# Replicated Storage with DRBD

Even if you’re serving up static websites, having to manually synchronize the contents of that website to all the machines in the cluster is not ideal. For dynamic websites, such as a wiki, its not even an option. Not everyone care afford network-attached storage but somehow the data needs to be kept in sync. Enter DRBD which can be thought of as network based RAID-1. See http://www.drbd.org/ for more details.

## Install the DRBD Packages

Since its inclusion in the upstream 2.6.33 kernel, everything needed to use DRBD ships with &DISTRO; &DISTRO_VERSION;. All you need to do is install it:

    [root@pcmk-1 ~]# yum install -y drbd-pacemaker drbd-udev
    Loaded plugins: presto, refresh-packagekit
    Setting up Install Process
    Resolving Dependencies
    --> Running transaction check
    ---> Package drbd-pacemaker.x86_64 0:8.3.7-2.fc13 set to be updated
    --> Processing Dependency: drbd-utils = 8.3.7-2.fc13 for package: drbd-pacemaker-8.3.7-2.fc13.x86_64
    --> Running transaction check
    ---> Package drbd-utils.x86_64 0:8.3.7-2.fc13 set to be updated
    --> Finished Dependency Resolution

    Dependencies Resolved

    =================================================================================
     Package                Arch           Version              Repository      Size
    =================================================================================
    Installing:
     drbd-pacemaker         x86_64         8.3.7-2.fc13         fedora          19 k
    Installing for dependencies:
     drbd-utils             x86_64         8.3.7-2.fc13         fedora         165 k

    Transaction Summary
    =================================================================================
    Install       2 Package(s)
    Upgrade       0 Package(s)

    Total download size: 184 k
    Installed size: 427 k
    Downloading Packages:
    Setting up and reading Presto delta metadata
    fedora/prestodelta                                        | 1.7 kB     00:00
    Processing delta metadata
    Package(s) data still to download: 184 k
    (1/2): drbd-pacemaker-8.3.7-2.fc13.x86_64.rpm             |  19 kB     00:01
    (2/2): drbd-utils-8.3.7-2.fc13.x86_64.rpm                 | 165 kB     00:02
    ---------------------------------------------------------------------------------
    Total                                             45 kB/s | 184 kB     00:04
    Running rpm_check_debug
    Running Transaction Test
    Transaction Test Succeeded
    Running Transaction
      Installing     : drbd-utils-8.3.7-2.fc13.x86_64                            1/2
      Installing     : drbd-pacemaker-8.3.7-2.fc13.x86_64                        2/2

    Installed:
      drbd-pacemaker.x86_64 0:8.3.7-2.fc13

    Dependency Installed:
      drbd-utils.x86_64 0:8.3.7-2.fc13

    Complete!
    [root@pcmk-1 ~]#

## Configure DRBD

Before we configure DRBD, we need to set aside some disk for it to use.

### Create A Partition for DRBD

If you have more than 1Gb free, feel free to use it. For this guide however, 1Gb is plenty of space for a single html file and sufficient for later holding the GFS2 metadata.

    [root@pcmk-1 ~]# lvcreate -n drbd-demo -L 1G VolGroup
      Logical volume "drbd-demo" created
    [root@pcmk-1 ~]# lvs
      LV        VG       Attr   LSize   Origin Snap%  Move Log Copy%  Convert
      drbd-demo VolGroup -wi-a- 1.00G
      lv_root   VolGroup -wi-ao   7.30G
      lv_swap   VolGroup -wi-ao 500.00M

Repeat this on the second node, be sure to use the same size partition.

    [root@pcmk-2 ~]# lvs
      LV      VG       Attr   LSize   Origin Snap%  Move Log Copy%  Convert
      lv_root VolGroup -wi-ao   7.30G
      lv_swap VolGroup -wi-ao 500.00M
    [root@pcmk-2 ~]# lvcreate -n drbd-demo -L 1G VolGroup
      Logical volume "drbd-demo" created
    [root@pcmk-2 ~]# lvs
      LV        VG       Attr   LSize   Origin Snap%  Move Log Copy%  Convert
      drbd-demo VolGroup -wi-a- 1.00G
      lv_root   VolGroup -wi-ao   7.30G
      lv_swap   VolGroup -wi-ao 500.00M

### Write the DRBD Config

There is no series of commands for build a DRBD configuration, so simply copy the configuration below to /etc/drbd.conf

Detailed information on the directives used in this configuration (and other alternatives) is available from http://www.drbd.org/users-guide/ch-configure.html

Be sure to use the names and addresses of your nodes if they differ from the ones used in this guide.

    global {
      usage-count yes;
    }
    common {
      protocol C;
    }
    resource wwwdata {
      meta-disk internal;
      device    /dev/drbd1;
      syncer {
        verify-alg sha1;
      }
      net {
        allow-two-primaries;
      }
      on pcmk-1 {
        disk      /dev/mapper/VolGroup-drbd--demo;
        address   192.168.122.101:7789;
      }
      on
    pcmk-2 {
        disk      /dev/mapper/VolGroup-drbd--demo;
        address   192.168.122.102:7789;
      }
    }

TODO: Explain the reason for the allow-two-primaries option

### Initialize and Load DRBD

With the configuration in place, we can now perform the DRBD initialization

    [root@pcmk-1 ~]# drbdadm create-md wwwdata
    md_offset 12578816
    al_offset 12546048
    bm_offset 12541952

    Found some data
     ==> This might destroy existing data! <==

    Do you want to proceed?
    [need to type 'yes' to confirm] yes

    Writing meta data...
    initializing activity log
    NOT initialized bitmap
    New drbd meta data block successfully created.
    success

Now load the DRBD kernel module and confirm that everything is sane

    [root@pcmk-1 ~]# modprobe drbd
    [root@pcmk-1 ~]# drbdadm up wwwdata
    [root@pcmk-1 ~]# cat /proc/drbd
    version: 8.3.6 (api:88/proto:86-90)
    GIT-hash: f3606c47cc6fcf6b3f086e425cb34af8b7a81bbf build by root@pcmk-1, 2009-12-08 11:22:57

     1: cs:WFConnection ro:Secondary/Unknown ds:Inconsistent/DUnknown C r----
        ns:0 nr:0 dw:0 dr:0 al:0 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:b oos:12248
    [root@pcmk-1 ~]#

    Repeat on the second node
    drbdadm --force create-md wwwdata
    modprobe drbd
    drbdadm up wwwdata
    cat /proc/drbd
    [root@pcmk-2 ~]# drbdadm --force create-md wwwdata
    Writing meta data...
    initializing activity log
    NOT initialized bitmap
    New drbd meta data block successfully created.
    success
    [root@pcmk-2 ~]# modprobe drbd
    WARNING: Deprecated config file /etc/modprobe.conf, all config files belong into /etc/modprobe.d/.
    [root@pcmk-2 ~]# drbdadm up wwwdata
    [root@pcmk-2 ~]# cat /proc/drbd
    version: 8.3.6 (api:88/proto:86-90)
    GIT-hash: f3606c47cc6fcf6b3f086e425cb34af8b7a81bbf build by root@pcmk-1, 2009-12-08 11:22:57

     1: cs:Connected ro:Secondary/Secondary ds:Inconsistent/Inconsistent C r----
        ns:0 nr:0 dw:0 dr:0 al:0 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:b oos:12248

Now we need to tell DRBD which set of data to use. Since both sides contain garbage, we can run the following on pcmk-1:

    [root@pcmk-1 ~]# drbdadm -- --overwrite-data-of-peer primary wwwdata
    [root@pcmk-1 ~]# cat /proc/drbd
    version: 8.3.6 (api:88/proto:86-90)
    GIT-hash: f3606c47cc6fcf6b3f086e425cb34af8b7a81bbf build by root@pcmk-1, 2009-12-08 11:22:57
     1: cs:SyncSource ro:Primary/Secondary ds:UpToDate/Inconsistent C r----
        ns:2184 nr:0 dw:0 dr:2472 al:0 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:b oos:10064
            [=====>..............] sync'ed: 33.4% (10064/12248)K
            finish: 0:00:37 speed: 240 (240) K/sec
    [root@pcmk-1 ~]# cat /proc/drbd
    version: 8.3.6 (api:88/proto:86-90)
    GIT-hash: f3606c47cc6fcf6b3f086e425cb34af8b7a81bbf build by root@pcmk-1, 2009-12-08 11:22:57
     1: cs:Connected ro:Primary/Secondary ds:UpToDate/UpToDate C r----
        ns:12248 nr:0 dw:0 dr:12536 al:0 bm:1 lo:0 pe:0 ua:0 ap:0 ep:1 wo:b oos:0

pcmk-1 is now in the Primary state which allows it to be written to. Which means its a good point at which to create a filesystem and populate it with some data to serve up via our WebSite resource.

### Populate DRBD with Data

    [root@pcmk-1 ~]# mkfs.ext4 /dev/drbd1
    mke2fs 1.41.4 (27-Jan-2009)
    Filesystem label=
    OS type: Linux
    Block size=1024 (log=0)
    Fragment size=1024 (log=0)
    3072 inodes, 12248 blocks
    612 blocks (5.00%) reserved for the super user
    First data block=1
    Maximum filesystem blocks=12582912
    2 block groups
    8192 blocks per group, 8192 fragments per group
    1536 inodes per group
    Superblock backups stored on blocks:
            8193

    Writing inode tables: done
    Creating journal (1024 blocks): done
    Writing superblocks and filesystem accounting information: done

    This filesystem will be automatically checked every 26 mounts or
    180 days, whichever comes first.  Use tune2fs -c or -i to override.

    Now mount the newly created filesystem so we can create our index file
    mount /dev/drbd1 /mnt/
    cat <<-END >/mnt/index.html
    <html>
    <body>My Test Site - drbd</body>
    </html>
    END
    umount /dev/drbd1
    [root@pcmk-1 ~]# mount /dev/drbd1 /mnt/
    [root@pcmk-1 ~]# cat <<-END >/mnt/index.html
    > <html>
    > <body>My Test Site - drbd</body>
    > </html>
    > END
    [root@pcmk-1 ~]# umount /dev/drbd1

## Configure the Cluster for DRBD

One handy feature of the crm shell is that you can use it in interactive mode to make several changes atomically.

First we launch the shell. The prompt will change to indicate you’re in interactive mode.

    [root@pcmk-1 ~]# crm
    cib crm(live)#

Next we must create a working copy or the current configuration. This is where all our changes will go. The cluster will not see any of them until we say its ok. Notice again how the prompt changes, this time to indicate that we’re no longer looking at the live cluster.

    cib crm(live)# cib new drbd
    INFO: drbd shadow CIB created
    crm(drbd)#

Now we can create our DRBD clone and display the revised configuration.

    crm(drbd)# configure primitive WebData ocf:linbit:drbd params drbd_resource=wwwdata \
            op monitor interval=60s
    crm(drbd)# configure ms WebDataClone WebData meta master-max=1 master-node-max=1 \
            clone-max=2 clone-node-max=1 notify=true
    crm(drbd)# configure show
    node pcmk-1
    node pcmk-2
    primitive WebData ocf:linbit:drbd \
     params drbd_resource="wwwdata" \
     op monitor interval="60s"
    primitive WebSite ocf:heartbeat:apache \
            params configfile="/etc/httpd/conf/httpd.conf" \
            op monitor interval="1min"
    primitive ClusterIP ocf:heartbeat:IPaddr2 \
            params ip="192.168.122.101" cidr_netmask="32" \
            op monitor interval="30s"
    ms WebDataClone WebData \
     meta master-max="1" master-node-max="1" clone-max="2" clone-node-max="1" notify="true"
    location prefer-pcmk-1 WebSite 50: pcmk-1
    colocation website-with-ip inf: WebSite ClusterIP
    order apache-after-ip inf: ClusterIP WebSite
    property $id="cib-bootstrap-options" \
            dc-version="1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f" \
            cluster-infrastructure="openais" \
            expected-quorum-votes=”2” \
            stonith-enabled="false" \
            no-quorum-policy="ignore"
    rsc_defaults $id="rsc-options" \
            resource-stickiness=”100”

Once we’re happy with the changes, we can tell the cluster to start using them and use crm_mon to check everything is functioning.

    crm(drbd)# cib commit drbd
    INFO: commited 'drbd' shadow CIB to the cluster
    crm(drbd)# quit
    bye
    [root@pcmk-1 ~]# crm_mon
    ============
    Last updated: Tue Sep  1 09:37:13 2009
    Stack: openais
    Current DC: pcmk-1 - partition with quorum
    Version: 1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f
    2 Nodes configured, 2 expected votes
    3 Resources configured.
    ============

    Online: [ pcmk-1 pcmk-2 ]

    ClusterIP        (ocf::heartbeat:IPaddr):        Started pcmk-1
    WebSite (ocf::heartbeat:apache):        Started pcmk-1
    Master/Slave Set: WebDataClone
     Masters: [ pcmk-2 ]
     Slaves: [ pcmk-1 ]

Include details on adding a second DRBD resource

Now that DRBD is functioning we can configure a Filesystem resource to use it. In addition to the filesystem’s definition, we also need to tell the cluster where it can be located (only on the DRBD Primary) and when it is allowed to start (after the Primary was promoted).

Once again we’ll use the shell’s interactive mode

    [root@pcmk-1 ~]# crm
    crm(live)# cib new fs
    INFO: fs shadow CIB created
    crm(fs)# configure primitive WebFS ocf:heartbeat:Filesystem \
            params device="/dev/drbd/by-res/wwwdata" directory="/var/www/html" fstype="ext4"
    crm(fs)# configure colocation fs_on_drbd inf: WebFS WebDataClone:Master
    crm(fs)# configure order WebFS-after-WebData inf: WebDataClone:promote WebFS:start

We also need to tell the cluster that Apache needs to run on the same machine as the filesystem and that it must be active before Apache can start.

    crm(fs)# configure colocation WebSite-with-WebFS inf: WebSite WebFS
    crm(fs)# configure order WebSite-after-WebFS inf: WebFS WebSite

Time to review the updated configuration:

    [root@pcmk-1 ~]# crm configure show
    node pcmk-1
    node pcmk-2
    primitive WebData ocf:linbit:drbd \
            params drbd_resource="wwwdata" \
            op monitor interval="60s"
    primitive WebFS ocf:heartbeat:Filesystem \
            params device="/dev/drbd/by-res/wwwdata" directory="/var/www/html" fstype="ext4"
    primitive WebSite ocf:heartbeat:apache \
            params configfile="/etc/httpd/conf/httpd.conf" \
            op monitor interval="1min"
    primitive ClusterIP ocf:heartbeat:IPaddr2 \
            params ip="192.168.122.101" cidr_netmask="32" \
            op monitor interval="30s"
    ms WebDataClone WebData \
            meta master-max="1" master-node-max="1" clone-max="2" clone-node-max="1" notify="true"
    location prefer-pcmk-1 WebSite 50: pcmk-1
    colocation WebSite-with-WebFS inf: WebSite WebFS
    colocation fs_on_drbd inf: WebFS WebDataClone:Master
    colocation website-with-ip inf: WebSite ClusterIP
    order WebFS-after-WebData inf: WebDataClone:promote WebFS:start
    order WebSite-after-WebFS inf: WebFS WebSite
    order apache-after-ip inf: ClusterIP WebSite
    property $id="cib-bootstrap-options" \
            dc-version="1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f" \
            cluster-infrastructure="openais" \
            expected-quorum-votes=”2” \
            stonith-enabled="false" \
            no-quorum-policy="ignore"
    rsc_defaults $id="rsc-options" \
            resource-stickiness=”100”

After reviewing the new configuration, we again upload it and watch the cluster put it into effect.

    crm(fs)# cib commit fs
    INFO: commited 'fs' shadow CIB to the cluster
    crm(fs)# quit
    bye
    [root@pcmk-1 ~]# crm_mon
    ============
    Last updated: Tue Sep  1 10:08:44 2009
    Stack: openais
    Current DC: pcmk-1 - partition with quorum
    Version: 1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f
    2 Nodes configured, 2 expected votes
    4 Resources configured.
    ============

    Online: [ pcmk-1 pcmk-2 ]

    ClusterIP        (ocf::heartbeat:IPaddr):        Started pcmk-1
    WebSite (ocf::heartbeat:apache): Started pcmk-1
    Master/Slave Set: WebDataClone
            Masters: [ pcmk-1 ]
            Slaves: [ pcmk-2 ]
    WebFS (ocf::heartbeat:Filesystem): Started pcmk-1

### Testing Migration

We could shut down the active node again, but another way to safely simulate recovery is to put the node into what is called “standby mode”. Nodes in this state tell the cluster that they are not allowed to run resources. Any resources found active there will be moved elsewhere. This feature can be particularly useful when updating the resources’ packages.

Put the local node into standby mode and observe the cluster move all the resources to the other node. Note also that the node’s status will change to indicate that it can no longer host resources.

    [root@pcmk-1 ~]# crm node standby
    [root@pcmk-1 ~]# crm_mon
    ============
    Last updated: Tue Sep  1 10:09:57 2009
    Stack: openais
    Current DC: pcmk-1 - partition with quorum
    Version: 1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f
    2 Nodes configured, 2 expected votes
    4 Resources configured.
    ============

    Node pcmk-1: standby
    Online: [ pcmk-2 ]

    ClusterIP        (ocf::heartbeat:IPaddr):        Started pcmk-2
    WebSite (ocf::heartbeat:apache):        Started pcmk-2
    Master/Slave Set: WebDataClone
            Masters: [ pcmk-2 ]
            Stopped: [ WebData:1 ]
    WebFS   (ocf::heartbeat:Filesystem):    Started pcmk-2

Once we’ve done everything we needed to on pcmk-1 (in this case nothing, we just wanted to see the resources move), we can allow the node to be a full cluster member again.

    [root@pcmk-1 ~]# crm node online
    [root@pcmk-1 ~]# crm_mon
    ============
    Last updated: Tue Sep  1 10:13:25 2009
    Stack: openais
    Current DC: pcmk-1 - partition with quorum
    Version: 1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f
    2 Nodes configured, 2 expected votes
    4 Resources configured.
    ============

    Online: [ pcmk-1 pcmk-2 ]

    ClusterIP        (ocf::heartbeat:IPaddr):        Started pcmk-2
    WebSite (ocf::heartbeat:apache):        Started pcmk-2
    Master/Slave Set: WebDataClone
            Masters: [ pcmk-2 ]
            Slaves: [ pcmk-1 ]
    WebFS   (ocf::heartbeat:Filesystem):    Started pcmk-2

Notice that our resource stickiness settings prevent the services from migrating back to pcmk-1.