Managing Geo-replication

Geo-replication provides a continuous, asynchronous, and incremental replication service from one site to another over Local Area Networks (LANs), Wide Area Network (WANs), and across the Internet.

Geo-replication uses a master–slave model, whereby replication and mirroring occurs between the following partners:

Master – a GlusterFS volume
Slave – a slave which can be of the following types:
- A local directory which can be represented as file URL like file:///path/to/dir. You can use shortened form, for example, /path/to/dir.
- A GlusterFS Volume - Slave volume can be either a local volume like gluster://localhost:volname (shortened form - :volname) or a volume served by different host like gluster://host:volname (shortened form - host:volname).
Note

Both of the above types can be accessed remotely using SSH tunnel. To use SSH, add an SSH prefix to either a file URL or gluster type URL. For example, ssh://root@remote-host:/path/to/dir (shortened form - root@remote-host:/path/to/dir) or ssh://root@remote-host:gluster://localhost:volname (shortened from - root@remote-host::volname).

This section introduces Geo-replication, illustrates the various deployment scenarios, and explains how to configure the system to provide replication and mirroring in your environment.

Replicated Volumes vs Geo-replication

The following table lists the difference between replicated volumes and geo-replication:

Replicated Volumes	Geo-replication
Mirrors data across clusters	Mirrors data across geographically distributed clusters
Provides high-availability	Ensures backing up of data for disaster recovery
Synchronous replication (each and every file operation is sent across all the bricks)	Asynchronous replication (checks for the changes in files periodically and syncs them on detecting differences)

Preparing to Deploy Geo-replication

This section provides an overview of the Geo-replication deployment scenarios, describes how you can check the minimum system requirements, and explores common deployment scenarios.

Exploring Geo-replication Deployment Scenarios

Geo-replication provides an incremental replication service over Local Area Networks (LANs), Wide Area Network (WANs), and across the Internet. This section illustrates the most common deployment scenarios for Geo-replication, including the following:

Geo-replication over LAN
Geo-replication over WAN
Geo-replication over the Internet
Multi-site cascading Geo-replication

Geo-replication over LAN

You can configure Geo-replication to mirror data over a Local Area Network.

geo-rep_lan

Geo-replication over WAN

You can configure Geo-replication to replicate data over a Wide Area Network.

geo-rep_wan

Geo-replication over Internet

You can configure Geo-replication to mirror data over the Internet.

geo-rep03_internet

Multi-site cascading Geo-replication

You can configure Geo-replication to mirror data in a cascading fashion across multiple sites.

geo-rep04_cascading

Geo-replication Deployment Overview

Deploying Geo-replication involves the following steps:

Verify that your environment matches the minimum system requirement.
Determine the appropriate deployment scenario.
Start Geo-replication on master and slave systems, as required.

Checking Geo-replication Minimum Requirements

Before deploying GlusterFS Geo-replication, verify that your systems match the minimum requirements.

The following table outlines the minimum requirements for both master and slave nodes within your environment:

Component	Master	Slave
Operating System	GNU/Linux	GNU/Linux
Filesystem	GlusterFS 3.2 or higher	GlusterFS 3.2 or higher (GlusterFS needs to be installed, but does not need to be running), ext3, ext4, or XFS (any other POSIX compliant file system would work, but has not been tested extensively)
Python	Python 2.4 (with ctypes external module), or Python 2.5 (or higher)	Python 2.4 (with ctypes external module), or Python 2.5 (or higher)
Secure shell	OpenSSH version 4.0 (or higher)	SSH2-compliant daemon
Remote synchronization	rsync 3.0.7 or higher	rsync 3.0.7 or higher
FUSE	GlusterFS supported versions	GlusterFS supported versions

Setting Up the Environment for Geo-replication

Time Synchronization

On bricks of a geo-replication master volume, all the servers' time must be uniform. You are recommended to set up NTP (Network Time Protocol) service to keep the bricks sync in time and avoid out-of-time sync effect.

For example: In a Replicated volume where brick1 of the master is at 12.20 hrs and brick 2 of the master is at 12.10 hrs with 10 minutes time lag, all the changes in brick2 between this period may go unnoticed during synchronization of files with Slave.

To setup Geo-replication for SSH

Password-less login has to be set up between the host machine (where geo-replication Start command will be issued) and the remote machine (where slave process should be launched through SSH).

On the node where geo-replication sessions are to be set up, run the following command:
```
# ssh-keygen -f /var/lib/glusterd/geo-replication/secret.pem
```
Press Enter twice to avoid passphrase.

Run the following command on master for all the slave hosts:

# ssh-copy-id -i /var/lib/glusterd/geo-replication/secret.pem.pub @

Setting Up the Environment for a Secure Geo-replication Slave

You can configure a secure slave using SSH so that master is granted a restricted access. With GlusterFS, you need not specify configuration parameters regarding the slave on the master-side configuration. For example, the master does not require the location of the rsync program on slave but the slave must ensure that rsync is in the PATH of the user which the master connects using SSH. The only information that master and slave have to negotiate are the slave-side user account, slave's resources that master uses as slave resources, and the master's public key. Secure access to the slave can be established using the following options:

Restricting Remote Command Execution
Using Mountbroker for Slaves
Using IP based Access Control

Backward Compatibility

Your existing Ge-replication environment will work with GlusterFS, except for the following:

The process of secure reconfiguration affects only the glusterfs instance on slave. The changes are transparent to master with the exception that you may have to change the SSH target to an unprivileged account on slave.
The following are the some exceptions where this might not work:
- Geo-replication URLs which specify the slave resource when configuring master will include the following special characters: space, *, ?, [;
- Slave must have a running instance of glusterd, even if there is no gluster volume among the mounted slave resources (that is, file tree slaves are used exclusively).

Restricting Remote Command Execution

If you restrict remote command execution, then the Slave audits commands coming from the master and the commands related to the given geo-replication session is allowed. The Slave also provides access only to the files within the slave resource which can be read or manipulated by the Master.

To restrict remote command execution:

Identify the location of the gsyncd helper utility on Slave. This utility is installed in PREFIX/libexec/glusterfs/gsyncd, where PREFIX is a compile-time parameter of glusterfs. For example, --prefix=PREFIX to the configure script with the following common values/usr, /usr/local, and /opt/glusterfs/glusterfs_version.
Ensure that command invoked from master to slave passed through the slave's gsyncd utility.

You can use either of the following two options:
- Set gsyncd with an absolute path as the shell for the account which the master connects through SSH. If you need to use a privileged account, then set it up by creating a new user with UID 0.
- Setup key authentication with command enforcement to gsyncd. You must prefix the copy of master's public key in the Slave account's authorized_keys file with the following command:
  
  command=<path to gsyncd>.
  
  For example, command="PREFIX/glusterfs/gsyncd" ssh-rsa AAAAB3Nza....

Using Mountbroker for Slaves

mountbroker is a new service of glusterd. This service allows an unprivileged process to own a GlusterFS mount by registering a label (and DSL (Domain-specific language) options ) with glusterd through a glusterd volfile. Using CLI, you can send a mount request to glusterd to receive an alias (symlink) of the mounted volume.

A request from the agent , the unprivileged slave agents use the mountbroker service of glusterd to set up an auxiliary gluster mount for the agent in a special environment which ensures that the agent is only allowed to access with special parameters that provide administrative level access to the particular volume.

To setup an auxiliary gluster mount for the agent:

In all Slave nodes, create a new group. For example, geogroup.
In all Slave nodes, create a unprivileged account. For example, geoaccount. Make it a member of geogroup.
In all Slave nodes, Create a new directory owned by root and with permissions 0711. For example, create a create mountbroker-root directory /var/mountbroker-root.
In any one of Slave node, Run the following commands to add options to glusterd vol file(/etc/glusterfs/glusterd.vol) in rpm installations and /usr/local/etc/glusterfs/glusterd.vol in Source installation.

sh gluster system:: execute mountbroker opt mountbroker-root /var/mountbroker-root gluster system:: execute mountbroker opt geo-replication-log-group geogroup gluster system:: execute mountbroker opt rpc-auth-allow-insecure on
In any one of the Slave node, Add Mountbroker user to glusterd vol file using,

sh gluster system:: execute mountbroker user geoaccount slavevol

where slavevol is the Slave Volume name

If you host multiple slave volumes on Slave, for each of them and add the following options to the volfile using,

sh gluster system:: execute mountbroker user geoaccount2 slavevol2 gluster system:: execute mountbroker user geoaccount3 slavevol3

To add multiple volumes per mountbroker user,

sh gluster system:: execute mountbroker user geoaccount1 slavevol11,slavevol12,slavevol13 gluster system:: execute mountbroker user geoaccount2 slavevol21,slavevol22 gluster system:: execute mountbroker user geoaccount3 slavevol31 6. Restart glusterd service on all Slave nodes.
Setup a passwdless SSH from one of the master node to the user on one of the slave node. For example, to geoaccount.
Create a geo-replication relationship between master and slave to the user by running the following command on the master node:

sh gluster volume geo-replication <master_volume> <mountbroker_user>@<slave_host>::<slave_volume> create push-pem [force]
In the slavenode, which is used to create relationship, run /usr/libexec/glusterfs/set_geo_rep_pem_keys.sh as a root with user name, master volume name, and slave volume names as the arguments.

sh /usr/libexec/glusterfs/set_geo_rep_pem_keys.sh <mountbroker_user> <master_volume> <slave_volume>

Using IP based Access Control

You can use IP based access control method to provide access control for the slave resources using IP address. You can use method for both Slave and file tree slaves, but in the section, we are focusing on file tree slaves using this method.

To set access control based on IP address for file tree slaves:

Set a general restriction for accessibility of file tree resources:
```
# gluster volume geo-replication '/*' config allow-network ::1,127.0.0.1
```
This will refuse all requests for spawning slave agents except for requests initiated locally.
If you want the to lease file tree at /data/slave-tree to Master, enter the following command:
```
# gluster volume geo-replicationconfig allow-network
```
MasterIP is the IP address of Master. The slave agent spawn request from master will be accepted if it is executed at /data/slave-tree.

If the Master side network configuration does not enable the Slave to recognize the exact IP address of Master, you can use CIDR notation to specify a subnet instead of a single IP address as MasterIP or even comma-separated lists of CIDR subnets.

If you want to extend IP based access control to gluster slaves, use the following command:

    # gluster volume geo-replication '*' config allow-network ::1,127.0.0.1

Starting Geo-replication

This section describes how to configure and start Gluster Geo-replication in your storage environment, and verify that it is functioning correctly.

Starting Geo-replication

To start Gluster Geo-replication

Use the following command to start geo-replication between the hosts:

# gluster volume geo-replication  start

For example:

# gluster volume geo-replication Volume1 example.com:/data/remote_dir start
Starting geo-replication session between Volume1
example.com:/data/remote_dir has been successful

Note

You may need to configure the service before starting Gluster Geo-replication.

Verifying Successful Deployment

You can use the gluster command to verify the status of Gluster Geo-replication in your environment.

To verify the status Gluster Geo-replication

Verify the status by issuing the following command on host:

# gluster volume geo-replication  status

For example:

# gluster volume geo-replication Volume1 example.com:/data/remote_dir status
# gluster volume geo-replication Volume1 example.com:/data/remote_dir status
MASTER    SLAVE                            STATUS
______    ______________________________   ____________
Volume1 root@example.com:/data/remote_dir  Starting....

Displaying Geo-replication Status Information

You can display status information about a specific geo-replication master session, or a particular master-slave session, or all geo-replication sessions, as needed.

To display geo-replication status information

Use the following command to display information of all geo-replication sessions:
```
# gluster volume geo-replication Volume1 example.com:/data/remote_dir status
```
Use the following command to display information of a particular master slave session:
```
# gluster volume geo-replication  status
```
For example, to display information of Volume1 and example.com:/data/remote_dir
```
# gluster volume geo-replication Volume1 example.com:/data/remote_dir status
```
The status of the geo-replication between Volume1 and example.com:/data/remote_dir is displayed.
Display information of all geo-replication sessions belonging to a master
```
# gluster volume geo-replication MASTER status
```
For example, to display information of Volume1
```
# gluster volume geo-replication Volume1 example.com:/data/remote_dir status
```
The status of a session could be one of the following:
Initializing: This is the initial phase of the Geo-replication session; it remains in this state for a minute in order to make sure no abnormalities are present.
Not Started: The geo-replication session is created, but not started.
Active: The gsync daemon in this node is active and syncing the data.
Passive: A replica pair of the active node. The data synchronization is handled by active node. Hence, this node does not sync any data.
Faulty: The geo-replication session has experienced a problem, and the issue needs to be investigated further.
Stopped: The geo-replication session has stopped, but has not been deleted.

The Crawl Status can be one of the following:
Changelog Crawl: The changelog translator has produced the changelog and that is being consumed by gsyncd daemon to sync data.
Hybrid Crawl: The gsyncd daemon is crawling the glusterFS file system and generating pseudo changelog to sync data.
Checkpoint Status: Displays the status of the checkpoint, if set. Otherwise, it displays as N/A.

Configuring Geo-replication

To configure Gluster Geo-replication

Use the following command at the Gluster command line:

# gluster volume geo-replication  config [options]

For example:

Use the following command to view list of all option/value pair:

# gluster volume geo-replication Volume1 example.com:/data/remote_dir config

Configurable Options

The following table provides an overview of the configurable options for a geo-replication setting:

Option	Description
gluster-log-file LOGFILE	The path to the geo-replication glusterfs log file.
gluster-log-level LOGFILELEVEL	The log level for glusterfs processes.
log-file LOGFILE	The path to the geo-replication log file.
log-level LOGFILELEVEL	The log level for geo-replication.
ssh-command COMMAND	The SSH command to connect to the remote machine (the default is SSH).
rsync-command COMMAND	The rsync command to use for synchronizing the files (the default is rsync).
use-tarssh true	The use-tarssh command allows tar over Secure Shell protocol. Use this option to handle workloads of files that have not undergone edits.
volume_id=UID	The command to delete the existing master UID for the intermediate/slave node.
timeout SECONDS	The timeout period in seconds.
sync-jobs N	The number of simultaneous files/directories that can be synchronized.
ignore-deletes	If this option is set to 1, a file deleted on the master will not trigger a delete operation on the slave. As a result, the slave will remain as a superset of the master and can be used to recover the master in the event of a crash and/or accidental delete.
checkpoint [LABEL\|now]	Sets a checkpoint with the given option LABEL. If the option is set as now, then the current time will be used as the label.

Stopping Geo-replication

You can use the gluster command to stop Gluster Geo-replication (syncing of data from Master to Slave) in your environment.

To stop Gluster Geo-replication

Use the following command to stop geo-replication between the hosts:

# gluster volume geo-replication  stop

For example:

# gluster volume geo-replication Volume1 example.com:/data/remote_dir stop
Stopping geo-replication session between Volume1 and
example.com:/data/remote_dir has been successful

Restoring Data from the Slave

You can restore data from the slave to the master volume, whenever the master volume becomes faulty for reasons like hardware failure.

The example in this section assumes that you are using the Master Volume (Volume1) with the following configuration:

machine1# gluster volume info
Type: Distribute
Status: Started
Number of Bricks: 2
Transport-type: tcp
Bricks:
Brick1: machine1:/export/dir16
Brick2: machine2:/export/dir16
Options Reconfigured:
geo-replication.indexing: on

The data is syncing from master volume (Volume1) to slave directory (example.com:/data/remote_dir). To view the status of this geo-replication session run the following command on Master:

# gluster volume geo-replication Volume1 root@example.com:/data/remote_dir status

Before Failure

Assume that the Master volume had 100 files and was mounted at /mnt/gluster on one of the client machines (client). Run the following command on Client machine to view the list of files:

client# ls /mnt/gluster | wc –l
100

The slave directory (example.com) will have same data as in the master volume and same can be viewed by running the following command on slave:

example.com# ls /data/remote_dir/ | wc –l
100

After Failure

If one of the bricks (machine2) fails, then the status of Geo-replication session is changed from "OK" to "Faulty". To view the status of this geo-replication session run the following command on Master:

    # gluster volume geo-replication Volume1 root@example.com:/data/remote_dir status

Machine2 is failed and now you can see discrepancy in number of files between master and slave. Few files will be missing from the master volume but they will be available only on slave as shown below.

Run the following command on Client:

    client # ls /mnt/gluster | wc –l
    52

Run the following command on slave (example.com):

    Example.com# # ls /data/remote_dir/ | wc –l
    100

To restore data from the slave machine

Use the following command to stop all Master's geo-replication sessions:

# gluster volume geo-replication  stop

For example:

machine1# gluster volume geo-replication Volume1
example.com:/data/remote_dir stop

Stopping geo-replication session between Volume1 &
example.com:/data/remote_dir has been successful

Note

Repeat # gluster volume geo-replication stopcommand on all active geo-replication sessions of master volume.

Use the following command to replace the faulty brick in the master:

# gluster volume replace-brick  start

For example:

machine1# gluster volume replace-brick Volume1 machine2:/export/dir16 machine3:/export/dir16 start
Replace-brick started successfully

Use the following command to commit the migration of data:

# gluster volume replace-brick  commit force

For example:

machine1# gluster volume replace-brick Volume1 machine2:/export/dir16 machine3:/export/dir16 commit force
Replace-brick commit successful

Use the following command to verify the migration of brick by viewing the volume info:

# gluster volume info

For example:

machine1# gluster volume info
Volume Name: Volume1
Type: Distribute
Status: Started
Number of Bricks: 2
Transport-type: tcp
Bricks:
Brick1: machine1:/export/dir16
Brick2: machine3:/export/dir16
Options Reconfigured:
geo-replication.indexing: on

Run rsync command manually to sync data from slave to master volume's client (mount point).

For example:
```
example.com# rsync -PavhS --xattrs --ignore-existing /data/remote_dir/ client:/mnt/gluster
```
Verify that the data is synced by using the following command:

On master volume, run the following command:
```
Client # ls | wc –l
100
```
On the Slave run the following command:
```
example.com# ls /data/remote_dir/ | wc –l
100
```
Now Master volume and Slave directory is synced.

Use the following command to restart geo-replication session from master to slave:

# gluster volume geo-replication  start

For example:

machine1# gluster volume geo-replication Volume1
example.com:/data/remote_dir start
Starting geo-replication session between Volume1 &
example.com:/data/remote_dir has been successful

Best Practices

Manually Setting Time

If you have to change the time on your bricks manually, then you must set uniform time on all bricks. Setting time backward corrupts the geo-replication index, so the recommended way to set the time manually is:

Stop geo-replication between the master and slave using the following command:
```
# gluster volume geo-replication stop
```
Stop the geo-replication indexing using the following command:
```
# gluster volume set  geo-replication.indexing of
```
Set uniform time on all bricks.
Use the following command to restart your geo-replication session:
```
# gluster volume geo-replication start
```

Running Geo-replication commands in one system

It is advisable to run the geo-replication commands in one of the bricks in the trusted storage pool. This is because, the log files for the geo-replication session would be stored in the *Server* where the Geo-replication start is initiated. Hence it would be easier to locate the log-files when required.

Isolation

Geo-replication slave operation is not sandboxed as of now and is ran as a privileged service. So for the security reason, it is advised to create a sandbox environment (dedicated machine / dedicated virtual machine / chroot/container type solution) by the administrator to run the geo-replication slave in it. Enhancement in this regard will be available in follow-up minor release.