raid_ephemeral.sh

#!/bin/bash -ue
#
# this script will attempt to detect any ephemeral drives on an EC2 node and create a RAID-0 stripe.
# It should be run once to setup the RAID, and automatically mount on reboot without further intervention.
#
# Beware, This script is NOT fully idempotent.
#
mount_point=${1:-"/mnt"}

METADATA_URL_BASE="http://169.254.169.254/latest"

DEBIAN_FRONTEND=noninteractive apt-get -y install mdadm curl xfsprogs

# Configure Raid - take into account xvdb or sdb
root_drive=`df -h | grep -v grep | awk 'NR==2{print $1}'`

if [ "${root_drive}" == "/dev/xvda1" ]; then
    echo "Detected 'xvd' drive naming scheme (root: ${root_drive})"
    DRIVE_SCHEME='xvd'
else
    echo "Detected 'sd' drive naming scheme (root: ${root_drive})"
    DRIVE_SCHEME='sd'
fi

# figure out how many ephemerals we have by querying the metadata API, and then:
#  - convert the drive name returned from the API to the hosts DRIVE_SCHEME, if necessary
#  - verify a matching device is available in /dev/
drives=""
ephemeral_count=0
ephemerals=$(curl --silent ${METADATA_URL_BASE}/meta-data/block-device-mapping/ | grep ephemeral)

for e in ${ephemerals}; do
    echo "Probing ${e} .."
    device_name=$(curl --silent ${METADATA_URL_BASE}/meta-data/block-device-mapping/${e})
    # might have to convert 'sdb' -> 'xvdb'
    device_name=$(echo ${device_name} | sed "s/sd/$DRIVE_SCHEME/")
    device_path="/dev/${device_name}"
    echo "Converting device name from ${device_name} to ${device_path}"
    
    # test that the device actually exists since you can request more ephemeral drives than are available
    # for an instance type and the meta-data API will happily tell you it exists when it really does not.
    if [ -b ${device_path} ]; then
        echo "Detected ephemeral disk: ${device_path}"
        drives="$drives ${device_path}"
        ephemeral_count=$((ephemeral_count + 1 ))
    else
        echo "Ephemeral disk ${e}, ${device_path} is not present. skipping"
    fi
done

# need at least one disk
if [ "${ephemeral_count}" = 0 ]; then
    echo "No ephemeral disk detected. exiting"
    exit 0
fi

# check for existing RAID
if [[ -e /dev/md0 ]]
then
    echo "Existing RAID array detected. exiting"
    exit 0
fi

# unmount any mounted ephemeral disks
for drive in ${drives}; do
    if [[ $(cat /proc/mounts  | grep ${drive}) ]]
    then
        umount ${drive}
    fi
done

# umount mount point if mounted
if [[ $(cat /proc/mounts  | grep ${mount_point}) ]]
then
    umount ${mount_point}
fi

# create the new mount point if needed
if [[ ! -e ${mount_point} ]]
then
    mkdir -p ${mount_point}
fi

# overwrite first few blocks in case there is a filesystem, otherwise mdadm will prompt for input
for drive in ${drives}; do
    dd if=/dev/zero of=${drive} bs=4096 count=1024
done

# Prepare mount point
if [[ ! -e /mnt/md0 ]]
then
    mkdir /mnt/md0
fi

# Create /dev/md0
partprobe || true
mdadm --create --verbose /dev/md0 --level=0 -c256 --force --raid-devices=${ephemeral_count} ${drives}
blockdev --setra 65536 /dev/md0

# configure mdadm.conf
/bin/cat<<EOF > /etc/mdadm/mdadm.conf
# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#

# by default (built-in), scan all partitions (/proc/partitions) and all
# containers for MD superblocks. alternatively, specify devices to scan, using
# wildcards if desired.
DEVICE partitions containers

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# instruct the monitoring daemon where to send mail alerts
MAILADDR root@localhost

# definitions of existing MD arrays
EOF

mdadm -Es >> /etc/mdadm/mdadm.conf


# Tuning for ec2 instance-store vols: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html#disk-performance
#
# "When configuring software-based RAID, make sure to change the minimum reconstruction speed"
echo $((30*1024)) > /proc/sys/dev/raid/speed_limit_min
#
# "Because of the way that Amazon EC2 virtualizes disks, the first write to any location on a standard instance store
# volume performs more slowly than subsequent writes. For most applications, amortizing this cost over the lifetime of
# the instance is acceptable. However, if you require high disk performance, we recommend that you pre-warm your drives
# by writing once to every drive location before production use."
# 
# "Note: The I2 high I/O instance type uses direct-attached solid state drives that provide maximum performance at launch
# time, without pre-warming."
#
# Pre-warm instance-store vols
# - This is VERY SLOW
# - Repeat until per-block write performance stabilizes across runs (?)
# - Try multiple instances to detect and throw out bad apples...
#time pv -terb /dev/zero -s "`blockdev --getsize64 /dev/md0`" | dd of=/dev/md0 bs=1M || true

# Create + mount fs
mkfs -t xfs /dev/md0
#mount -t xfs -o noatime /dev/md0 ${mount_point}

# Remove old /mnt (xvdb/sdb) entry from fstab
chmod 777 /etc/fstab
sed -i "/${DRIVE_SCHEME}b/d" /etc/fstab

# Make raid appear on reboot
# only insert once into fstab
fstab_entry="/dev/md0 ${mount_point} xfs defaults,noatime,nobootwait 0 0"
if ! grep -q "${fstab_entry}" /etc/fstab
then
    echo "${fstab_entry}" | tee -a /etc/fstab
fi

mount ${mount_point}

update-initramfs -u