Shrink EBS Root

This post was published 2 years, 5 months ago. Due to the rapidly evolving world of technology, some concepts may no longer be applicable.

My EC2 instances are setup to have only the operating system and program files on the root volume, with all other data (logs, mail, etc.) on a second EBS volume. This leads to a very stable root volume, which sees a minimum of changes. Fully configured, my root volume (using Amazon’s Linux) is 1.2GB. The default size of the root volume is 8GB. Given the above, it serves little purpose for me to have so much space allocated to my root volume, and unused. I opted to shrink my root volume to 4GB, and my in future reduce this even more.

Before proceeding, it is worth noting that Amazon’s Linux uses ext4 as its root filesystem. Ext2 and ext3 root file systems can be resized in the same way, however other file systems require a different procedure.

  1. Snapshot root volumeThis step is done either as a backup or to create a temporary EBS volume containing the data we will copy to the new, smaller, volume.
  2. Create a new (empty) EBS volume of the target sizeThis will become our new root volume – so, in my case, I created a 4GB EBS volume (it should be in the same availaility zone as the instance you want to attach it to)
  3. Prepare your original root volumeEither:
    • Stop (not terminate) the instance it is attached to, and detach the volume OR
    • Create a new EBS volume using the snapshot created earlier
  4. Attach the volumes from the previous 2 steps to an instance
    While you can attach them to the original instance, these volumes should not be mounted (only attached)
    In the examples below, /dev/xvda1 refers to the original root volume, and /dev/xvdg refers to the new volume.
  5. Run a file system check on the original volume (or volume derived from snapshot)
    e2fsck -f /dev/xvda1
  6. Copy the data to the new volume
    • Option 1: Use rsync
      Format the new volume: mkfs.ext4 /dev/xvdg
      Mount the two volumes, and use rsync -aHAXxSP /source /target

      • The ‘a’ option (archive) is recursive, copies symlinks, preserves permissions, times, groups, owners, device files, and special files
      • the ‘H’ option copies hardlinks
      • the ‘A’ option copies ACLs
      • the ‘X’ option copies extended attributes
      • the ‘x’ option don’t cross file system boundaries
      • the ‘S’ option handle sparse files efficiently
      • the ‘P’ option displays progress

       

    • Option 2: Use dd
      1. Resize the file system of the original volume to its minimum size
        Since this is an ext4 file system, we use resize2fs

        • the ‘M’ option shrinks to the minimum size
        • the ‘p’ option displays progress
        resize2fs -M -p /dev/xvda1

        The above command will output the new file system size. For instance:

        "Resizing the filesystem on /dev/xvda1 to 319011 (4k) blocks.
      2. Calculate the number of chunks
        The filesystem sits at the start of the partition, and is continuous – the size corresponds to output of resize2fs from above. We want to copy everything from the start to that point.

        Since EBS usage charges for I/O, we want to use a somewhat large chunk size – I used 16MB.

        blocks*4/(chunk_size_in_mb*1024) – round up a bit for safety (I ended up with 78 blocks, which I rounded to 80)

      3. Perform the actual copy of data
        dd bs=16M if=/dev/xvda1 of=/dev/xvdg count=80

        Note: dd uses ‘M’ as 1048576B and ‘MB’ as 1000000B

      4. Resize the file system on the new volume to its maximum size
        resize2fs -p /dev/xvdg
  7. Check the new file system for consistency
    e2fsck -f /dev/xvdg
  8. Now that the data has been copied over and everything checked. We can replace our root volume on the target instance.
    If the target instance is running, stop (not terminate) it
    If you haven’t already, detach the root volume from the target instance.
    Attach the new EBS volume to the target instance as /dev/sda1You can determine the root device by running:

    ec2-describe-instance-attribute `curl http://169.254.169.254/latest/meta-data/instance-id -s` --root-device-name

Start your instance and run df -h to verify the size of your root volume.

20 thoughts on “Shrink EBS Root

  1. Yeah… doesn’t work for more recent ubuntu. Just detaching the old and attaching the new root as per above fails. From the system log, printed though the management console:

    Begin: Running /scripts/init-premount ... done.
    Begin: Mounting root file system ... Begin: Running /scripts/local-top ... done.
    Gave up waiting for root device.  Common problems:
     - Boot args (cat /proc/cmdline)
       - Check rootdelay= (did the system wait long enough?)
       - Check root= (did the system wait for the right device?)
     - Missing modules (cat /proc/modules; ls /dev)
    ALERT!  /dev/disk/by-label/uec-rootfs does not exist.  Dropping to a shell!
    • Thanks for the feedback. I don’t work much with Ubuntu, so its always good to hear how things work on it (and I am a fan of Alestic too, so I’ll certainly check it out).

  2. Thank you. Worked well on RHEL 5.4. Only the -M is not supported in that version of “resize2fs”. Simply had to give it a minimum size at the end. e.g. “resize2fs -p /dev/xvda1 8G”

    • Good to know you found it useful – it isn’t uncommon for the tools on various systems to differ in the parameters they accept, but it can definitely make things annoying at times.

  3. This worked perfectly. I shrank the EBS volume of my Ubuntu 12.04 x64 instance from 8GB to 4GB successfully. I used the “dd” option instead of rsync. Thank you.

  4. Hi Cyberx86,

    I tried to use both the methods – rsync and dd

    Rsync

    after running the command, I stopped the instance. Then I detached my original root volume and attached the new volume. After this I restarted the instance. When I tried ssh into system using putty it said authentication failure

    dd method:

    When I used resize2fs -M -p /dev/xvda1 it said Online shrinking is not possible

    Can you please me what wrong I am doing

    Thanks

    • After you detach the root volume, you have to attach it to a new instance as a non-root volume. You perform the resize on this new instance. (The device should not be /dev/xvda1 – that is likely the root volume; it should be something like /dev/xvdf). As the error you got suggests, you cannot do a resize on a volume that is in use – which is why it is attached to another instance as a secondary volume (e.g. the other instance is running off its own root volume – different from the one you are resizing). Hope that helps, good luck.

  5. Hi Cyberx86,

    Thanks for helping me with shrinking my EBS root volume.

    I was just curious if you ever assigned two EIPs to a ubuntu instance in ec2. I tried doing that but after assigning 2 EIPs I was only able to access the instance using one EIP. With other I was getting connection timeout. The reason I need two EIPs from a single instance is that I want to run two websites from the same instance.

    I followed the following procedure for associating two EIPs to a single instance:-
    I created a VPC, associated two EINs to the instance and associated two EIPs to the EINs.

    Is there some additional steps which I need to perform to get both EIPs working

    Thanks
    Lavesh

    • Two websites usually don’t usually need 2 IPs (the exception being some SSL setups) – you can setup virtual hosts and have many websites running under the same IP.

      You can’t attach multiple EIPs to a non-VPC instance. In VPC, you only need one ENI – but you have a secondary private IP address on that ENI. When you associate the EIP with the instance, you choose which private IP it will be mapped to. Finally, you need to modify /etc/network/interfaces to include the new addresses. AWS provides a good overview of the procedure in their documentation. If you have trouble getting it working, I would recommend asking on ServerFault.

  6. Hi Cyberx86,

    I was successfully able to setup software raid for ebs drives.

    Wanted to know is it possible to make EBS root a part of software RAID 1 using mdadm.

    I was not able to find any solutions on the net. So, wanted to know if its possible

    Thanks,
    Lavesh

    • I believe it is possible, but I would advise against it as it has many complexities that are unnecessary (the problem comes from grub and the kernel not being able to read the array prior to it being initialized). Instead, I would suggest creating a separate RAID array, and binding mount points to the relevant locations. Essentially, don’t store anything other than the operating system and core packages on your root volume – databases, code, uploads, logs, etc. can all go on your RAID array (and when you use mount with bind, you are able to make the RAID appear transparent to the system (e.g. you can bind /var/log to /mnt/raid/logs). If you care about the contents of your root volume (which really, you shouldn’t) then take snapshots of it.

      • Essentially I was doing the same thing which you mentioned. Created a separate raid array and attached it to OS. This raid array stores all my code, db, logs everything.

        The reason I asked you if it is possible to raid root is what if the root volume goes down. what can i do in that case

        • Since your files are all on a separate volume, then it is just a matter of launching a new instance. You can do one of two things – make an image and launch an instance from that (not ideal – since the image will get outdated quite quickly) or take snapshots and create a new EBS volume from the snapshots. I would suggest daily snapshots of something like a root volume, and hourly snapshots of important data (remember, they are differential, so you only store the difference). If you are running only one instance, you will have a bit of downtime – but it should only take about 5 minutes to launch a new instance referencing your latest snapshot as the root volume. Frankly, if the downtime is not acceptable, then you need to look into setup a load-balanced, stateless, high-availability cluster – where any single node can go down, and the other nodes just pick up the slack until new nodes are brought online.

          • can the new instance created pick up the existing raid array. Also, if I am creating a daily snapshot and the new instance which gets created continue using existing db

            Was just curious if my root volume only has OS related stuff then why do I need to take daily snapshot as nothing is changing over there

          • The new instance can pick up the raid array (since the root volume stores the configuration information for the array) – you just need to ensure you mount the relevant EBS volumes with the same device names. As for the daily snapshot, you are right that it is not needed – however, things do change – you tweak your configurations, update packages, install new packages. If nothing has changed, your snapshot will take up no space, if something has changed, only the difference will be stored in the snapshot. Since the snapshots are differential, it makes it worthwhile to take a snapshot frequently, regardless of whether or not things change.

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