Description of problem:
Following https://docs.openshift.org/latest/install_config/install/prerequisites.html#configuring-docker-storage to set up docker-pool volume as docker back-end storage.

Here is my env info:
# cat /etc/sysconfig/docker-storage
DOCKER_STORAGE_OPTIONS=--storage-driver devicemapper --storage-opt dm.fs=xfs --storage-opt dm.thinpooldev=/dev/mapper/rhel72-docker--pool --storage-opt dm.use_deferred_removal=true

# ps -ef|grep docker
root      15538      1  0 Jan20 ?        00:08:01 /usr/bin/docker daemon --insecure-registry=172.31.0.0/16 --selinux-enabled --storage-driver devicemapper --storage-opt dm.fs=xfs --storage-opt dm.thinpooldev=/dev/mapper/rhel72-docker--pool --storage-opt dm.use_deferred_removal=true -b=lbr0 --mtu=1450 --add-registry rcm-img-docker01.build.eng.bos.redhat.com:5001 --add-registry registry.access.redhat.com --insecure-registry 0.0.0.0/0

# lvs
  LV          VG     Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  docker-pool rhel72 twi-aot--- 17.44g             73.35  9.57                            
  root        rhel72 -wi-ao---- 10.00g                                                    
  swap        rhel72 -wi-ao----  2.00g  

# df -h
Filesystem                                               Size  Used Avail Use% Mounted on
/dev/mapper/rhel72-root                                   10G  2.7G  7.4G  27% /
devtmpfs                                                 1.9G     0  1.9G   0% /dev
tmpfs                                                    1.9G     0  1.9G   0% /dev/shm
tmpfs                                                    1.9G  191M  1.7G  11% /run
tmpfs                                                    1.9G     0  1.9G   0% /sys/fs/cgroup
/dev/vda1                                                497M  214M  284M  43% /boot

If deploy docker-registry with an external storage. e.g: NFS, that means docker images will be pulled in docker-pool volume, sti build will push image to external storage, root partition will be used less and less.

After setup env, and follow https://docs.openshift.org/latest/admin_guide/garbage_collection.html to configure "Image Garbage Collection", then restart node service.
kubeletArguments: 
  image-gc-high-threshold:
  - '20'
  image-gc-low-threshold:
  - '10'


Seen from node log, found ImageManager is monitoring root disk usage by default.
<--snip-->
Jan 20 18:49:20 openshift-149 atomic-openshift-node: I0120 18:49:20.682843   32634 image_manager.go:202] [ImageManager]: Disk usage on "/dev/mapper/rhel72-root" (/) is at 26% which is over the high threshold (20%). Trying to free 672878592 bytes
Jan 20 18:49:20 openshift-149 docker: time="2016-01-20T18:49:20.683168834+08:00" level=info msg="GET /images/json"
Jan 20 18:49:20 openshift-149 docker: time="2016-01-20T18:49:20.768170253+08:00" level=info msg="GET /containers/json?all=1"
Jan 20 18:49:20 openshift-149 atomic-openshift-node: I0120 18:49:20.770445   32634 image_manager.go:254] [ImageManager]: Removing image "05f86996004c05346d746261b53a406a43e9016753f0ab3bd3a62756828db551" to free 235319119 bytes
<--snip-->

Sometimes, to do image garbage on root disk is NOT what user want to do. In the above scenarios, I want to do image garbage on docker-pool volume.
"Image garbage collection" configuration should allow user to specify which disk partition to be done against.


Version-Release number of selected component (if applicable):
# openshift version
openshift v3.1.1.5
kubernetes v1.1.0-origin-1107-g4c8e6f4
etcd 2.1.2


How reproducible:
Always

Steps to Reproduce:
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Actual results:


Expected results:


Additional info: