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Partition Resizing


Chromium OS will be able to resize partitions without erasing them, easing delivery of system updates.

Use case

A big new operating system release is on the way, and it needs more system space than the current system partitions have. We can push an update that makes the system partitions bigger (by shrinking the stateful partition); then when the big new release arrives, enough space is available on the disk.

Partition layout

A drive currently contains up to four partitions, in the following order:
  1. One partition for state resident on the drive, called the "stateful partition." Any space required in the stateful partition is pre-allocated by the system. For example, a large file will be allocated for the autoupdater to use to store downloaded updates.
  2. An optional swap partition.
  3. Two partitions for the root file system. For more information, see the File System/Autoupdate document.
In the future, drives may be able to have more partitions, as needed.

Procedure for resizing partitions

The following diagram shows all the steps that must occur to resize partitions and leave them in the desired state.

Keep in mind that at any point the system may be unexpectedly rebooted, and the OS must recover gracefully (in particular, it must not brick). Also, keep in mind that the OS doesn't allow changing the contents or location of the partition we booted into.

The following procedure assumes that at the start of the process, the system has booted into the last partition on the disk. If, instead, the system is currently booted into the second-to-last partition, then the partition-resizing process usually waits to begin until the next update or reboot occurs naturally. However, if the resizing is urgent, the system can copy the A partition into B and then reboot before starting the illustrated procedure.

If there's a swap partition, it isn't resized.

Note that, at the end of the following process, the A and B partitions are always the same size as each other.

When does resizing occur?

Partition resizing is initiated by the system, not by the user.

Some notes:
  • The approach described in this document depends on ext4 being able to make the stateful partition smaller.
  • If the stateful partition contains a lot of data, it may not always be possible to resize it down to the desired size; in that case the system will have to delete state information before resizing.
  • The resizing process currently assumes that the machine can't be used while partitions are being resized. Therefore, during the time while the system is shrinking the stateful partition, the user will have to leave the machine turned on but won't be able to use it. The system will display a "working...please wait" message on the screen.
  • Open issue: Will ext4 support resizing partitions while the file system is mounted? If it does, then we may be able to allow the user to use the machine while resizing partitions.
Note that, to support encrypted home directories, we will already have to have a "working...please wait" message on the screen at logout when we re-sparsify the home directory. This would be an appropriate time to resize the stateful partition file system if needed.

Jed Hartman,
Nov 19, 2009, 9:03 AM