ZFS filesystem is getting a wider recognition on Linux.
In ZFS, you can enable compression at the filesystem level. This will store the data in compressed format, which will save lot of disk space.
In this article, we’ll explain how to create the filesystem from the ZFS storage pool and enable compression on ZFS.
This is the 2nd article in the ZFS filesystem series.
In the first part of this series, we explained the fundamentals of ZFS, and how to install ZFS on linux. We also created a ZFS pool.
Create ZFS Filesystem
First, view all the current ZFS filesystems using zfs list command as shown below. In this example, we currently have one ZFS filesystem.
# zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 296K 5.84G 30K /mypool
Now, create a new ZFS filesystem using zfs create command.
# zfs create mypool/fs1
As we see below, the new ZFS filesystem is now created successfully.
# zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 170K 5.84G 30K /mypool mypool/fs1 30K 5.84G 30K /mypool/fs1
Set ZFS quote and Reservation
When you create a ZFS filesystem, by default it consumes all the space in the pool. So, you must specify a quota and reservation for the filesystem.
To set a quote, use zfs set command as shown below. Here we are specifying the quota as 1GB for this filesystem.
# zfs set quota=1G mypool/fs1
Next, set the reservation for the filesystem. In this example, fs1 is reserved 256M out of 5.59G so that no one can use this space and also it can extend up to 1G based on the quota we set if there is free space available.
# 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
Create ZFS alternate Mount Point
Instead of mounting it using the “mypool/fs1” name, you can also set an alternative mount point with any name that you wish for a filesystem.
For example, the following command will set the mount point as “/testmnt”, instead of “mypool/fs1”.
# zfs set mountpoint=/testmnt mypool/fs1
As we see from the following output, the first column NAME indicates the real name of the ZFS filesystem. The last column MOUNTPOINT indicates the alternative mount point that we created above.
# zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 256M 5.59G 32.5K /mypool mypool/fs1 30K 1024M 30K /testmnt
When you execute df command, you’ll see the alternative mount point as shown below.
# 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
Enable Compression on ZFS Filesystem
To set compression on a ZFS dataset, you can set the compression property as shown below. Once this property is set, any large files stored on this ZFS filesystem will be compressed.
# zfs set compression=lzjb mypool/fs1
The following are the valid compression properties:
You can enable compression on an existing filesystem as well. In that case, the compression will be applied only to the new and modified data; and any existing data will remain uncompressed.
Verify ZFS Compression
In following example, we have copied the 61M tar file to the ZFS filesystem mypool/fs1 mounted under /testmnt.
# ls -lh /testmnt/test.tar -rw-r--r--. 1 root root 61M Nov 11 09:44 /testmnt/test.tar
If you look at the total size of USED space from the zfs list command, you will see only 20.9M space is consumed which indicates that compression is turned on and working.
# zfs list NAME USED AVAIL REFER MOUNTPOINT mypool 256M 5.59G 32.5K /mypool mypool/fs1 20.9M 1003M 20.9M /testmnt
You can also get the compression ratio using the following command.
# zfs get compressratio mypool/fs1 NAME PROPERTY VALUE SOURCE mypool/fs1 compressratio 2.90x -
Apart from compression, ZFS filesystem has several advanced features. In the next article of the ZFS series, we’ll discuss about how to take ZFS clones and snapshots.
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ZFS on linux supports lz4 compression, which is supposedly better than lzjb.