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EC2 persistent boots with pivot root

S Rich | | March 2, 2010

Amazon recently allowed Elastic Block Store to boot persistent images. However, there are two concerns I have with the method. * The EBS boot volumes must be EBS Snapshots, which cost more than regular EBS volumes. (http://aws.amazon.com/ec2/#pricing) * The EBS boot volumes currently do not work within the Virtual Private Cloud (VPC) infrastructure. (http://aws.amazon.com/vpc/faqs/#45) To work around these two issues, it is possible to “pivot” from a normal AMI’s local root volume (/dev/sda1) to an arbitrary EBS volume that contains a full OS installation. An additional benefit is that a very small AMI may be used to launch the EBS backed instance, so that the instance launches much faster than a large AMI. The goal of this post is to provide instructions on how to build a small AMI that is capable of launching a full-blown persistent Linux installation that is backed on EBS storage. BusyBox, “The Swiss Army Knife of Embedded Systems”, provides a solid foundation for our small AMI. To build a BusyBox AMI and an instance backed by EBS, simply follow these directions: 1. Start an instance from an AMI that is running the desired kernel and software configuration. (You can use an existing public AMI or a custom AMI that you created. Personally, I prefer to create my own AMI’s, so I know what is in them.) 2. Log into the instance. 3. Download the latest version of BusyBox. wget “http://busybox.net/downloads/busybox-1.15.3.tar.bz2” 4. Create a busyroot directory. mkdir busyroot 5. Extract BusyBox. bunzip2 busybox-1.15.3.tar.bz2 tar xvf busybox-1.15.3.tar 6. Configure BusyBox. (You can experiment here to reduce the image size, but the configuration listed below works.) cd busybox-1.15.3 make config Select “y” for the STATIC option and the default values for everything else. Build BusyBox as a static binary (no shared libs) (STATIC) [N/y/?] y 7. Make and install BusyBox. make CONFIG_PREFIX=$HOME/busyroot install chmod 4755 $HOME/busyroot/bin/busybox 8. Create required directories. cd $HOME/busyroot mkdir dev sys etc proc mnt mnt/new-root 9. Create the necessary devices. (We will use /dev/sdj for the EBS volume, but this could be any block device not used by the normal AMI.) MAKEDEV -d $HOME/busyroot/dev -x sdj MAKEDEV -d $HOME/busyroot/dev -x console MAKEDEV -d $HOME/busyroot/dev -x null MAKEDEV -d $HOME/busyroot/dev -x zero 10. Create the init file. mv $HOME/busyroot/sbin/init $HOME/busyroot/sbin/init.orig cat <<'EOL' > $HOME/busyroot/sbin/init #!/bin/busybox sh PATH=/bin:/usr/bin:/sbin:/usr/sbin NEWDEV=”/dev/sdj” NEWTYP=”ext3″ NEWMNT=”/mnt/new-root” OLDMNT=”/mnt/old-root” OPTIONS=”noatime,ro” SLEEP=10 echo “Remounting writable.” mount -o remount,rw / [ ! -d $NEWMNT ] && echo “Creating directory $NEWMNT.” && mkdir -p $NEWMNT while true ; do echo “sleeping…” sleep $SLEEP echo “Trying to mount $NEWDEV writable.” mount -t $NEWTYP -o rw $NEWDEV $NEWMNT || continue echo “Mounted.” break; done [ ! -d $NEWMNT/$OLDMNT ] && echo “Creating directory $NEWMNT/$OLDMNT.” && mkdir -p $NEWMNT/$OLDMNT echo “Remounting $NEWMNT $OPTIONS.” mount -o remount,$OPTIONS $NEWMNT echo “Trying to pivot.” cd $NEWMNT pivot_root . ./$OLDMNT for dir in /dev /proc /sys; do echo “Moving mounted file system ${OLDMNT}${dir} to $dir.” mount –move ./${OLDMNT}${dir} ${dir} done echo “Trying to chroot.” exec chroot . /bin/sh -c “umount ./$OLDMNT; exec /sbin/init $*” < /dev/console > /dev/console 2>&1 EOL chmod 755 $HOME/busyroot/sbin/init 11. Create the fstab file. cat <<'EOL' > $HOME/busyroot/etc/fstab /dev/sda1 / ext3 defaults 1 1 none /dev/pts devpts gid=5,mode=620 0 0 none /proc proc defaults 0 0 none /sys sysfs defaults 0 0 EOL 12. Create a 4MB loopback file. cd dd if=/dev/zero of=busybox.fs bs=1M count=4 mkfs.ext3 busybox.fs 13. Mount the loopback file. mkdir $HOME/busyimg mount -o loop $HOME/busybox.fs $HOME/busyimg 14. Copy the staged files and directories to the image. (Technically, the BusyBox image could have been built directly in $HOME/busyimg, but we were not sure how big the image was going to be.) cp -rp $HOME/busyroot/* $HOME/busyimg 15. Un-mount the image. sync umount -d $HOME/busyimg 16. Set environment variables. export EC2_HOME=/opt/ec2-api-tools export EC2_CERT=/path/to/your/cert.pem export EC2_PRIVATE_KEY=/path/to/your/pk.pem export AWS_ACCOUNT_NUMBER=”NNNN-NNNN-NNNN” export AWS_ACCESS_KEY_ID=your_key export AWS_SECRET_ACCESS_KEY=your_secret_key export EC2_BUCKET=”your_bucket” export JAVA_HOME=/usr/java/default export ARCH=`uname -i` export AKI=`curl -s http://169.254.169.254/latest/meta-data/kernel-id` export ARI=`curl -s http://169.254.169.254/latest/meta-data/ramdisk-id` export INSTANCE_ID=`curl -s http://169.254.169.254/latest/meta-data/instance-id` export AVAIL_ZONE=`curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone` export SEC_GROUP=`curl -s http://169.254.169.254/latest/meta-data/security-groups` export PUB_KEY=`wget -q -O – “http://169.254.169.254/latest/meta-data/public-keys” | awk -F= ‘{print $2}’` 17. Bundle the image. ec2-bundle-image -i $HOME/busybox.fs -d /tmp -k $EC2_PRIVATE_KEY -c $EC2_CERT -u $AWS_ACCOUNT_NUMBER -r $ARCH –kernel $AKI –ramdisk $ARI 18. Upload the image. ec2-upload-bundle -b $EC2_BUCKET -m /tmp/busybox.fs.manifest.xml -a $AWS_ACCESS_KEY_ID -s $AWS_SECRET_ACCESS_KEY 19. Register the AMI. BUSYBOX_AMI=`ec2-register “$EC2_BUCKET/busybox.fs.manifest.xml” | awk ‘{print $2}’` echo “BUSYBOX_AMI: $BUSYBOX_AMI” 20. Create an EBS volume of the desired size (10G or more) in the desired availability zone. VOLUME_ID=`ec2-create-volume -s 10 -z $AVAIL_ZONE | awk ‘{print $2}’` echo “VOLUME_ID: $VOLUME_ID” 21. Attach the volume to the current instance as /dev/sdj. ec2-attach-volume $VOLUME_ID -i $INSTANCE_ID -d /dev/sdj 22. Create an EXT3 file system on /dev/sdj. mkfs.ext3 /dev/sdj 22. Mount the EBS volume. mkdir /mnt/ebs_boot mount /dev/sdj /mnt/ebs_boot 23. Copy the current AMI to the EBS volume. rsync -avHx / /mnt/ebs_boot 24. Fix the /etc/fstab file. vi /mnt/ebs_boot/etc/fstab Remove the local file systems. /dev/sda1 / ext3 defaults 1 1 /dev/sdb /mnt ext3 defaults 1 2 /dev/sda3 swap swap defaults 0 0 Add the /dev/sdj file system. /dev/sdj / ext3 defaults 1 1 25. Fix the /etc/inittab file. The cloud AMI’s are normally configured for runlevel 4. vi /mnt/ebs_boot/etc/inittab Edit the following line if necessary: id:4:initdefault: 26. Un-mount the EBS volume. sync umount /mnt/ebs_boot 27. Detach the volume. ec2-detach-volume $VOLUME_ID -i $INSTANCE_ID -d /dev/sdj 28. Create a new instance running the BusyBox AMI. BUSYBOX_ID=`ec2-run-instances $BUSYBOX_AMI -z $AVAIL_ZONE -k $PUB_KEY -g $SEC_GROUP | awk ‘{print $6}’` 29. Wait until the instance is running… ec2-describe-instances $BUSYBOX_ID 30. Attach the EBS volume to the BusyBox instance as /dev/sdj. ec2-attach-volume $VOLUME_ID -i $BUSYBOX_ID -d /dev/sdj 31. Reboot the BusyBox instance to make sure it picks up the new device. ec2-reboot-instances $BUSYBOX_ID 32. Check the BusyBox instance’s console output to make sure it came up as expected. ec2-get-console-output $BUSYBOX_ID 33. Log into the new EBS backed instance. That should be it. You now have a persistent instance that is backed by EBS storage!

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