Compression allows you to reduce the size of a file. By decreasing use of the inodes table, it means more files and folders can be stored, maximizing the space available on physical storage. In Linux, system administrators have used tools such as zip, tar, gzip, bzip2, and other open source utilities for this task.
Red Hat Enterprise Linux (RHEL) 8 introduced the Virtual Data Optimizer (VDO). This system compresses data dynamically. VDO is integrated into Logical Volume Management (LVM) in RHEL 9, but in RHEL 8, it has its own special toolchain. I'll cover that in this article. In a future article, I'll look at using LVM2 to create VDO volumes on RHEL 9.
VDO uses two kernel modules,
uds, to implement a three-stage compression scheme:
- Zero-Block Elimination (or thin provisioning): First, it verifies a VDO device exists and can use the underlying physical storage as and when necessary. VDO locates all data blocks containing zero bytes and records this information in metadata. Non-zero blocks are passed on to the next phase.
- Deduplication: This phase ensures that multiple repeated copies of data are eliminated, leaving only distinct data blocks on disk.
- Compression: This last phase uses the
kvdokernel module to compress data blocks using the LZ4 compression algorithm.
Install VDO in RHEL 8
Install VDO in RHEL 8 by installing the relevant packages:
$ sudo yum install vdo kmod-kvdo -y
Enable VDO using the
vdo command, which is a userspace command for interacting with VDO. To create a VDO volume called
my_vdo, based on the physical disk
/dev/sdb, with the logical space of 100 GB:
$ sudo vdo create --name=my_vdo \ --device=/dev/sdb --vdoLogicalSize=100G
--vdoLogicalSize flag defines the size the VDO volume presents to users. It's usually larger than the actual physical storage. If left unspecified, by default, it has a 1:1 ratio to the physical storage.
To list the VDO volumes created on your system, use the
$ sudo vdo list
For details on a specific VDO, use the
$ sudo vdo status --name=my_vdo
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Next, format the VDO volume with the XFS filesystem (or the filesystem of your choice). You can also use LVM to manage the volume. Here is an
$ sudo mkfs.xfs -K /dev/mapper/my_vdo
udevadm to register the new device:
$ sudo udevadm settle
Finally, mount the volume using the
$ sudo mount /dev/mapper/my_vdo /mnt
/etc/fstab file to set the mount options for the device:
$ sudo mount /dev/mapper/vdo_kvm /mnt
You can then update the
/etc/fstab file for the new device with the mount options. In the example above, the
/etc/fstab entry will be:
$ sudo vim /etc/fstab /dev/mapper/vdo_kvm /mnt xfs defaults 0 0
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vdostats command to display data deduplication and compression details on the mounted VDO filesystem:
$ sudo vdostats --human-readable
With VDO, you don't have to compress files manually to conserve space. Instead, space is maximized by dynamic compression as data is created and modified. If you don't have VDO enabled on your RHEL 8 system, give it a try! If you're using RHEL 9 already, stay tuned for my next article.