Devicemapper curiosity

General Discussions
1

Hi guys,

this days I’m testing out Docker 1.11.2 on a CentOS7 with a direct-lvm devicemapper. I created the thin pool and fed it to docker and everything works fine.

Now I’m curious.

How can I inspect the volumes related to the images that are listed in the “docker images” output? I can find the volumes of running containers using “dmsetup info” but nothing related to the images.

Data is there because I can see that space is being consumed in the pool but couldn’t find any volumes.

Thanks!

2

I don’t think that makes sense. Images don’t have attached volumes. Without trying to break into Docker space, docker volume ls will list volumes that exist in current Docker.

Dockerfile reference | Docker Docs does suggest that the VOLUME instruction has some special semantics (it makes the named directory immutable for the remainder of the Dockerfile). I haven’t read the source, but it wouldn’t surprise me if the implementation of the “docker run populates volumes on startup” feature was just to copy data out of the existing image.

3

Interesting. So, my question is: where the files of the image live before being copied to a brand new volume created for a container. And, yet, I wonder what this figure means:

It seems to me that the ubuntu:latest image is an actual snapshot and it should exist as such.

I’m trying to read the code but I don’t know Go and it’s difficult.

Thanks for your reply.

4

Just to add a bit of information, here is the effect of pulling/deleting an image from the hub on the thin pool:

[root@atlas mapper]# lvs
  LV        VG        Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  docker-tp atlas-vg0 twi-a-t--- 45.00g             0.04   0.01
[root@atlas mapper]# docker pull ubuntu:latest
latest: Pulling from library/ubuntu
43db9dbdcb30: Pull complete
85a9cd1fcca2: Pull complete
c23af8496102: Pull complete
e88c36ca55d8: Pull complete
Digest: sha256:7ce82491d6e35d3aa7458a56e470a821baecee651fba76957111402591d20fc1
Status: Downloaded newer image for ubuntu:latest
[root@atlas mapper]# lvs
  LV        VG        Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  docker-tp atlas-vg0 twi-a-t--- 45.00g             0.44   0.01
[root@atlas mapper]#
[root@atlas mapper]# docker rmi ubuntu
Untagged: ubuntu:latest
Deleted: sha256:ac526a356ca46f915e822e2f8051b9cf3404c756b725661c51191564ae4e6ea7
Deleted: sha256:4247fb52a8da87346107c6a63ce0017826f344b4da5e2f13d55651f3a159716e
Deleted: sha256:2ee485d050041e742f27c9af6a1a1680208ad9d61256abb005e9b8a391e72fd7
Deleted: sha256:860708544664c614b8ab025d5d01ba8f37694e939fd02ce861538bfd7c64cc43
Deleted: sha256:ea9f151abb7e06353e73172dad421235611d4f6d0560ec95db26e0dc240642c1
[root@atlas mapper]# lvs
  LV        VG        Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  docker-tp atlas-vg0 twi-a-t--- 45.00g             0.04   0.01
[root@atlas mapper]#

As you can see, the thinpool is storing the image but I can’t find any device in /dev/mapper or using dmsetup ls. That’s really strange.

Thanks.