Wednesday, 4 April 2018

Shrinking Your Storage Requirements with VDO

Whether you're using proprietary storage arrays or software defined storage, the actual cost of capacity can sometimes provoke responses like, "why do you you need all that space?" or "OK, but that's all the storage you're going to get, so make it last".

The problem is that storage is a commodity resource, it's like toner or ink in a printer. When you run out, things will stop and lots of people tend to lose their sense of humor. Controlling storage growth has been going on for over 10 years in the proprietary storage space, with one of the most successful companies being NetApp who introduced data deduplication with their ASIS (advanced Single Instance Storage) feature back in 2007. The message was that if you wanted to reduce storage consumption, you basically had to buy the more expensive "stuff" in the first place.

This was the "status quo" until Red Hat acquired Permabit in mid compression and deduplication features are heading towards a Linux server near you!

That's the history lesson, now let's look at how you can kick the tyres on open sourced based compression and deduplication. For the remainder of this article, I'll walk through the steps you need to quickly get "dedupe" up and running with Fedora.


Since we're just testing, create a vm and install Fedora 27. Use libvirt, parallels, virtualbox...whatever takes your fancy - or maybe just use a cloud image in AWS. The choice is yours! Just try to ensure the vm has something like; 2 vcpus, 4GB RAM, an OS disk (20GB) and a data disk for vdo testing.

Once installed you'll need to enable an additional repository to pick up the vdo deduplication modules (kvdo - kernel virtual data optimizer)

dnf copr enable rhawalsh/dm-vdo
dnf install vdo kmod-kvdo


In my test environment, I'm using a 20g vdisk for my vdo testing.
[root@f27-vdo ~]# lsblk
vda    252:0    0   4G  0 disk 
└─vda1 252:1    0   4G  0 part /
vdb    252:16   0  20G  0 disk 

Now with the kvdo module in place, let's create a vdo volume of 100G using the 20G /dev/vdb device

[root@f27-vdo ~]# vdo create --name=vdo0 --device=/dev/vdb \
Creating VDO vdo0
Starting VDO vdo0
Starting compression on VDO vdo0
VDO instance 0 volume is ready at /dev/mapper/vdo0

Not exactly complicated :) Couple of things worth noting though;
  • by default new volumes are created with compression and deduplication enabled. If you don't like that you can play with the  --compression or --deduplication flags.
  • a vdo volume is actually a device mapper device, in this case /dev/mapper/vdo0. It's this 'dm' device that you'll use from here on in.


Now you have a vdo volume, next step is to get it deployed and understand how to report on space savings. The first thing is filesystem formatting. Make sure you use the -K switch to avoid issuing discards, remember a vdo volume is in effect a thin provisioned volume.

[root@f27-vdo ~]# mkfs.xfs -K /dev/mapper/vdo0

With the filesystem in place, the next step would normally be updating fstab...right? Well not this time. For vdo volumes, the boot time startup sequence between fstab and the vdo service is a problem - so we need to use a mount service to ensure vdo volumes are mounted correctly. 
The vdo rpm provides a sample mount service definition (/usr/share/doc/vdo/examples/systemd/VDO.mount.example). For this example, I'm going to mount the vdo volume at /mnt/vdo0

mkdir /mnt/vdo0
cp /usr/share/doc/vdo/examples/systemd/VDO.mount.example /etc/systemd/system/mnt-vdo0.mount

Then update the mount unit to look like this
Description = Mount filesystem that lives on VDO0
name = mnt-vdo0.mount
Requires = vdo.service systemd-remount-fs.service
After =
Conflicts =

What = /dev/mapper/vdo0
Where = /mnt/vdo0
Type = xfs
Options = discard

WantedBy =

Reminder: mount services are named to reflect their intended mount location within the filesystem.

Now reload systemd, enable the mount and start it
systemctl daemon-reload
systemctl enable mnt-vdo0.mount
systemctl start mnt-vdo0.mount
[root@f27-vdo ~]# df -h /mnt/vdo0
Filesystem         Size Used Avail Use% Mounted on
/dev/mapper/vdo0   100G 135M 100G    1% /mnt/vdo0

At this point you've used the vdo command to create the volume, but there is also a command to look at the volume's statistics called vdostats. To give us something to look at I copied the same 200MB disk image to the volume 20 times, which will also help to explain vdo overheads.

[root@f27-vdo ~]# df -h /mnt/vdo0
Filesystem        Size  Used Avail Use% Mounted on
/dev/mapper/vdo0  100G  4.5G   96G   5% /mnt/vdo0

[root@f27-vdo ~]# vdostats --hu vdo0
Device               Size   Used   Available   Use% Space saving%
vdo0                20.0G   4.2G       15.8G    21%           95%

Wait a a logical layer, the filesystem says that it's 4.5G used, but at the physical vdo layer it's saying practically the same thing AND that there's a 95% saving! So which is right? The answer is both :) The vdo subsystem persists metadata on the volume (lookup maps etc), which accounts for a chunk of the physical space used, and the savings value is derived purely from the logical blocks "in" and the physical, unique blocks written. If you need to understand more you can dive into the sysfs filesystem. 
Each vdo volume stores and maintains statistics under  /sys/kvdo/<vol_name>/statistics (which is where vdostats gets it's information from!)

The most useful stats I've found to understand how space is consumed are;

  • overhead_blocks_used : metadata for the volume. The overhead is proportional to the physical size of the volume; for example, on an 8TB device, the overhead was around 9GB
  • data_blocks_used: this is the count of the physical blocks consumed by user data
  • logical_blocks_used: the count of blocks consumed at the filesystem level
In my case, the "overhead_blocks_used" was 4GB, and the "data_blocks_used" around 200MB. The savings% value is derived from  data_blocks_used / logical_blocks_used, since it only applies to actual user data written to the volume, which equates to around 95%. Now it makes sense!

Final Words

Deduplication is a complex beast, but hopefully the above will at least get you up and running with this new Linux feature.

If you decide to use vdo across a number of servers, running vdostats isn't really a viable option. For that it would be more useful to leave the command line behind at look at solutions like prometheus and grafana to track capacity usage and generate alerts. Spoiler alert!...that's the subject of my next post :)

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