EXT4

Migration Guide: Moving from EXT4 to ZFS

Migrating from EXT4 to ZFS offers significant advantages, including improved data integrity, built-in snapshots, integrated volume management, and advanced RAID support (RAID-Z). EXT4 is a widely used file system, but ZFS offers a comprehensive solution for storage management with additional features beyond what EXT4 and traditional volume managers like LVM can provide.

Why Migrate from EXT4 to ZFS?

Before initiating the migration process, it is essential to understand the key benefits of ZFS over EXT4:

• Data Integrity: ZFS employs end-to-end checksumming, allowing for the detection and automatic repair of data corruption, whereas EXT4 relies on journaling, which primarily protects metadata.
• Snapshots: ZFS supports efficient, native snapshots, enabling point-in-time recovery, a feature that EXT4 lacks.
• Integrated RAID: ZFS includes built-in RAID-Z, eliminating the need for external RAID solutions or hardware RAID.
• Compression and Deduplication: ZFS provides native data compression and deduplication, helping reduce disk usage, especially in environments where redundant data is common.

Prerequisites

Before starting the migration process, ensure that the following prerequisites are met:

• Data Backup: A full backup of the data on the EXT4 file system must be performed to prevent any data loss. The migration steps will involve formatting disks, which will erase data.
• Available Disk Space: ZFS will require a new pool setup, so either additional disks or enough free space must be available to move the data.
• Install ZFS Utilities: Install the necessary ZFS packages depending on the operating system.

For Ubuntu or Debian-based systems, ZFS can be installed with the following command:

$ sudo apt install zfsutils-linux

For RHEL-based systems (CentOS, Fedora), install ZFS with:

$ sudo yum install zfs

For FreeBSD, ZFS is natively included, so no additional installation steps are required.

Step 1: Preparing the New ZFS Pool

1. Identify Available Disks: First, identify the disks or partitions that will be used for the new ZFS pool. Available disks can be listed with:

$ lsblk

Make note of the device names to be used (e.g., /dev/sdb, /dev/sdc).

2. Create the ZFS Pool: Create a new ZFS pool on the target disks. For a single disk setup:

$ sudo zpool create mypool /dev/sdb

For a mirrored pool:

$ sudo zpool create mypool mirror /dev/sdb /dev/sdc

For RAID-Z1:

$ sudo zpool create mypool raidz /dev/sdb /dev/sdc /dev/sdd

Replace mypool with the chosen pool name.

Step 2: Migrating Data from EXT4 to ZFS

Once the ZFS pool is created, the next step is to transfer the data from the existing EXT4 file system to the new ZFS pool.

1. Mount the ZFS Pool: After pool creation, ZFS automatically mounts the pool under /mypool. Verify the mount point with:

$ zfs list

2. Copy Data from EXT4 to ZFS: If the EXT4 file system is not mounted, mount it first:

$ sudo mount /dev/sda1 /mnt/ext4

Use rsync to transfer the data while preserving file attributes, permissions, and symbolic links:

$ sudo rsync -aHAXv /mnt/ext4/ /mypool/

This command will copy the contents of the EXT4 file system located at /mnt/ext4 to the new ZFS pool at /mypool.

Step 3: Configuring the ZFS Pool

After the data migration, the ZFS pool can be further configured based on specific requirements.

Enable Compression

ZFS supports various compression algorithms such as lz4 (the default) and gzip. Compression can be enabled to save disk space and optimize performance:

$ sudo zfs set compression=lz4 mypool

Compression effectiveness can be checked with:

$ zfs get compression,compressratio mypool

Enable Deduplication (Optional)

In environments with redundant data, enabling deduplication may reduce storage needs. However, it is memory-intensive and should be used cautiously:

$ sudo zfs set dedup=on mypool

Set Additional Pool Properties

Other ZFS dataset properties, such as mount points, ACLs, and quota settings, can also be customized.

Step 4: Verifying and Monitoring the ZFS Pool

After completing the migration, verify the integrity of the ZFS pool using ZFS's built-in monitoring tools.

Check Pool Status

Check the pool’s health status to ensure everything is functioning correctly:

$ sudo zpool status

Scrub the Pool

Perform a ZFS scrub to check the data’s integrity and fix any issues:

$ sudo zpool scrub mypool

A scrub verifies the data against its checksums and repairs any errors found using redundant data.

Step 5: Post-Migration Cleanup

Once the data migration is confirmed and the ZFS pool is functioning correctly, it is possible to remove or repurpose the old EXT4 partitions.

• Unmount the EXT4 File System:

$ sudo umount /mnt/ext4

• Reclaim or Reformat the EXT4 Partition: Using tools like fdisk or parted, the old EXT4 partitions can be deleted or reformatted as necessary.

EXT4 to ZFS Command and Utility Mapping

This table maps common commands and utilities used in EXT4 to their equivalents in ZFS.