Jan 282019
 

My cloud computing cluster like all cloud computing clusters of course needs a storage back-end. There were a number of options I could have chosen, but the one I went with in the end was Ceph, and so far, it’s ran pretty well.

Lately though, I was starting to get some odd crashes out of ceph-osd. I was running release 10.2.3, which is quite dated now, this is one of the earlier Jewel releases. Adding to the fun, I’m running btrfs as my filesystem on the OS and the OSD, and I’m running it all on Gentoo. On top of this, my monitor nodes are my OSDs as well.

Not exactly a “supported” configuration, never mind the hacks done at hardware level.

There was also a nagging issue about too many placement groups in the Ceph cluster. When I first established the cluster, I christened it by dragging a few of my lxc containers off the old server and making them VMs in the cluster. This was done using libvirt and virt-manager. These got thrown into a storage pool called transitional-inst, with a VLAN set aside for the VMs to use. When I threw OpenNebula on, I created another Ceph pool, one for its images. The configuration of these lead to the “too many placement groups” warning, which until now, I just ignored.

This weekend was a long weekend, for controversial reasons… and so I thought I’ll take a snapshot of all my VMs, download those snapshots to a HDD as raw images, then see if I can fix these issues, and migrate to Ceph Luminous (v12.2.10) at the same time.

Backing up

I was going to be doing some nasty things to the cluster, so I thought the first thing to do was to back up all images. This was done by using rbd snap create pool/image@date to create a snapshot of an image, then rbd export pool/image@date /path/to/storage/pool-image.img before blowing away the snapshot with rbd snap rm pool/image@date.

This was done for all images on the Ceph cluster, stashing them on a 4TB hard drive I had bought for the purpose.

Getting things ready

My cluster is actually set up as a distcc cluster, with Apache HTTP server instances sharing out distfiles and binary package repositories, so if I build packages on one, I can have the others fetch the binary packages that it built. I started with a node, and got it to update all packages except Ceph. Made sure everything was up-to-date.

Then, I ran emerge -B =ceph-10.2.10-r2. This was the first step in my migration, I’d move to the absolute latest Jewel release available in Gentoo. Once it built, I told all three storage nodes to install it (emerge -g =ceph-10.2.10-r2). This was followed up by a re-start of the mon daemons on each node (one at a time), then the mds daemons, finally the osd daemons.

Resolving the “too many placement groups” warning

To resolve this, I first researched the problem. An Internet search lead me to this Stack Overflow post. In it, it was suggested the problem could be alleviated by making a new pool with the correct settings, then copying the images over to it and blowing away the old one.

As it happens, I had an easier solution… move the “transitional” images to OpenNebula. I created empty data blocks in OpenNebula for the three images, then used qemu-img convert -p /path/to/image.img rbd:pool/image to upload the images.

It was then a case of creating a virtual machine template to boot them. I put them in a VLAN with the other servers, and when each one booted, edited the configuration with the new TCP/IP settings.

Once all those were moved across, I blew away the old VMs and the old pool. The warning disappeared, and I was left with a HEALTH_OK message out of Ceph.

The Luminous moment

At this point I was ready to try migrating. I had a good read of the instructions beforehand. They seemed simple enough. I prepared as I did before by updating everything on the system except Ceph, then, telling Portage to build a binary package of Ceph itself.

Then I deployed the binary to the three nodes.

First step was to re-start the monitors… this went smoothly, I just did a /etc/init.d/ceph-mon.${HOST} restart on each one individually, and after a brief moment, quorum was re-established. I then deployed a manager daemon to each one — basically I just “copied” my monitor symbolic link, changing mon to mgr, added it to OpenRC’s list, then started them. No problems.

The OSDs though were still running the Jewel release.

I proceeded as before, trying a re-start of the first OSD. After a while it hadn’t come back…

2019-01-27 14:42:59.745860 7f28fac06e00 -1 filestore(/var/lib/ceph/osd/ceph-0) _detect_fs(1197): deprecated btrfs support is not ena
bled

Ohh bugger, so no btrfs support. This is where the fun began. At this point I was a bit flustered and thought I’d have to either migrate these nodes to XFS, or to BlueStore. So immediately I started looking at the BlueStore migration documentation, as I did not want to risk re-starting the other two OSDs and losing access to my data!

A hasty BlueStore migration

So, I started this by doing the ceph osd set out 0 to start my now downed OSD 0 on the path of migration. The fact it was already down didn’t click with me. I then tried running ceph osd safe-to-destroy 0, only to be told Error EINVAL: (22) Invalid argument.

Uhh ohh, this isn’t good. I waited a bit, but also part of me said: there should be a copy of everything on this node, on at least one of the other two nodes. I had configured it to maintain at least two copies of everything, so even if this node went up in smoke, the data should be recoverable.

With great trepidation, I continued and tried destroying the OSD, then creating a BlueStore one in its place… only to have the ceph-volume command blow up. It couldn’t find the keyring, then when I got that sorted out, it was failing to talk to systemd, then when I found the --no-systemd argument, it still failed because of LVM. I therefore realised I needed two things:

  1. I needed the bootstrap-osd keyring that ceph-deploy normally creates.
  2. The lvmetad daemon must be running.

For (1), this is taken care of with the following commands:

# ceph auth add client.bootstrap-osd --cap mon 'profile bootstrap-osd
# mkdir /var/lib/ceph/bootstrap-osd
# ceph auth get client.bootstrap-osd > /var/lib/ceph/bootstrap-osd/ceph.keyring

As for (2), install sys-fs/lvm and add lvmetad to your start-up services. Also add lvm, as you’ll want that at boot. (I learned this later.)

After doing that, the following command worked:

ceph-volume lvm create --bluestore --data /dev/sdb \
--osd-id 0 --no-systemd

The --no-systemd is important on Gentoo with OpenRC as there is no systemctl binary. Once I did that, I found I could start my OSD again. Data recovery began at once. The data recovery was an overnight effort — it took with my hardware until 3PM today to migrate all the placement groups over to the newly re-formatted OSD.

Migrating the other nodes

For now, they still run btrfs. In my “ohh crap” state, I didn’t see the little hint given:

2019-01-27 14:40:55.147888 7f8feb7a2e00 -1 *** experimental feature 'btrfs' is not enabled ***
This feature is marked as experimental, which means it
 - is untested
 - is unsupported
 - may corrupt your data
 - may break your cluster is an unrecoverable fashion
To enable this feature, add this to your ceph.conf:
  enable experimental unrecoverable data corrupting features = btrfs

2019-01-27 14:40:55.147901 7f8feb7a2e00 -1 filestore(/var/lib/ceph/osd/ceph-0) _detect_fs(1197): deprecated btrfs support is not enabled
2019-01-27 14:40:55.147906 7f8feb7a2e00 -1 filestore(/var/lib/ceph/osd/ceph-0) mount(1523): error in _detect_fs: (1) Operation not permitted
2019-01-27 14:40:55.147926 7f8feb7a2e00 -1 osd.0 0 OSD:init: unable to mount object store

Not feeling like a 24-hour wait, I did as it told me:

osd pool default size = 2  # Write an object n times.
osd pool default min size = 1 # Allow writing n copy in a degraded state.
osd pool default pg num = 128
osd pool default pgp num = 128
osd crush chooseleaf type = 1
osd max backfills = 10

# Allow btrfs to work:
enable experimental unrecoverable data corrupting features = btrfs

Now, my other OSDs re-started successfully, and I could finally finish off by restarting the metadata daemons and completing the migration. I’m now left with two OSDs with BTRFS and one with BlueStore.

For now, I’ll leave it that way, next week end, I might migrate a second node to BlueStore.

The reboot test

I needed to ensure the nodes would come back without my intervention. So starting with the two BTRFS nodes, I rebooted each one individually. The OSD on that node first went offline, then the monitor, finally the cluster noticed the metadata and manager services had gone. Then, upon successful boot, the services returned.

So far so good. Now the BlueStore node.

First reboot, my OSD didn’t come back. On investigation, I saw the following logs:

2019-01-28 16:25:59.312369 7fd58d4f0e00 -1  ** ERROR: unable to open OSD superblock on /var/lib/ceph/osd/ceph-0: (2) No such file or
directory
2019-01-28 16:26:14.865883 7fe92f942e00 -1 ** ERROR: unable to open OSD superblock on /var/lib/ceph/osd/ceph-0: (2) No such file or
directory
2019-01-28 16:26:30.419863 7fd4fa026e00 -1 ** ERROR: unable to open OSD superblock on /var/lib/ceph/osd/ceph-0: (2) No such file or directory

/var/lib/ceph/osd/ceph-0 was completely empty! Bugger, do I have to endure those 24 hours again? As it happened, no. I don’t know how the files in that directory disappeared, I did observe a tmpfs pseudovolume mounted at that directory earlier when trying to create the OSD … maybe that didn’t get unmounted before OSD creation, anyway, the files were gone.

A bit of digging revealed a ceph-bluestore-tool utility, with options like repair. At first I tried to wing it using that, but no dice. Then looking at the man page I noticed the sub-command prime-osd-dir. BINGO.

At first I threw the raw device at it, but as it happens, ceph-volume had deployed LVM to the raw disk, then put BlueStore on top of that. Starting lvm got the volume group recognised, so I added that to my boot-up services (see why I mentioned it earlier). It had created a sym-link to the LVM volume in /dev/ceph-${UUID1}/osd-block-${UUID2}.

No idea where the two UUIDs came from, but I tried this:

# ceph-bluestore-tool prime-osd-dir \
    --dev /dev/ceph-d62d0d95-2e13-4c59-834d-03a87b88c85e/osd-block-62b4be3e-3935-4d51-ab5c-dde077f99ea3 \
    --path /var/lib/ceph/osd/ceph-0

That populated the directory with files, so I tried again starting the OSD.

2019-01-28 16:59:23.680039 7fd93fcbee00 -1 bluestore(/var/lib/ceph/osd/ceph-0/block) _read_bdev_label failed to open /var/lib/ceph/osd/ceph-0/block: (13) Permission denied
2019-01-28 16:59:23.680082 7fd93fcbee00 -1  ** ERROR: unable to open OSD superblock on /var/lib/ceph/osd/ceph-0: (2) No such file or directory
2019-01-28 16:59:39.229888 7f4a585b4e00 -1 bluestore(/var/lib/ceph/osd/ceph-0/block) _read_bdev_label failed to open /var/lib/ceph/osd/ceph-0/block: (13) Permission denied
2019-01-28 16:59:39.229918 7f4a585b4e00 -1  ** ERROR: unable to open OSD superblock on /var/lib/ceph/osd/ceph-0: (2) No such file or directory

Ah ha, chown -R ceph:ceph /var/lib/ceph/osd/ceph-0, and all sprang to life. The OSD came up.

Testing the fixes, a second re-boot

Since the OSD now was starting, and working, I did a second re-boot test, only to have history partially repeat itself.

The files were still there this time, but it was failing with a permissions error opening the block device. Sure enough, it was now owned by root.

Changed the permissions, and the OSD came up.

Fixing this was a job for udev:

cat /etc/udev/rules.d/99ceph.rules
SUBSYSTEM=="block", KERNEL=="sda7", OWNER="ceph", GROUP="ceph", MODE="0600"
SUBSYSTEM=="block", ENV{DM_VG_NAME}=="ceph-*", OWNER="ceph", GROUP="ceph", MODE="0600"

The first line is left-over from when /dev/sda7 was my journal. Not sure what I’ll do with this partition now, I’ll think of something (maybe Docker). The second line tells udev to change the permissions on the volume group that Ceph created.

Having done this, I rebooted again. This time, all worked. The OSD came up without my intervention.

Recap

So, the pitfalls I ran across in my Jewel-Luminous migration on Gentoo.

btrfs OSDs

I had btrfs volumes for my OSDs, which are now frowned upon and considered experimental. It isn’t necessary to migrate to BlueStore or XFS straight away, but for the OSDs to boot, you will need the following line in your /etc/ceph/ceph.conf before restarting your OSDs:

enable experimental unrecoverable data corrupting features = btrfs

ceph-volume expects the bootstrap-osd key.

To use ceph-volume, it for some reason expects to see the bootstrap-osd key in a hard-coded location. It won’t work with the default admin key.

This bootstrap key can be generated as follows:

# ceph auth add client.bootstrap-osd --cap mon 'profile bootstrap-osd
# mkdir /var/lib/ceph/bootstrap-osd
# ceph auth get client.bootstrap-osd > /var/lib/ceph/bootstrap-osd/ceph.keyring

Before creating a BlueStore OSD, make sure lvmetad and lvm are started (and set to start at boot)

You can get away with just lvmetad for the initial creation, but you’ll want lvm running at boot anyway to ensure all the logical volume groups get started at boot before Ceph goes looking for them.

So before attempting OSD creation, ensure LVM is installed, and set to start at boot.

ceph-osd runs as the ceph user

So your udev rules need to reflect that. Luckily, ceph-volume seems to prefer creating LVM volume groups named ceph-${UUID}. I don’t know what decides the UUID value, but thankfully udev supports globbing. The following udev rule (put it in /etc/udev/rules.d/99ceph.rules or wherever seems appropriate) will keep permissions in check:

SUBSYSTEM=="block", ENV{DM_VG_NAME}=="ceph-*", OWNER="ceph", GROUP="ceph", MODE="0600"

(The above should be all on one line.)

Before rebooting a BlueStore node, back up your OSD data directories

Shouldn’t be strictly necessary, but now I’ve been bitten, I’m going to be taking extra care of that data directory on my other two nodes when I migrate them. I don’t fancy playing around with ceph-bluestore-tool frantically trying to get an OSD back up again.