ec2_ephemeral_swap

#!/bin/sh

# PROVIDE: ec2_ephemeral_swap
# REQUIRE: devd
# BEFORE: savecore
# BEFORE: ec2_ephemeral_diskseen

# Define ec2_ephemeral_swap_enable=YES in /etc/rc.conf to enable slicing
# of the ephemeral disks to create swap space when the system next boots.
#
# Define ec2_ephemeral_swap_size=N in /etc/rc.conf to use N MB of swap space
# instead of auto-sizing based on the amount of RAM.
#
: ${ec2_ephemeral_swap_enable=NO}
: ${ec2_ephemeral_swap_size=AUTO}

. /etc/rc.subr

name="ec2_ephemeral_swap"
rcvar=${name}_enable
start_cmd="${name}_run"
stop_cmd=":"

ec2_ephemeral_swap_run()
{
	local RAM RAMMB SWAPMB MAXPAGES PAGESZ MAXSWAPMB SWAPLIMITMB
	local EC2DISKSEEN EC2DISKS NEWSWAPDISKS OLDSWAPDISKS NSWAPDISKS
	local EC2DISK SWAPDISK SWAPPERDISK DISKSZ SKIPLEN DISKSZMB SWAPSZ

	# Compute "ideal" swap size:
	#     2*RAM if RAM <= 4 GB
	#     8 GB  if 4 GB <= RAM <= 8 GB
	#     RAM   if 8 GB <= RAM
	RAM=`sysctl -n hw.physmem`
	RAMMB=`expr $RAM / 1048576`
	if [ $RAMMB -lt 4096 ]; then
		SWAPMB=`expr $RAMMB \* 2`
	elif [ $RAMMB -lt 8192 ]; then
		SWAPMB=8192
	else
		SWAPMB=$RAMMB
	fi
	debug "Predicted ideal swap size is $SWAPMB MB"

	# If a target swap size was specified, use that instead of
	# our "ideal" value.
	case ${ec2_ephemeral_swap_size} in
	[Aa][Uu][Tt][Oo])
		;;
	*)
		SWAPMB=${ec2_ephemeral_swap_size}
		;;
	esac
	debug "Target swap size is $SWAPMB MB"

	# Reduce this size if the kernel hasn't reserved enough space to
	# keep track of that much swap.
	MAXPAGES=`sysctl -n vm.swap_maxpages`
	PAGESZ=`sysctl -n hw.pagesize`
	MAXSWAPMB=$((MAXPAGES * PAGESZ / 1048576))
	SWAPLIMITMB=$((MAXSWAPMB / 2))
	if [ $SWAPLIMITMB -lt $SWAPMB ]; then
		echo -n "Reducing ephemeral swap target from $SWAPMB MB to"
		echo " $SWAPLIMITMB MB due to kernel swap_maxpages limit."
		SWAPMB=$SWAPLIMITMB
	fi

	# Get a list of "previously seen" disks.  We won't allocate swap
	# partitions on these disks, in case they're already being used.
	# (If we boot and find "new" ephemeral disks, it should be safe
	# to use those, since there isn't time for anything else to use
	# those disks before us.)
	#
	# This list is populated by the ec2_ephemeral_diskseen script,
	# which is enabled by default in order to avoid problems if a
	# system boots with ec2_ephemeral_swap disabled, a disk gets used,
	# and then the system reboots with ec2_ephemeral_swap enabled.
	if [ -f /var/db/ec2_ephemeral_diskseen ]; then
		EC2DISKSEEN=`cat /var/db/ec2_ephemeral_diskseen`
	else
		echo -n "Disabling EC2 ephemeral swap since"
		echo " ec2_ephemeral_diskseen not initialized"
		return 0
	fi

	# Get a list of EC2 ephemeral disks.
	EC2DISKS=`ls /dev/aws/disk/ephemeral`
	debug "EC2 ephemeral disks are $EC2DISKS"

	# Decide whether we should use each of them.
	NEWSWAPDISKS=""
	OLDSWAPDISKS=""
	NSWAPDISKS=0
	for EC2DISK in $EC2DISKS; do
		# Did we see this disk last time we booted?
		case $EC2DISKSEEN in
		*$EC2DISK* )
			debug "EC2 ephemeral disk seen before: $EC2DISK"

			# Only use this disk if it already has swap configured.
			if [ -c /dev/gpt/ephemeral_swap-$EC2DISK ]; then
				debug "Already has ephemeral swap: $EC2DISK"
				NSWAPDISKS=$(($NSWAPDISKS + 1))
			fi
			;;
		*)
			debug "New EC2 ephemeral disk found: $EC2DISK"

			# Translate to device name.
			SWAPDISK=`realpath /dev/aws/disk/ephemeral/$EC2DISK`

			# Paranoia: Make sure the first and last 4k of the
			# disk are zeroed; Nitro does this for "new" ephemeral
			# disks and most uses of the disk will result in one
			# or both of those being modified.
			DISKSZ=`diskinfo ${SWAPDISK} | awk '{ print $3 }'`
			SKIPLEN=$((DISKSZ / 512 - 8))
			HS=`dd if=$SWAPDISK bs=4k count=1 status=none | sha256`
			HE=`dd if=$SWAPDISK bs=512 count=8 skip=${SKIPLEN} status=none | sha256`
			if [ $HS = "ad7facb2586fc6e966c004d7d1d16b024f5805ff7cb47c7a85dabd8b48892ca7" ] &&
			   [ $HE = "ad7facb2586fc6e966c004d7d1d16b024f5805ff7cb47c7a85dabd8b48892ca7" ]; then
				NEWSWAPDISKS="$NEWSWAPDISKS $EC2DISK:$SWAPDISK"
				NSWAPDISKS=$(($NSWAPDISKS + 1))
			else
				echo "New ephemeral disk $EC2DISK is not zeroed!"
			fi
			;;
		esac
	done

	# If we have no ephemeral disks, give up.
	if [ $NSWAPDISKS -eq 0 ]; then
		echo -n "No ephemeral disks are available,"
		echo " so no swap space is being created."
		return 0
	fi

	# How much swap space do we want per disk?
	SWAPPERDISK=`expr $SWAPMB / $NSWAPDISKS`

	# NOTE: FreeBSD 12.x and earlier have limits on the size of each
	# swap device; the kernel will automatically reduce the amount of
	# swap used on oversized devices.  We could impose the limit here,
	# and potentially save some GB of disk space (by making sure that
	# we don't create swap partitions which will only be partially
	# used); but most EC2 instance types won't trip over this anyway.

	debug "Want $SWAPPERDISK MB swap on each of $NSWAPDISKS disks"
	debug "Initializing disks: $NEWSWAPDISKS"

	# Create swap partitions and label them as ephemeral_swap-foo
	for DISK in $NEWSWAPDISKS; do
		# Extract EC2 and local device names
		EC2DISK=${DISK%%\:*}
		SWAPDISK=${DISK##*\:}

		# Check if the disks are too small for the desired amount of
		# swap space.  Reduce "disk size" by 10 MB to account for
		# partitioning overhead.
		DISKSZ=`diskinfo ${SWAPDISK} | awk '{ print $3 }'`
		DISKSZMB=$((DISKSZ / 1024 / 1024 - 10))
		if [ $DISKSZMB -gt $SWAPPERDISK ]; then
			SWAPSZ=$SWAPPERDISK
		else
			debug "Reducing swap on $SWAPDISK to ${DISKSZMB}MB to fit"
			SWAPSZ=$DISKSZMB
		fi

		# Create the swap partition
		gpart create -s GPT $SWAPDISK
		gpart add -t freebsd-swap -s ${SWAPSZ}M \
		    -l ephemeral_swap-$EC2DISK $SWAPDISK
	done

	# Turn on swap on all the ephemeral disks which have swap partitions.
	for EC2DISK in $EC2DISKS; do
		if [ -c /dev/gpt/ephemeral_swap-$EC2DISK ]; then
			# Enable swap on this disk
			echo "Enabling swapping to /dev/gpt/ephemeral_swap-$EC2DISK"
			swapon /dev/gpt/ephemeral_swap-$EC2DISK
		fi
	done

	# Turn on dumping, if ${dumpdev} is AUTO (the dumpon rc.d script runs
	# before us in the boot order, so we duplicate here some of the work
	# that script does).
	case ${dumpdev} in
	[Aa][Uu][Tt][Oo])
		for EC2DISK in $EC2DISKS; do
			# Skip if this is not a disk we're swapping to.
			if ! [ -c /dev/gpt/ephemeral_swap-$EC2DISK ]; then
				continue;
			fi

			# If we don't have a dump disk yet, use this one.
			if ! [ -e /dev/dumpdev ]; then
				echo "Enabling crash dumps to /dev/gpt/ephemeral_swap-$EC2DISK"
				dumpon /dev/gpt/ephemeral_swap-$EC2DISK
				ln -sf /dev/gpt/ephemeral_swap-$EC2DISK /dev/dumpdev
			fi
		done
		;;
	esac
}

load_rc_config $name
run_rc_command "$1"