE/VZDump/QemuServer.pm
> +++ b/PVE/VZDump/QemuServer.pm
> @@ -827,6 +827,7 @@ sub enforce_vm_running_for_backup {
>   	    skiplock => 1,
>   	    skiptemplate => 1,
>   	    paused => 1,
> +	    minimal => !$self->{vm_was_running},
>   	};
>   	PVE::QemuServer::vm_start($self->{storecfg}, $vmid, $params);
>       };
>