Comment 6 for bug 973194

** Impact **

If user faces locking related problem when two nova-compute hosts sharing same disk area via nfs, try to download same image into cache concurrently - Then base_dir_name can be set to "_base_$my_ip" in nova.conf

Default value for base_dir_name is "_base" thus retaining existing behavior.

** Development Fix **

This issue has been address in the development trunk: https://review.openstack.org/6262 and fixed in quantal.

** Stable Fix **

This issue has been address in the stable tree: https://review.openstack.org/7269

** Test Case **

1. There are two nova-compute Hosts : HostA and HostB
2. NFS server : HostC exports "/shared_instances_path"
3. Both HostA and HostB mount HostC:/shared_instances_path at /shared_instances_path
4. Both HostA and HostB have "instances_path=/shared_instances_path" in their nova.conf
5. HostC:/shared_instances_path is empty to begin with
6. Two VMs are launched from the same image at the same time (Parallel VM creation)
7. Since there are no cached images in <instances_path>/_base folder - both nova-compute hosts try to download kernel and ramdisk images to the same location, same filename.

This seems like problem related to file locking.
Since these are two different compute hosts, @utils.synchronized is not useful.

Expected Response:

Parallel VM Creation as explained above should not fail.

Actual Response:

using same disk area from multiple nova-compute crashes instance images.

** Regression **

Minimal this is geared to small systems like ARM machines.