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Line 1: − Typical external resource when dumping a container (especially LXC/Docker) -- is a mount point whose root sits outside of the container's root. This situation was intended to be resolved using [[plugins]] but turned out to be ''so'' frequent, that we introduced a non-plugin way of handling them.+ + + Line 20:
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minor fixups
__TOC__
One of typical external resources when dumping a container (especially LXC/Docker) is a mount point whose root sits outside of the container's root. This situation was intended to be resolved using [[plugins]] but turned out to be common enough to introduce a built-in way of handling it.
== What is external bind mount ==
== What is external bind mount ==
== How to teach CRIU to dump them ==
== How to teach CRIU to dump them ==
By default CRIU doesn't dump such mountpoints, because there's no way CRIU will be able to restore it -- the root of these mounts is out of the scope of what CRIU dumped. In the logs you would see the message like
By default CRIU doesn't dump such mountpoints, because there's no way CRIU will be able to restore it -- the root of these mounts is out of scope of what CRIU dumped. In the logs you would see a message like
34:/bar doesn't have a proper root mount
34:/bar doesn't have a proper root mount
which will mean, that the mountpoint /bar has inaccessible root.
which means the mountpoint /bar has inaccessible root.
To dump and restore them there's the <code>--external mnt[KEY]:VAL</code> option that sets up external mounts root mapping.
On dump, KEY is a mountpoint inside container, and corresponding VAL is a string that will be written into the image as mountpoint's root value.
On restore, KEY is the value from the image (VAL from dump), and the VAL is the path on host that will be bind-mounted into container (to the mountpoint path from image).
For example, if we want to dump the task above we should call
criu dump ... --external mnt[/bar]:barmount
The word <code>barmount</code> is an arbitrary identifier, that will be put in the image file instead of the original root path
criu show -f mountpoints.img -F mnt_id,root,mountpoint
mnt_id: 0x22 root: barmount mountpoint: /bar
On restore we should tell CRIU where to bind mount the <code>barmount</code> from like this
criu restore ... --external mnt[barmount]:/foo
With this CRIU will bind mount the /foo into proper mountpoint.
== Auto detection ==
In case one wants CRIU to autodetect and dump all the external bind mounts, and there is no need to change host mount points on restore, one can use a special syntax:
criu dump ... --external mnt[]:''flags''
Note here is nothing inside square brackets, and the optional <code>:''flags''</code> argument can contain the following characters:
; <code>m</code>
: Also enable dumping of external master mounts (as in <code>mount --make-slave</code>)
; <code>s</code>
: Also enable dumping of external shared mounts (as in <code>mount --make-shared</code>)
By default, neither master nor shared external mounts are not dumped (if found, dump is aborted). Note if <code>''flags''</code> are not given, semicolon is optional.
=== Examples ===
criu dump ... --external 'mnt[]'
Auto detect and dump all external bind mounts.
criu dump ... --external 'mnt[]:s'
Auto detect and dump all external bind mounts, including the shared ones.
criu dump ... --external 'mnt[]:sm'
Auto detect and dump all external bind mounts, including the shared and the master ones.
== Old days ==
For now the same behavior is configured with the <code>--ext-mount-map KEY:VAL</code> option. Soon this option will be [[deprecation|deprecated]].
[[Category:HOWTO]]
[[Category:External]]