How to Create ZFS Filesystems and Enable Compression on Linux

Abstract: ZFS file system is gaining recognition on Linux. In ZFS you can enable compression at the filesystem level, saving disk space. This article explains how to create a ZFS filesystem from a pool and enable compression.

Creating a ZFS Filesystem

First, list existing ZFS filesystems with zfs list. Example output shows a pool named mypool and an existing filesystem mypool/fs1.

# zfs list
NAME         USED  AVAIL  REFER  MOUNTPOINT
mypool       170K  5.84G   30K   /mypool
mypool/fs1   30K   5.84G   30K   /mypool/fs1

Then create a new filesystem and set a quota of 1 GB:

# zfs set quota=1G mypool/fs1

The new filesystem appears in the list:

# zfs list
NAME         USED  AVAIL  REFER  MOUNTPOINT
mypool       170K  5.84G   30K   /mypool
mypool/fs1   30K   5.84G   30K   /mypool/fs1

Setting ZFS Quota and Reservation

By default a ZFS filesystem can consume all space in the pool, so you must set a quota and a reservation. The quota is set with zfs set quota=1G mypool/fs1. The reservation reserves 256 M so that space cannot be used by others, while still allowing growth up to the quota.

# zfs set reservation=256M mypool/fs1
# zfs list
NAME         USED  AVAIL  REFER  MOUNTPOINT
mypool       256M  5.59G  32.5K  /mypool
mypool/fs1   30K   1024M  30K    /mypool/fs1

Creating an Alternate Mountpoint

You can assign any mountpoint name to the filesystem. For example, set the mountpoint to /testmnt instead of the default mypool/fs1:

# zfs set mountpoint=/testmnt mypool/fs1

The zfs list output shows the new mountpoint, and the df -h command confirms it:

# df -h
Filesystem   Size  Used Avail Use% Mounted on
... 
mypool       5.6G  128K 5.6G   1% /mypool
mypool/fs1   1.0G  128K 1.0G   1% /testmnt

Enabling Compression on a ZFS Dataset

Enable compression by setting the compression property, e.g., lzjb. Valid values are on, off, lzjb, gzip, gzip[1-9], and zle.

# zfs set compression=lzjb mypool/fs1

Compression applies to new and modified data; existing data remains uncompressed.

Verifying ZFS Compression

Copy a 61 M tar file to the mounted dataset and check space usage. The zfs list shows only 20.9 M used, confirming compression.

# ls -lh /testmnt/test.tar
-rw-r--r-- 1 root root 61M Nov 11 09:44 /testmnt/test.tar
# zfs list
NAME         USED   AVAIL   REFER   MOUNTPOINT
mypool       256M   5.59G   32.5K   /mypool
mypool/fs1   20.9M  1003M   20.9M   /testmnt

Retrieve the compression ratio:

# zfs get compressratio mypool/fs1
NAME        PROPERTY       VALUE  SOURCE
mypool/fs1  compressratio  2.90x  -

The article notes that ZFS also offers advanced features such as clones and snapshots, which will be covered in the next installment.