Percona XtraDB Cluster on Ceph

CephThis post discusses how XtraDB Cluster and Ceph are a good match, and how their combination allows for faster SST and a smaller disk footprint.

My last post was an introduction to Red Hat’s Ceph. As interesting and useful as it was, it wasn’t a practical example. Like most of the readers, I learn about and see the possibilities of technologies by burning my fingers on them. This post dives into a real and novel Ceph use case: handling of the Percona XtraDB Cluster SST operation using Ceph snapshots.

If you are familiar with Percona XtraDB Cluster, you know that a full state snapshot transfer (SST) is required to provision a new cluster node. Similarly, SST can also be triggered when a cluster node happens to have a corrupted dataset. Those SST operations consist essentially of a full copy of the dataset sent over the network. The most common SST methods are Xtrabackup and rsync. Both of these methods imply a significant impact and load on the donor while the SST operation is in progress.

For example, the whole dataset will need to be read from the storage and sent over the network, an operation that requires a lot of IO operations and CPU time. Furthermore, with the rsync SST method, the donor is under a read lock for the whole duration of the SST. Consequently, it can take no write operations. Such constraints on SST operations are often the main motivations beyond the reluctance of using Percona XtraDB cluster with large datasets.

So, what could we do to speed up SST? In this post, I will describe a method of performing SST operations when the data is not local to the nodes. You could easily modify the solution I am proposing for any non-local data source technology that supports snapshots/clones, and has an accessible management API. Off the top of my head (other than Ceph) I see AWS EBS and many SAN-based storage solutions as good fits.

The challenges of clone-based SST

If we could use snapshots and clones, what would be the logical steps for an SST? Let’s have a look at the following list:

  1. New node starts (joiner) and unmounts its current MySQL datadir
  2. The joiner and asks for an SST
  3. The donor creates a consistent snapshot of its MySQL datadir with the Galera position
  4. The donor sends to the joiner the name of the snapshot to use
  5. The joiner creates a clone of the snapshot name provided by the donor
  6. The joiner mounts the snapshot clone as the MySQL datadir and adjusts ownership
  7. The joiner initializes MySQL on the mounted clone

As we can see, all these steps are fairly simple, but hide some challenges for an SST method base on cloning. The first challenge is the need to mount the snapshot clone. Mounting a block device requires root privileges – and SST scripts normally run under the MySQL user. The second challenge I encountered wasn’t expected. MySQL opens the datadir and some files in it before the SST happens. Consequently, those files are then kept opened in the underlying mount point, a situation that is far from ideal. Fortunately, there are solutions to both of these challenges as we will see below.

SST script

So, let’s start with the SST script. The script is available in my Github at:

https://github.com/y-trudeau/ceph-related-tools/raw/master/wsrep-sst/wsrep_sst_ceph

You should install the script in the /usr/bin directory, along with the other user scripts. Once installed, I recommend:

The script has a few parameters that can be defined in the [sst] section of the my.cnf file.

cephlocalpool
The Ceph pool where this node should create the clone. It can be a different pool from the one of the original dataset. For example, it could have a replication factor of 1 (no replication) for a read scaling node. The default value is: mysqlpool
cephmountpoint
What mount point to use. It defaults to the MySQL datadir as provided to the SST script.
cephmountoptions
The options used to mount the filesystem. The default value is: rw,noatime
cephkeyring
The Ceph keyring file to authenticate against the Ceph cluster with cephx. The user under which MySQL is running must be able to read the file. The default value is: /etc/ceph/ceph.client.admin.keyring
cephcleanup
Whether or not the script should cleanup the snapshots and clones that are no longer is used. Enable = 1, Disable = 0. The default value is: 0
Root privileges

In order to allow the SST script to perform privileged operations, I added an extra SST role: “mount”. The SST script on the joiner will call itself back with sudo and will pass “mount” for the role parameter. To allow the elevation of privileges, the follow line must be added to the /etc/sudoers file:

Files opened by MySQL before the SST

Upon startup, MySQL opens files at two places in the code before the SST completes. The first one is in the function mysqld_main , which sets the current working directory to the datadir (an empty directory at that point).  After the SST, a block device is mounted on the datadir. The issue is that MySQL tries to find the files in the empty mount point directory. I wrote a simple patch, presented below, and issued a pull request:

With this patch, I added a new my_setwd call right after the SST completed. The Percona engineering team approved the patch, and it should be added to the upcoming release of Percona XtraDB Cluster.

The Galera library is the other source of opened files before the SST. Here, the fix is just in the configuration. You must define the base_dir Galera provider option outside of the datadir. For example, if you use /var/lib/mysql as datadir and cephmountpoint, then you should use:

Of course, if you have other provider options, don’t forget to add them there.

Walkthrough

So, what are the steps required to use Ceph with Percona XtraDB Cluster? (I assume that you have a working Ceph cluster.)

1. Join the Ceph cluster

The first thing you need is a working Ceph cluster with the needed CephX credentials. While the setup of a Ceph cluster is beyond the scope of this post, we will address it in a subsequent post. For now, we’ll focus on the client side.

You need to install the Ceph client packages on each node. On my test servers using Ubuntu 14.04, I did:

These commands also installed all the dependencies. Next, I copied the Ceph cluster configuration file /etc/ceph/ceph.conf:

and the authentication file /etc/ceph/ceph.client.admin.keyring from another node. I made sure these files were readable by all. You can define more refined privileges for a production system with CephX, the security layer of Ceph.

Once everything is in place, you can test if it is working with this command:

Which gives the current state of the Ceph cluster.

2. Create the Ceph pool

Before we can use Ceph, we need to create a first RBD image, put a filesystem on it and mount it for MySQL on the bootstrap node. We need at least one Ceph pool since the RBD images are stored in a Ceph pool.  We create a Ceph pool with the command:

Here, we have defined the pool mysqlpool with 512 placement groups. On a larger Ceph cluster, you might need to use more placement groups (again, a topic beyond the scope of this post). The pool we just created is replicated. Each object in the pool will have two copies as defined by the osd_pool_default_size parameter in the ceph.conf file. If needed, you can modify the size of a pool and its replication factor at any moment after the pool is created.

3. Create the first RBD image

Now that we have a pool, we can create a first RBD image:

and “map” the RBD image to a host block device:

The commands return the local RBD block device that corresponds to the RBD image. The other steps are not specific to RBD images, we need to create a filesystem and prepare the mount points.

The rest of the steps are not specific to RBD images. We need to create a filesystem and prepare the mount points:

You need to mount the RBD device and run the mysql_install_db tool only on the bootstrap node. You need to create the directories /var/lib/mysql and /var/lib/galera on the other nodes and adjust the permissions similarly.

4. Modify the my.cnf files

You will need to set or adjust the specific