Zfs

ZFS is a combined file system and logical volume manager designed by Sun Microsystems. ZFS can scale to very large file system sizes and supports compression.

ZFS greatly simplifies the process of increasing a file system size as required. The simplest approach is to add new volumes to your VM and then add them to your ZFS file system to grow the size of your file system. This can be done while the file system is live and file I/O is occurring on the file system.

Installing ZFS¶

Ubuntu¶

Starting with the image Ubuntu-18.04-Bionic-x64-2018-09.

Ensure your package list is up-to-date and also do upgrades of your installed packages. While it isn't strictly necessary to upgrade your installed packages, it is a good idea.

[name@server]$ sudo apt-get update
[name@server]$ sudo apt-get dist-upgrade -y

Next, install ZFS.

[name@server]$ sudo apt-get install zfsutils-linux

CentOS¶

Starting with the image CentOS-7-x64-2018-09.

[name@server]$ sudo yum install http://download.zfsonlinux.org/epel/zfs-release.el7_5.noarch.rpm
...
Total size: 2.9 k
Installed size: 2.9 k
Is this ok [y/d/N]: y
...

Note

This process can be more complicated; see, for example, this guide.

Fedora¶

Starting with the image Fedora-Cloud-Base-29-1.2.

Content Incomplete

This section is yet to be written.

Using ZFS¶

Creating a zpool¶

[name@server]$ sudo zpool create -f data /dev/vdb /dev/vdc

This will create a new mount point at /data backed by the volumes attached at /dev/vdb and /dev/vdc. The file system will have a size slightly smaller than the combined sizes of all attached volumes. As data is written to the /data mount point, it will be dynamically stripped across all drives. This is good for performance and allows you to make use of all your storage; however, it doesn't provide any additional data replication (see this FreeBSD handbook section for information about creating mirrored zpools).

ZFS can compress data as it is written to the file system and uncompress it when it is read. To turn on and choose a compression algorithm for a zpool, use the following command.

[name@server]$ sudo zfs set compression=lz4 data

This will use the LZ4 compression algorithm on the zpool data. If your data is largely binary, you might not see a large reduction in storage use; however, if your data is something more compressible, such as ASCII data, you may see a larger reduction in storage use. Using compression can also improve file I/O performance because less data needs to be read and written. However, this can depend on the particular compression algorithm chosen, and some particularly computationally intensive algorithms may actually reduce file I/O performance. The LZ4 algorithm was chosen because it is a reasonable compromise between speed and amount of compression achieved; other compression algorithms may provide better compression or speed, but likely not both.

To see a list of properties for your pool, use the command below.

[name@server]$ sudo zfs get all data
NAME  PROPERTY              VALUE                  SOURCE
data  type                  filesystem             -
data  creation              Fri Mar  1 17:57 2019  -
data  used                  82.5K                  -
data  available             19.3G                  -
data  referenced            24K                    -
data  compressratio         1.00x                  -
data  mounted               yes                    -
data  quota                 none                   default
...

Zpools can be further subdivided into datasets. A dataset allows you to operate on smaller portions of your zpool independently. For example, you can set different compression algorithms for the dataset. To create a dataset within the data zpool, do the following.

[name@server]$ sudo zfs create data/www

Listing zpools¶

The existing pools can be listed with the command zpool list.

[name@server]$ zpool list
NAME           SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
data            19G   156K    19G        -         -     0%     0%  1.00x    ONLINE  -

Growing a zpool¶

Growing a zpool is a relatively simple task when compared to other file systems and logical volume managers. It is a two-step process: 1) add a new volume to your VM, and 2) add the new device to your zpool – that's it. Below is the command to add a new device to the zpool data.

[name@server]$ sudo zpool add data /dev/vde

The zpool will now have access to the added storage contributed by the newly added device. This process usually takes less than a minute even for very large, several-TB-sized zpools and volumes.

Check pool status:

[name@server]$ sudo zpool status data

Destroying a zpool¶

[name@server]$ sudo zpool destroy data

Notes¶

While in theory it should be possible to use ZFS with resizing volumes in OpenStack, in practice this has not been straightforward and is better to be avoided if possible.
While there is no hard limit to how many volumes you can attach to your VM, it is best to keep the number of volumes attached to a reasonable number. 19 attached volumes has been tested and shown to work well with the Queens version of OpenStack.
Having a pool of two or more volumes may provide improved I/O performance.

Zfs