Tag: VDO

Postgresql with File System Compression – VDO and ZFS

Our database storage is sizable. To reduce the financial impact of storing so much data, we opted to use a compressed file system. This allows us to maintain, for example, 8TB of data in under 2TB of space. Unfortunately, the ZFS file system we use to compress our data is no longer “built in” with newer version of RedHat.

There are alternatives. BTRFS is a long-standing option, however it’s got reliability issues (we piloted BTRFS on one of the read only replicas, and the compression ratio is nowhere near as good — the 2TB of ZFS data filled the 10TB BTRFS disk even using the better compression option. And I/O was so slow there was a continual replication backlog). RedHat introduced Virtual Disk Optimizer to replace ZFS. In theory, it’s better since it also deduplicates data (e.g. if every one of us saved the same PPT presentation to the disk, only one copy would actually be stored). That’s great for email and file shares where a lot of people are likely to store the same information. Not so useful on a database server where there’s little to de-duplicate. It does, however, compress data … so we decided to try it out.

The results, unfortunately, are not spectacular. VDO does not allow you to do much customization of the compression. It’s on or off. I’ve found some people tweaking it up in unsupported ways, but the impetus behind trying VDO was that it’s supported by RedHat. Making unsupported changes to it defeats that purpose. And the compression that we’re seeing is far less than we get in ZFS. Our existing servers run between 4.5x and 6x compression

In VDO, however, we don’t even get a 2x compression factor. 11TB of information is stored in 8TB of space! That’s 1.4x

So, while we found the performance of VDO to be satisfactory and it’s really easy to use in newer RedHat releases … we’d have to increase our 20TB LUNs to 80TB to continue storing the data we store today. That seems like A Really Bad Idea(tm).

Seems like I’m going to have to sort out using OpenZFS on the new servers.

Postgresql and Timescale with RedHat VDO

RedHat is phasing out ZFS – there are several reasons for this move, but primarily ZFS is a closed source Solaris (now Oracle) codebase. While OpenZFS exists, it’s not quite ‘the same’. RedHat’s preferred solution is Virtual Data Optimizer (VDO). This page walks through the process of installing PostgreSQL and creating a database cluster on VDO and installing TimescaleDB extension on the database cluster for RedHat Enterprise 8 (RHEL8)

Before we create a VDO disk, we need to install it

yum install vdo kmod-kvdo

Then we need to create a vdo – here a VDO named ‘PGData’ is created on /dev/sdb – a 9TB volume on which we will hold 16TB

vdo create --name=PGData --device=/dev/sdb --vdoLogicalSize=16T

Check to verify that the object was created – it is /dev/mapper/PGData in this instance

vdo list

Now format the volume using xfs.

mkfs.xfs /dev/mapper/PGData

And finally add a mount point

# Create the mount point folder
mkdir /pgpool
# Update fstab to mount the new volume to that mount pint
cat /etc/fstab
/dev/mapper/PGData /pgpool xfs defaults,x-systemd.requires=vdo.service 0 0
# Load the updated fstab
systemctl daemon-reload
# and mount the volume
mount -a

it should be mounted at ‘/pgpool/’

The main reason for using VDO with Postgres is because of its compression feature – this is automatically enabled, although we may need to tweak settings as we test it.

We now have a place in our pool where we want our Postgres database to store its data. So let’s go ahead and install PostgreSQL,

here we are using RHEL8 and installing PostgreSQL 12

# Install the repository RPM:
dnf install -y https://download.postgresql.org/pub/repos/yum/reporpms/EL-8-x86_64/pgdg-redhat-repo-latest.noarch.rpm
dnf clean all
# Disable the built-in PostgreSQL module:
dnf -qy module disable postgresql
# Install PostgreSQL:
dnf install -y postgresql12-server

Once the installation is done we need to initiate the database cluster and start the server . Since we want our Postgres to store data in our VDO volume we need to initialize it into our custom directory, we can do that in many ways,

In all cases we need to make sure that the mount point of our zpool i.e., ‘/pgpool/pgdata/’ is owned by the ‘postgres’ user which is created when we install PostgreSQL. We can do that by running the below command before running below steps for starting the postgres server

mkdir /pgpool/pgdata
chown -R postgres:postgres /pgpool

Customize the systemd service by editing the postgresql-12 unit file and updateding the PGDATA environment variable

vdotest-uos:pgpool # grep Environment /usr/lib/systemd/system/postgresql-12.service
# Note: avoid inserting whitespace in these Environment= lines, or you may

and  then initialize, enable and start our server as below

/usr/pgsql-12/bin/postgresql-12-setup initdb
systemctl enable postgresql-12
systemctl start postgresql-12

Here ‘/usr/pgsql-12/bin/’ is the bin directory of postgres installation you can substitute it with your bin directory path.


We can also directly give the data directory value while initializing db using below command

/usr/pgsql-12/bin/initdb -D /pgpool/pgdata/

and then start the server using

systemctl start postgresql-12

Now we have installed postgreSQL and started the server, we will install the Timescale extension for Postgres now.

add the time scale repo with below command

tee /etc/yum.repos.d/timescale_timescaledb.repo <<EOL
sudo yum update -y

then install  it using below command

yum install -y timescaledb-postgresql-12

After installing we need to add ‘timescale’ to shared_preload_libraries in our postgresql.conf, Timescale gives us ‘timescaledb-tune‘ which can be used for this and also configuring different settings for our database. Since we initialize our PG database cluster in a custom location we need to point the direction of postgresql.conf to timescaledb-tune it also requires a path to our pg_config file we can do both by following command.

timescaledb-tune --pg-config=/usr/pgsql-12/bin/pg_config --conf-path=/pgpool/pgdata/postgresql.conf

After running above command we need to restart our Postgres server, we can do that by one of the below commands

systemctl restart postgresql-12

After restarting using one of the above commands connect to the database you want to use Timescale hypertables in and run below statement to load Timescale extension


you can check if Timescale is loaded by passing ‘\dx’ command to psql which will load the extension list.

in order to configure PostgreSQL to allow remote connection we need to do couple of changes as below