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Setting up GlusterFS Volumes

A volume is a logical collection of bricks where each brick is an export directory on a server in the trusted storage pool. To create a new volume in your storage environment, specify the bricks that comprise the volume. After you have created a new volume, you must start it before attempting to mount it.

See Setting up Storage for how to set up bricks.

Volume Types

  • Volumes of the following types can be created in your storage environment:

    • Distributed - Distributed volumes distribute files across the bricks in the volume. You can use distributed volumes where the requirement is to scale storage and the redundancy is either not important or is provided by other hardware/software layers.

    • Replicated – Replicated volumes replicate files across bricks in the volume. You can use replicated volumes in environments where high-availability and high-reliability are critical.

    • Distributed Replicated - Distributed replicated volumes distribute files across replicated bricks in the volume. You can use distributed replicated volumes in environments where the requirement is to scale storage and high-reliability is critical. Distributed replicated volumes also offer improved read performance in most environments.

    • Dispersed - Dispersed volumes are based on erasure codes, providing space-efficient protection against disk or server failures. It stores an encoded fragment of the original file to each brick in a way that only a subset of the fragments is needed to recover the original file. The number of bricks that can be missing without losing access to data is configured by the administrator on volume creation time.

    • Distributed Dispersed - Distributed dispersed volumes distribute files across dispersed subvolumes. This has the same advantages of distribute replicate volumes, but using disperse to store the data into the bricks.

To create a new volume

  • Create a new volume :

    # gluster volume create [stripe | replica | disperse] [transport tcp | rdma | tcp,rdma]

    For example, to create a volume called test-volume consisting of server3:/exp3 and server4:/exp4:

    # gluster volume create test-volume server3:/exp3 server4:/exp4
    Creation of test-volume has been successful
    Please start the volume to access data.
    

Creating Distributed Volumes

In a distributed volume files are spread randomly across the bricks in the volume. Use distributed volumes where you need to scale storage and redundancy is either not important or is provided by other hardware/software layers.

Note: Disk/server failure in distributed volumes can result in a serious loss of data because directory contents are spread randomly across the bricks in the volume.

distributed_volume

To create a distributed volume

  1. Create a trusted storage pool.

  2. Create the distributed volume:

    # gluster volume create [transport tcp | rdma | tcp,rdma]

    For example, to create a distributed volume with four storage servers using tcp:

    # gluster volume create test-volume server1:/exp1 server2:/exp2 server3:/exp3 server4:/exp4
    Creation of test-volume has been successful
    Please start the volume to access data.
    

    (Optional) You can display the volume information:

    # gluster volume info
    Volume Name: test-volume
    Type: Distribute
    Status: Created
    Number of Bricks: 4
    Transport-type: tcp
    Bricks:
    Brick1: server1:/exp1
    Brick2: server2:/exp2
    Brick3: server3:/exp3
    Brick4: server4:/exp4
    

    For example, to create a distributed volume with four storage servers over InfiniBand:

    # gluster volume create test-volume transport rdma server1:/exp1 server2:/exp2 server3:/exp3 server4:/exp4
    Creation of test-volume has been successful
    Please start the volume to access data.
    

    If the transport type is not specified, tcp is used as the default. You can also set additional options if required, such as auth.allow or auth.reject.

    Note: Make sure you start your volumes before you try to mount them or else client operations after the mount will hang.

Creating Replicated Volumes

Replicated volumes create copies of files across multiple bricks in the volume. You can use replicated volumes in environments where high-availability and high-reliability are critical.

Note: The number of bricks should be equal to of the replica count for a replicated volume. To protect against server and disk failures, it is recommended that the bricks of the volume are from different servers.

replicated_volume

To create a replicated volume

  1. Create a trusted storage pool.

  2. Create the replicated volume:

    # gluster volume create [replica ] [transport tcp | rdma | tcp,rdma]

    For example, to create a replicated volume with two storage servers:

    # gluster volume create test-volume replica 2 transport tcp server1:/exp1 server2:/exp2
    Creation of test-volume has been successful
    Please start the volume to access data.
    

    If the transport type is not specified, tcp is used as the default. You can also set additional options if required, such as auth.allow or auth.reject.

    Note:

    • Make sure you start your volumes before you try to mount them or else client operations after the mount will hang.

    • GlusterFS will fail to create a replicate volume if more than one brick of a replica set is present on the same peer. For eg. a four node replicated volume where more than one brick of a replica set is present on the same peer.

      # gluster volume create <volname> replica 4 server1:/brick1 server1:/brick2 server2:/brick3 server4:/brick4
      volume create: <volname>: failed: Multiple bricks of a replicate volume are present on the same server. This setup is not optimal. Use 'force' at the end of the command if you want to override this behavior.
      

    Use the force option at the end of command if you still want to create the volume with this configuration.

Arbiter configuration for replica volumes

Arbiter volumes are replica 3 volumes where the 3rd brick acts as the arbiter brick. This configuration has mechanisms that prevent occurrence of split-brains.

It can be created with the following command:

`# gluster volume create  <VOLNAME>  replica 3 arbiter 1 host1:brick1 host2:brick2 host3:brick3`

More information about this configuration can be found at Features : afr-arbiter-volumes

Note that the arbiter configuration for replica 3 can be used to create distributed-replicate volumes as well.

Creating Distributed Replicated Volumes

Distributes files across replicated bricks in the volume. You can use distributed replicated volumes in environments where the requirement is to scale storage and high-reliability is critical. Distributed replicated volumes also offer improved read performance in most environments.

Note: The number of bricks should be a multiple of the replica count for a distributed replicated volume. Also, the order in which bricks are specified has a great effect on data protection. Each replica_count consecutive bricks in the list you give will form a replica set, with all replica sets combined into a volume-wide distribute set. To make sure that replica-set members are not placed on the same node, list the first brick on every server, then the second brick on every server in the same order, and so on.

distributed_replicated_volume

To create a distributed replicated volume

  1. Create a trusted storage pool.

  2. Create the distributed replicated volume:

    # gluster volume create [replica ] [transport tcp | rdma | tcp,rdma]

    For example, a four node distributed (replicated) volume with a two-way mirror:

    # gluster volume create test-volume replica 2 transport tcp server1:/exp1 server2:/exp2 server3:/exp3 server4:/exp4
    Creation of test-volume has been successful
    Please start the volume to access data.
    

    For example, to create a six node distributed (replicated) volume with a two-way mirror:

    # gluster volume create test-volume replica 2 transport tcp server1:/exp1 server2:/exp2 server3:/exp3 server4:/exp4 server5:/exp5 server6:/exp6
    Creation of test-volume has been successful
    Please start the volume to access data.
    

    If the transport type is not specified, tcp is used as the default. You can also set additional options if required, such as auth.allow or auth.reject.

    Note: - Make sure you start your volumes before you try to mount them or else client operations after the mount will hang.

    • GlusterFS will fail to create a distribute replicate volume if more than one brick of a replica set is present on the same peer. For eg. for a four node distribute (replicated) volume where more than one brick of a replica set is present on the same peer.
      # gluster volume create <volname> replica 2 server1:/brick1 server1:/brick2 server2:/brick3 server4:/brick4
      volume create: <volname>: failed: Multiple bricks of a replicate volume are present on the same server. This setup is not optimal. Use 'force' at the end of the command if you want to override this behavior.
      

    Use the force option at the end of command if you want to create the volume in this case.

Creating Dispersed Volumes

Dispersed volumes are based on erasure codes. It stripes the encoded data of files, with some redundancy added, across multiple bricks in the volume. You can use dispersed volumes to have a configurable level of reliability with minimum space waste.

Redundancy

Each dispersed volume has a redundancy value defined when the volume is created. This value determines how many bricks can be lost without interrupting the operation of the volume. It also determines the amount of usable space of the volume using this formula:

<Usable size> = <Brick size> * (#Bricks - Redundancy)

All bricks of a disperse set should have the same capacity, otherwise, when the smallest brick becomes full, no additional data will be allowed in the disperse set.

It's important to note that a configuration with 3 bricks and redundancy 1 will have less usable space (66.7% of the total physical space) than a configuration with 10 bricks and redundancy 1 (90%). However the first one will be safer than the second one (roughly the probability of failure of the second configuration if more than 4.5 times bigger than the first one).

For example, a dispersed volume composed of 6 bricks of 4TB and a redundancy of 2 will be completely operational even with two bricks inaccessible. However a third inaccessible brick will bring the volume down because it won't be possible to read or write to it. The usable space of the volume will be equal to 16TB.

The implementation of erasure codes in GlusterFS limits the redundancy to a value smaller than #Bricks / 2 (or equivalently, redundancy * 2 < #Bricks). Having a redundancy equal to half of the number of bricks would be almost equivalent to a replica-2 volume, and probably a replicated volume will perform better in this case.

Optimal volumes

One of the worst things erasure codes have in terms of performance is the RMW (Read-Modify-Write) cycle. Erasure codes operate in blocks of a certain size and it cannot work with smaller ones. This means that if a user issues a write of a portion of a file that doesn't fill a full block, it needs to read the remaining portion from the current contents of the file, merge them, compute the updated encoded block and, finally, writing the resulting data.

This adds latency, reducing performance when this happens. Some GlusterFS performance xlators can help to reduce or even eliminate this problem for some workloads, but it should be taken into account when using dispersed volumes for a specific use case.

Current implementation of dispersed volumes use blocks of a size that depends on the number of bricks and redundancy: 512 * (#Bricks - redundancy) bytes. This value is also known as the stripe size.

Using combinations of #Bricks/redundancy that give a power of two for the stripe size will make the disperse volume perform better in most workloads because it's more typical to write information in blocks that are multiple of two (for example databases, virtual machines and many applications).

These combinations are considered optimal.

For example, a configuration with 6 bricks and redundancy 2 will have a stripe size of 512 * (6 - 2) = 2048 bytes, so it's considered optimal. A configuration with 7 bricks and redundancy 2 would have a stripe size of 2560 bytes, needing a RMW cycle for many writes (of course this always depends on the use case).

To create a dispersed volume

  1. Create a trusted storage pool.

  2. Create the dispersed volume:

    # gluster volume create [disperse [<count>]] [redundancy <count>] [transport tcp | rdma | tcp,rdma]

    A dispersed volume can be created by specifying the number of bricks in a disperse set, by specifying the number of redundancy bricks, or both.

    If disperse is not specified, or the <count> is missing, the entire volume will be treated as a single disperse set composed by all bricks enumerated in the command line.

    If redundancy is not specified, it is computed automatically to be the optimal value. If this value does not exist, it's assumed to be '1' and a warning message is shown:

    # gluster volume create test-volume disperse 4 server{1..4}:/bricks/test-volume
    There isn't an optimal redundancy value for this configuration. Do you want to create the volume with redundancy 1 ? (y/n)
    

    In all cases where redundancy is automatically computed and it's not equal to '1', a warning message is displayed:

    # gluster volume create test-volume disperse 6 server{1..6}:/bricks/test-volume
    The optimal redundancy for this configuration is 2. Do you want to create the volume with this value ? (y/n)
    

    redundancy must be greater than 0, and the total number of bricks must be greater than 2 * redundancy. This means that a dispersed volume must have a minimum of 3 bricks.

    If the transport type is not specified, tcp is used as the default. You can also set additional options if required, like in the other volume types.

    Note:

    • Make sure you start your volumes before you try to mount them or else client operations after the mount will hang.

    • GlusterFS will fail to create a dispersed volume if more than one brick of a disperse set is present on the same peer.

      # gluster volume create <volname> disperse 3 server1:/brick{1..3}
      volume create: <volname>: failed: Multiple bricks of a replicate volume are present on the same server. This setup is not optimal.
      Do you still want to continue creating the volume? (y/n)
      

    Use the force option at the end of command if you want to create the volume in this case.

Creating Distributed Dispersed Volumes

Distributed dispersed volumes are the equivalent to distributed replicated volumes, but using dispersed subvolumes instead of replicated ones.

To create a distributed dispersed volume

  1. Create a trusted storage pool.

  2. Create the distributed dispersed volume:

    # gluster volume create disperse <count> [redundancy <count>] [transport tcp | rdma | tcp,rdma]

    To create a distributed dispersed volume, the disperse keyword and <count> is mandatory, and the number of bricks specified in the command line must must be a multiple of the disperse count.

    redundancy is exactly the same as in the dispersed volume.

    If the transport type is not specified, tcp is used as the default. You can also set additional options if required, like in the other volume types.

    Note:

    • Make sure you start your volumes before you try to mount them or else client operations after the mount will hang.

    • GlusterFS will fail to create a distributed dispersed volume if more than one brick of a disperse set is present on the same peer.

      # gluster volume create <volname> disperse 3 server1:/brick{1..6}
      volume create: <volname>: failed: Multiple bricks of a replicate volume are present on the same server. This setup is not optimal.
      Do you still want to continue creating the volume? (y/n)
      

    Use the force option at the end of command if you want to create the volume in this case.

Starting Volumes

You must start your volumes before you try to mount them.

To start a volume

  • Start a volume:

    # gluster volume start

    For example, to start test-volume:

    # gluster volume start test-volume
    Starting test-volume has been successful