Linux – mount a read-only btrfs volume across multiple machines

btrfscoreosdockerlinux

Is it possible to have a single btrfs mounted volume read-only mounted across multiple machines?

Can one of the hosts mount it read/write to provide updates to the others?

The reason I'm asking this is I'm wondering if it's sensible to mount /var/lib/docker to multiple CoreOS instances. What do people do in practice for private clouds? particularly netbooted instances.

Or should I create a unique block device mounted at /var/lib/docker for each physical host and link it to a private registry?

https://coreos.com/docs/cluster-management/setup/mounting-storage/

Best Answer

I can't imagine this would end well. btrfs isn't a cluster filesystem and, even though you're talking about doing read-only mounts, filesystem metadata cached in memory by each node mounting the filesystem will be out of date when the filesystem is written to. Without a cache invalidation mechanism (which a real cluster filesystem would have) you're going to start seeing inconsistencies on the reading nodes.

Related Solutions

LVM snapshot of btrfs volume changes mounted device

I have not looked massively into this but btrfs as a filesystem works on groups of disks, not individual devices.

I suspect that there is no way for btrfs to distinguish between the mounted snapshot and the real mounted filesystem when a mount occurs. It may as a matter of fact see the UUID of the underlying subvolume, assume its a mirror of the original volume and write to both volumes at the same time.

I would be surprised if this ever gets fixed seeing as for most intents and purposes btrfs snapshots supersede LVM snapshots.

Linux – Mount Docker container contents in host file system

Take a look at docker export.

To quickly list the files in your container:

docker export CONTAINER|tar -t

To export:

docker export CONTAINER>snapshot.tar
docker export CONTAINER|tar x PATH-IN-CONTAINER

Or to look at a file:

docker export CONTAINER|tar x --to-stdout PATH-IN-CONTAINER
# e.g. 
docker export consul|tar x --to-stdout etc/profile

Docker 1.8 supports cp:

https://docs.docker.com/reference/commandline/cp/

Usage:  docker cp [options] CONTAINER:PATH LOCALPATH|-
        docker cp [options] LOCALPATH|- CONTAINER:PATH

update: you should ssh to your docker machine when you run this.

Best Answer

Related Solutions

LVM snapshot of btrfs volume changes mounted device

Linux – Mount Docker container contents in host file system

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