I have another experience to share with Stackers that use Ceph as their storage backend. In this environment Ceph is used for Glance, Nova and Cinder. I won’t get into configuration details, I assume you’re familiar with this kind of setup. So let’s dive into the issue.
This is another article about OpenStack with Ceph as the storage backend. Like my other posts about this topic this is not about how to install and configure your private cloud but it’s more a collection of obstacles you could be facing. For me it’s also an online documentation in case I forgot what I did weeks, months or even years ago.
Now let’s get to it. We’ve been working with Ceph for quite a while now, it’s really comfortable launching instances within seconds. But from time to time we noticed that some instances took several minutes to boot, and there was nothing obvious happening on the compute nodes or in the Ceph cluster. So we didn’t really bother to debug it further, it’s not too bad if you have to wait one minute or so for a 6 GB instance to start.
Working with Ceph and OpenStack can make your life as a cloud administrator really easy, but sometimes you discover its downsides. From time to time I share some findings in this blog, it’s a nice documentation for me and hopefully it helps you preventing the same mistakes I did.
I discovered an orphaned instance in a user’s project, fortunately it was not an important one. The instance’s disk was not a volume but a clone from the Glance image (<INSTANCE_ID>_disk), so it depended on that base image. Only there was no base image in the backend anymore, somehow it must have been deleted even though there were existing clones. I assume it had to do with a cache tier incident a couple of months earlier, something must have destroyed the relationship between the image and its clones.
Anyway, how can you clean that up? You can’t simply delete the instance from OpenStack since nova will try to remove the clone from the backend, which will fail because of the missing parent:
ImageNotFound: [errno 2] error opening image 187cb991-1038-476e-b643-001db259eba7_disk at snapshot None
Digging in the database won’t help you except you could update it to deleted state so it won’t show up in the instance list. This would still leave you with the data objects in the Ceph pool. You cloud clean that up manually, of course, but don’t! There’s a better way for this! The steps described in the following could help with other issues, so I’ll leave it here anyway, but please read till the end!
# Get keys and values of the affected instance control:~ # rados -p images listomapvals rbd_directory | grep -A5 187cb991-1038-476e-b643-001db259eba7 name_187cb991-1038-476e-b643-001db259eba7_disk value (18 bytes) : 00000000 0e 00 00 00 32 31 66 32 39 38 31 66 34 34 37 39 |....21f2981f4479| 00000010 63 63 |cc| 00000012 control:~ # rados -p images listomapvals rbd_directory | grep -A5 21f2981f4479cc id_21f2981f4479cc value (45 bytes) : 00000000 29 00 00 00 31 38 37 63 62 39 39 31 2d 31 30 33 |)...187cb991-103| 00000010 38 2d 34 37 36 65 2d 62 36 34 33 2d 30 30 31 64 |8-476e-b643-001d| 00000020 62 32 35 39 65 62 61 37 5f 64 69 73 6b |b259eba7_disk| 0000002d # Remove omapkeys control:~ # rados -p images rmomapkey rbd_directory name_187cb991-1038-476e-b643-001db259eba7_disk control:~ # rados -p images rmomapkey rbd_directory id_21f2981f4479cc # Remove remaining objects control:~ # rados -p images rm rbd_id.187cb991-1038-476e-b643-001db259eba7_disk control:~ # rados -p images ls | grep 21f2981f4479cc | xargs rados -p images rm
This would work, you wouldn’t see orphaned instances anymore, but there’s still a chance that some of this leaves inconsistencies in your cluster, so once again: be careful with this approach!
There’s a much more elegant way to achieve this, so don’t try to delete the orphaned instance yet:
# Rebuild the instance from a different image control:~ # openstack server rebuild --image 187cb991-1038-476e-b643-001db259eba7
This will remove the corrupt references to a non-existing Glance image and update the relevant database entries. Now you can either keep using this rebuilt instance for existing or new purposes, or finally delete it.
There’s a nice command in the rbd client to list all images within a specified pool, showing all relationships between base images and clones:
control:~ # rbd -p images list --long 2018-04-24 15:26:46.454115 7ff10a7fc700 -1 librbd::image::RefreshParentRequest: failed to open parent image: (2) No such file or directory 2018-04-24 15:26:46.454130 7ff10a7fc700 -1 librbd::image::RefreshRequest: failed to refresh parent image: (2) No such file or directory 2018-04-24 15:26:46.454142 7ff10a7fc700 -1 librbd::image::OpenRequest: failed to refresh image: (2) No such file or directory 2018-04-24 15:26:46.454254 7ff10a7fc700 -1 librbd::io::AioCompletion: 0x5565e04c7910 fail: (2) No such file or directory NAME SIZE PARENT FMT PROT LOCK 284007bf-cd6b-42ee-9529-274d259e6812_disk 20480M [...]
If the output of that command starts with these errors you have orphans in your cluster. Check your instances and their base images to identify which one it is.
Disclaimer, and please leave a comment below
And as always with such articles: The steps above do work for us, but needn’t work for you. So if anything goes wrong while you try to reproduce above procedure, it’s not our fault, but yours. And it’s yours to fix it! But whether it works for you or not, please leave a comment below so that others will know.