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Help:Resize root partition of an OpenStack hosted virtual machine

From Wikitech
Do not attempt this unless you have strong system administration skills and are willing to lose the complete contents of the VM.
Cloud Services staff will not assist you in recovering from a failed attempt at resizing the root partition of a VM!
This page contains historical information. It may be outdated or unreliable.
This is not even possible with modern VMs.

Overview

This page provides instructions for resizing the root partition of an OpenStack hosted virtual machine.

Before you begin

Resizing the root partition of an OpenStack hosted virtual machine is not for the faint of heart, but it is possible.

Each VM has a disk image that contains three partitions by default:

  1. a 20G root partition
  2. a 0.5G swap partition
  3. the rest of the disk as an LVM container that is initially uninitialized.

The role::labs::lvm::srv Puppet role can be used to create an LVM partition filling the entire container that is mounted at /srv. This is often the best way to utilize the disk.

There are some use cases when you really want one very large root partition. There is no GUI method to achieve this with OpenStack/Horizon/Wikitech, but it can be attempted manually.

Basic steps

  1. Disable current swap partition
  2. Edit the /dev/vda partition map
    1. Delete all current partitions
    2. Create new larger partition 1 (Note: this new partition must start at same offset as original partition - if the new partition's start sector does not match, resizing will not work)
    3. Create new swap partition
    4. Save changes
  3. Reboot the VM
  4. Enable the new swap partition

Note: When editing the partition table, you must use fdisk -c=dos so that the new root partition will start on the same physical boundary as the initial partition.

Alternative to rebooting

Once the partition has been resized a reboot can be avoided by doing the following:

  1. Scan for the new partition: partprobe /dev/vda
    1. If you see the following error, your partition start sector was incorrect and the resizing will fail: Warning: WARNING: the kernel failed to re-read the partition table on /dev/sda (Device or resource busy). As a result, it may not reflect all of your changes until after reboot
  2. Grow the partition to use all of the new partition: resize2fs /dev/vda2
  3. df should now reflect the expanded disk.

Example session

$ ssh my-cool-instance
$ free
             total       used       free     shared    buffers     cached
Mem:       4048264    1900868    2147396       5848     193556     906968
-/+ buffers/cache:     800344    3247920
Swap:       499708          4     499704
$ sudo swapoff --all
$ free
             total       used       free     shared    buffers     cached
Mem:       4048264    1900952    2147312       5848     193564     906968
-/+ buffers/cache:     800420    3247844
Swap:            0          0          0
$ sudo fdisk -c=dos /dev/vda

WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (with command 'c').
Command (m for help): p

Disk /dev/vda: 42.9 GB, 42949672960 bytes
16 heads, 63 sectors/track, 83220 cylinders, total 83886080 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00031db0

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1              63    37998046    18998992   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/vda2        37998592    38998015      499712   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/vda3        38998016    83886079    22444032   8e  Linux LVM
Partition 3 does not end on cylinder boundary.

Command (m for help): d
Partition number (1-4): 3

Command (m for help): d
Partition number (1-4): 2

Command (m for help): d
Selected partition 1

Command (m for help): n
Partition type:
   p   primary (0 primary, 0 extended, 4 free)
   e   extended
Select (default p): p
Partition number (1-4, default 1): 1
First sector (63-83886079, default 63):
Using default value 63
Last sector, +sectors or +size{K,M,G} (63-83886079, default 83886079): 82886656

Command (m for help): n
Partition type:
   p   primary (1 primary, 0 extended, 3 free)
   e   extended
Select (default p): p
Partition number (1-4, default 2): 2
First sector (82886657-83886079, default 82886657):
Using default value 82886657
Last sector, +sectors or +size{K,M,G} (82886657-83886079, default 83886079):
Using default value 83886079

Command (m for help): t
Partition number (1-4): 2
Hex code (type L to list codes): 82
Changed system type of partition 2 to 82 (Linux swap / Solaris)

Command (m for help): p

Disk /dev/vda: 42.9 GB, 42949672960 bytes
16 heads, 63 sectors/track, 83220 cylinders, total 83886080 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00031db0

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1              63    82886656    41443297   83  Linux
/dev/vda2        82886657    83886079      499711+  82  Linux swap / Solaris

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.

WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.

$ sudo reboot
W: molly-guard: SSH session detected!
Please type in hostname of the machine to reboot: my-cool-instance

Broadcast message from bd808@my-cool-instance
        (/dev/pts/0) at 17:34 ...

The system is going down for reboot NOW!
$ packet_write_wait: Connection to UNKNOWN: Broken pipe

$ ssh my-cool-instance
Linux mw-base 3.13.0-85-generic #129-Ubuntu SMP Thu Mar 17 20:50:15 UTC 2016 x86_64
Ubuntu 14.04.3 LTS
The last Puppet run was at Thu Apr  7 17:36:00 UTC 2016 (1 minutes ago).
Last login: Thu Apr  7 17:37:30 2016 from bastion-01.bastion.eqiad1.wikimedia.cloud
$ free
             total       used       free     shared    buffers     cached
Mem:       4048240    1498572    2549668      11352      53488     748044
-/+ buffers/cache:     697040    3351200
Swap:            0          0          0
$ sudo mkswap /dev/vda2
Setting up swapspace version 1, size = 499704 KiB
no label, UUID=1fac41ba-958a-4650-b72c-aedccf59452e
$ sudo swapon --all
$ free
             total       used       free     shared    buffers     cached
Mem:       4048240    1657788    2390452      14364      59604     802148
-/+ buffers/cache:     796036    3252204
Swap:       499704          0     499704
$ df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            2.0G   12K  2.0G   1% /dev
tmpfs           396M  396K  395M   1% /run
/dev/vda1        39G   17G   21G  44% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
none            5.0M     0  5.0M   0% /run/lock
none            2.0G     0  2.0G   0% /run/shm
none            100M     0  100M   0% /run/user

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