Shrinking CentOS 7 EBS boot volume on AWS

I’m really bummed when I provision a boot volume that’s too large..

[rgibson@centos7 ~]$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       50G  6.5G   44G  13% /

At the current GP2 price of $0.05/gb, I’m paying three or four dollars more per month than I really should be. It’s really easy to make boot volumes bigger, but there’s not a lot of instruction out there on making them smaller. This method also has the advantage of removing the marketplace codes on the volume, making it easier to attach to a running instance later.

Shutting down the instance you want to shrink the boot volume for. You’ll need a second instance of a different OS, or at least one from a different AMI, as we can’t reliably boot up if there are multiple volumes attached with the same UUID. I used an Ubuntu instance.

First, detach the root volume you want to shrink from the original instance:

Detach original root volume

(Side note, this would be a good time to take a snapshot of this volume just in case you make a mistake in this process, such as wiping the partition table or running mkfs on the wrong volume…)

Attach it as a volume on your powered-off Ubuntu instance. I used sdf:

Attach volume to sdf on Ubuntu instance

Create a smaller replacement volume in the correct availability zone. This would be a good time to encrypt it as well…

Create new smaller volume. Encrypt it.

Attach this volume as sdg to the Ubuntu instance, and power it on. SSH in and create a new empty partition on the new volume. Pay attention that you’re working on the NEW volume and not messing with your source volume. Since we attached it as sdg it will be visible as /dev/xvdg:

ubuntu@ubuntu:~$ sudo -i 
root@ubuntu:~# fdisk /dev/xvdg 
Welcome to fdisk (util-linux 2.27.1).                                                                                                                                  
Changes will remain in memory only, until you decide to write them.                                                                                                    
Be careful before using the write command. 
Device does not contain a recognized partition table. 
Created a new DOS disklabel with disk identifier 0x1db62587. 
Command (m for help): n 
Partition type 
   p   primary (0 primary, 0 extended, 4 free) 
   e   extended (container for logical partitions) 
Select (default p): p 
Partition number (1-4, default 1):  
First sector (2048-41943039, default 2048):  
Last sector, +sectors or +size{K,M,G,T,P} (2048-41943039, default 41943039):  
Created a new partition 1 of type 'Linux' and of size 20 GiB. 
Command (m for help): w 
The partition table has been altered. 
Calling ioctl() to re-read partition table. 
Syncing disks. 

Format the new partition as XFS:

root@ubuntu:~# mkfs.xfs /dev/xvdg1
meta-data=/dev/xvdg1             isize=512    agcount=4, agsize=1310656 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=1, sparse=0
data     =                       bsize=4096   blocks=5242624, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal log           bsize=4096   blocks=2560, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

Make mount points for the original and new volumes, and mount them. I mounted the original drive read only:

root@ubuntu:~# mkdir /mnt/xvdf1
root@ubuntu:~# mkdir /mnt/xvdg1
root@ubuntu:~# mount -o ro /dev/xvdf1 /mnt/xvdf1
root@ubuntu:~# mount /dev/xvdg1 /mnt/xvdg1

Now we are going to use rsync to clone the contents of the original drive (xvdf1) into the replacement (xvdg1).

root@ubuntu:~# cd /mnt/xvdg1
root@ubuntu:/mnt/xvdg1# rsync -Pa ../xvdf1/ .

After all of the files are left, we need to fix the volume UUID. While we could update the UUID of the new drive to match the UUID of the old drive, there are advantages to using a new UUID. For example, if you had to perform an offline fix on this drive in the future, you could attach it to one of your other running CentOS instances and mount it. However, if you keep the old UUID you’ll have difficulty attaching it to another CentOS instance because the root UUIDs will match and the OS will get confused. Either way, we need to get a list of the UUID’s on the attached disks. This is done with the blkid command:

root@ubuntu:/mnt/xvdg1# blkid
/dev/xvda1: LABEL="cloudimg-rootfs" UUID="567ab888-a3b5-43d4-a92a-f594e8653924" TYPE="ext4" PARTUUID="1a7d4c6a-01"
/dev/xvdf1: UUID="0f790447-ebef-4ca0-b229-d0aa1985d57f" TYPE="xfs" PARTUUID="000aec37-01"
/dev/xvdg1: UUID="6094350f-7d18-4256-b52e-6dbf5f196219" TYPE="xfs" PARTUUID="1db62587-01"

xvda1 is the ubuntu system, so forget about that UUID. xvdf1 has the original UUID that came with the CentOS AMI. xvdg1’s UUID was created when we ran mkfs.xfs. While you *could* use xfs_admin to assign the UUID from xvdf1 onto xvdg1 and skip to installing grub, I think it’s better to put the new UUID in all the places it need to go, so that’s what I’m going to describe next. There are four files to modify, and we can use sed to perform the editing on each one of them. The format of the command is:

# sed -i -e 's/old_UUID/new_UUID/g' /path/to/file

We’re already in /mnt/xvdg1/ from when we ran rsync, so the file paths are relative to this point:

root@ubuntu:/mnt/xvdg1# sed -i -e 's/0f790447-ebef-4ca0-b229-d0aa1985d57f/6094350f-7d18-4256-b52e-6dbf5f196219/g' etc/fstab
root@ubuntu:/mnt/xvdg1# sed -i -e 's/0f790447-ebef-4ca0-b229-d0aa1985d57f/6094350f-7d18-4256-b52e-6dbf5f196219/g' boot/grub2/grub.cfg
root@ubuntu:/mnt/xvdg1# sed -i -e 's/0f790447-ebef-4ca0-b229-d0aa1985d57f/6094350f-7d18-4256-b52e-6dbf5f196219/g' boot/grub/grub.conf 
root@ubuntu:/mnt/xvdg1# sed -i -e 's/0f790447-ebef-4ca0-b229-d0aa1985d57f/6094350f-7d18-4256-b52e-6dbf5f196219/g' boot/grub/menu.lst

I really don’t think the last two files are necessary but the old UUID is in there so I changed it anyway. The last step is to chroot into the disk and to install grub. In order for that to work, we need to bind mount some places from the running Ubuntu. Grub needs to be installed on the new boot drive (/dev/xvdg), NOT the partition (/dev/xvdg1).

root@ubuntu:/mnt/xvdg1# mount --bind /dev dev
root@ubuntu:/mnt/xvdg1# mount --bind /proc proc
root@ubuntu:/mnt/xvdg1# mount --bind /sys sys
root@ubuntu:/mnt/xvdg1# chroot .
[root@centos /]# grub2-install /dev/xvdg
Installing for i386-pc platform.
Installation finished. No error reported.
[root@centos /]# exit

At this point, we are just about done. Shut down the ubuntu instance, detach the new drive we created, and attach it to the original instance as /dev/sda1.

Attach replacement as /dev/sda1

That’s it! Start your original instance, log in and verify the size:

[rgibson@centos ~]$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       20G  7.1G   13G  36% /

My used space is a little higher, I assume this may be because of sparse files on the root volume. Perhaps some piped form of tar -S would be preferable to rsync to clone the volume efficiently.

If your instance is unreachable or you otherwise can’t log in, diagnose it with AWS’s “view instance screenshot” which is probably the coolest feature they’ve added to EC2 lately. Delete your old volume and any snapshots when you’re comfortable doing so.

Happy shrinking!