OCFS2 File system and Snapshot with REFLINK

OCFS2 is a shared-disk file system designed for use in a cluster environment. It was developed by Oracle Corporation and is included in the Linux kernel.

OCFS2 stands for Oracle Cluster File System 2, and it is a successor to the original OCFS. Like its predecessor, OCFS2 allows multiple servers to access the same shared storage, enabling them to work together to provide highly available services to clients. This makes it well-suited for use in high-performance computing, virtualization, and database clustering applications.

OCFS2 offers a number of advanced features, including support for large files, POSIX compliance, journaling, and snapshotting. It also supports both metadata and data clustering, which allows for highly efficient data access and improves performance in high-concurrency workloads. Additionally, it provides features like multi-volume support, distributed lock management, and ACLs (Access Control Lists).

Overall, OCFS2 is a highly capable file system that provides a scalable and reliable platform for use in clustered environments.

OCFS2 supports both snapshots and reflink. While both features allow you to create copies of a file system, there are some differences between them.

As I mentioned earlier, snapshots in OCFS2 are read-only copies of a file system at a specific point in time. They are created using the ocfs2cluster command line tool and stored in a separate snapshot volume. Snapshots can be used for backup, testing, or other purposes, but they are read-only, which means that you can’t modify the contents of a snapshot.

On the other hand, reflink is a copy-on-write mechanism that allows you to create a new file or directory that shares data with an existing file or directory. Reflink uses a special system call called clone to create a new file or directory that references the same data as the original file or directory. This means that changes to the new file or directory are only stored in the new file or directory, not in the original file or directory.

While snapshots are read-only, reflink copies can be modified independently of the original file or directory. This can be useful for creating efficient backups or for making copies of large files without using up additional disk space.

In OCFS2, you can use reflink by using the cp --reflink command. Here’s an example:

cp --reflink /mnt/mydata/file.txt /mnt/mydata/file-copy.txt

In this example, file.txt is the original file, and file-copy.txt is the new file that will share the same data as file.txt. Any changes made to file-copy.txt will be stored separately from file.txt.

Overall, both snapshots and reflink can be useful features in OCFS2, depending on your specific needs. Snapshots are read-only copies of a file system at a specific point in time, while reflink allows you to create new files or directories that share data with existing files or directories.

here’s an example of how to use reflink with an Oracle VM (OVM) virtual machine (VM) in OCFS2:

  1. First, make sure that your OCFS2 file system is mounted with the reflink=1 option. This will enable reflink support on the file system.
  2. Next, create a new OVM virtual machine. You can use the OVM Manager GUI or the ovmcli command line tool to create the VM.
  3. Once the VM is created, use the ovm_vmcontrol command to stop the VM:
ovm_vmcontrol stop test_vm
  1. After the VM is stopped, use the cp command with the --reflink option to create a new copy of the VM’s virtual hard disk:
cp --reflink /path/to/vm/disk.img /path/to/new/disk.img

Replace /path/to/vm/disk.img with the path to the original virtual hard disk of your VM, and /path/to/new/disk.img with the path to the new virtual hard disk you want to create.

  1. Once the new virtual hard disk is created, you can use the ovm_vmcontrol command to update the VM’s configuration to use the new virtual hard disk:
ovm_vmcontrol edit test_vm disk disk1 file=/path/to/new/disk.img
  1. Finally, use the ovm_vmcontrol command to start the VM:
ovm_vmcontrol start VirtualMachineName

OCFS2 is used as the file system for storing the virtual machine images in Oracle Exadata Database Machine.

The snapshots can be created using the ocfs2cluster command line tool or the Exadata Snapshot Management feature in Oracle Enterprise Manager (OEM).

You can follow this guide oracle for use the snapshot ocfs2 as backup for the virtual machines : https://docs.oracle.com/en/engineered-systems/exadata-database-machine/dbmmn/managing-oracle-vm-domains.html#GUID-A71A5F50-539D-44DF-9379-AA5D2E837999

Once a snapshot is created, it can be used for various purposes, such as restoring the database to a previous point-in-time or for creating a clone of the database for testing or development purposes. The snapshot feature of OCFS2 can help simplify and speed up the backup and recovery process in an Exadata environment.

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