In this mode, if one of the hosts has made no modifications at all during split brain, DRBD will simply recover gracefully and declare the split brain resolved. Note that this is a fairly unlikely scenario. Even if both hosts only mounted the file system on the DRBD block device even read-only , the device contents typically would be modified e.
Whether or not automatic split brain recovery is acceptable depends largely on the individual application. Consider the example of DRBD hosting a database. The discard modifications from host with fewer changes approach may be fine for a web application click-through database.
By contrast, it may be totally unacceptable to automatically discard any modifications made to a financial database, requiring manual recovery in any split brain event. When local block devices such as hard drives or RAID logical disks have write caching enabled, writes to these devices are considered completed as soon as they have reached the volatile cache.
Controller manufacturers typically refer to this as write-back mode, the opposite being write-through. If a power outage occurs on a controller in write-back mode, the last writes are never committed to the disk, potentially causing data loss. To counteract this, DRBD makes use of disk flushes.
DRBD uses disk flushes for write operations both to its replicated data set and to its meta data. In effect, DRBD circumvents the write cache in situations it deems necessary, as in activity log updates or enforcement of implicit write-after-write dependencies. This means additional reliability even in the face of power failure.
It is important to understand that DRBD can use disk flushes only when layered on top of backing devices that support them. The same is true for device-mapper devices LVM2, dm-raid, multipath.
Controllers with battery-backed write cache BBWC use a battery to back up their volatile storage. On such devices, when power is restored after an outage, the controller flushes all pending writes out to disk from the battery-backed cache, ensuring that all writes committed to the volatile cache are actually transferred to stable storage.
See Disabling backing device flushes for details. For more details, see e. Since 8. The effect is that e. Thus, it is left to upper layers to deal with such errors this may result in a file system being remounted read-only, for example. This strategy does not ensure service continuity, and is hence not recommended for most users.
Performance in this mode will be reduced, but the service continues without interruption, and can be moved to the peer node in a deliberate fashion at a convenient time. DRBD distinguishes between inconsistent and outdated data. Inconsistent data is data that cannot be expected to be accessible and useful in any manner.
The prime example for this is data on a node that is currently the target of an on-going synchronization. Data on such a node is part obsolete, part up to date, and impossible to identify as either.
Thus, for example, if the device holds a filesystem as is commonly the case , that filesystem would be unexpected to mount or even pass an automatic filesystem check.
Outdated data, by contrast, is data on a secondary node that is consistent, but no longer in sync with the primary node. This would occur in any interruption of the replication link, whether temporary or permanent. Data on an outdated, disconnected secondary node is expected to be clean, but it reflects a state of the peer node some time past.
In order to avoid services using outdated data, DRBD disallows promoting a resource that is in the outdated state. DRBD has interfaces that allow an external application to outdate a secondary node as soon as a network interruption occurs.
DRBD will then refuse to switch the node to the primary role, preventing applications from using the outdated data. A complete implementation of this functionality exists for the Pacemaker cluster management framework where it uses a communication channel separate from the DRBD replication link. However, the interfaces are generic and may be easily used by any other cluster management application.
Whenever an outdated resource has its replication link re-established, its outdated flag is automatically cleared. A background synchronization then follows. When using three-way replication, DRBD adds a third node to an existing 2-node cluster and replicates data to that node, where it can be used for backup and disaster recovery purposes.
Three-way replication works by adding another, stacked DRBD resource on top of the existing resource holding your production data, as seen in this illustration:. Three-way replication can be used permanently, where the third node is continuously updated with data from the production cluster.
Alternatively, it may also be employed on demand, where the production cluster is normally disconnected from the backup site, and site-to-site synchronization is performed on a regular basis, for example by running a nightly cron job.
In that event, the writing application has to wait until some of the data written runs off through a possibly small bandwidth network link. The average write bandwidth is limited by available bandwidth of the network link. Write bursts can only be handled gracefully if they fit into the limited socket output buffer. However, when the bandwidth of the network link is the limiting factor, the gain in shortening transmit time outweighs the compression and decompression overhead.
Compression and decompression were implemented with multi core SMP systems in mind, and can utilize multiple CPU cores.
Truck based replication, also known as disk shipping, is a means of preseeding a remote site with data to be replicated, by physically shipping storage media to the remote site. This is particularly suited for situations where. In such situations, without truck based replication, DRBD would require a very long initial device synchronization on the order of weeks, months, or years. Truck based replication allows to ship a data seed to the remote site, and so drastically reduces the initial synchronization time.
See Using truck based replication for details on this use case. A somewhat special use case for DRBD is the floating peers configuration.
In floating peer setups, DRBD peers are not tied to specific named hosts as in conventional configurations , but instead have the ability to float between several hosts.
Now, as your storage demands grow, you will encounter the need for additional servers. Rather than having to buy 3 more servers at the same time, you can rebalance your data across a single additional node. In the figure above you can see the before and after states: from 3 nodes with three 25TiB volumes each for a net 75TiB , to 4 nodes, with net TiB. DRBD 9 makes it possible to do an online, live migration of the data; please see Data rebalancing for the exact steps needed.
The basic idea is that the DRBD backend can consist of 3, 4, or more nodes depending on the policy of required redundancy ; but, as DRBD 9 can connect more nodes than that. DRBD works then as a storage access protocol in addition to storage replication. All write requests executed on a primary DRBD client gets shipped to all nodes equipped with storage.
Read requests are only shipped to one of the server nodes. The DRBD client will evenly distribute the read requests among all available server nodes. In order to avoid split brain or diverging data of replicas one has to configure fencing. It turns out that in real world deployments node fencing is not popular because often mistakes happen in planning or deploying it.
In the moment a data-set has 3 replicas one can rely on the quorum implementation within DRBD instead of cluster manager level fencing. Pacemaker gets informed about quorum or loss-of-quorum via the master score of the resource. In case the service terminates in the moment it is exposed to an IO-error the on quorum loss behavior is very elegant.
In case the service does not terminate upon IO-error the systems needs to be configured to reboot a primary node that looses quorum. The fundamental problem with two node clusters is that in the moment they lose connectivity we have two partitions and none of them has quorum, which results in the cluster halting the service.
This problem can be mitigated by adding a third, diskless node to the cluster which will then act as a quorum tiebreaker. See Using a diskless node as a tiebreaker for more information. Veritas Cluster Server or Veritas Infoscale Availability is a commercial alternative to the Pacemaker open source software. These packages are available via repositories e.
LINBIT signs most of its kernel module object files, the following table gives an overview when signing for distributions started:. It can be enrolled via:. A password can be chosen freely. It will be used when the key is actually enrolled to the MOK list after the required reboot. Before you can pull images, you have to log in to the registry:. After a successful login, you can pull images. To test your login and the registry, start by issuing the following command:. A number of distributions provide DRBD, including pre-built binary packages.
Support for these builds, if any, is being provided by the associated distribution vendor. Their release cycle may lag behind DRBD source releases. It comes bundled with the High Availability package selection.
DRBD can be installed using yum note that you will need a correct repository enabled for this to work :. Releases generated by git tags on github are snapshots of the git repository at the given time. You most likely do not want to use these.
They might lack things such as generated man pages, the configure script, and other generated files. If you want to build from a tarball, use the ones provided by us.
All our projects contain standard build scripts e. Maintaining specific information per distribution e. This chapter outlines typical administrative tasks encountered during day-to-day operations. It does not cover troubleshooting tasks, these are covered in detail in Troubleshooting and error recovery.
After you have installed DRBD, you must set aside a roughly identically sized storage area on both cluster nodes. This will become the lower-level device for your DRBD resource. You may use any type of block device found on your system for this purpose.
Typical examples include:. You may also use resource stacking , meaning you can use one DRBD device as a lower-level device for another. Some specific considerations apply to stacked resources; their configuration is covered in detail in Creating a stacked three-node setup.
It is not necessary for this storage area to be empty before you create a DRBD resource from it. It is recommended, though not strictly required, that you run your DRBD replication over a dedicated connection. At the time of this writing, the most reasonable choice for this is a direct, back-to-back, Gigabit Ethernet connection. When DRBD is run over switches, use of redundant components and the bonding driver in active-backup mode is recommended.
It is generally not recommended to run DRBD replication via routers, for reasons of fairly obvious performance drawbacks adversely affecting both throughput and latency. In terms of local firewall considerations, it is important to understand that DRBD by convention uses TCP ports from upwards, with every resource listening on a separate port. For proper DRBD functionality, it is required that these connections are allowed by your firewall configuration.
You may have to adjust your local security policy so it does not keep DRBD from functioning properly. If you want to provide for DRBD connection load-balancing or redundancy, you can easily do so at the Ethernet level again, using the bonding driver.
The local firewall configuration allows both inbound and outbound TCP connections between the hosts over these ports. Normally, this configuration file is just a skeleton with the following contents:. It is also possible to use drbd.
Such a configuration, however, quickly becomes cluttered and hard to manage, which is why the multiple-file approach is the preferred one. Regardless of which approach you employ, you should always make sure that drbd. The DRBD source tarball contains an example configuration file in the scripts subdirectory. This section describes only those few aspects of the configuration file which are absolutely necessary to understand in order to get DRBD up and running. For the purposes of this guide, we assume a minimal setup in line with the examples given in the previous sections:.
Resources are configured to use fully synchronous replication Protocol C unless explicitly specified otherwise. The configuration above implicitly creates one volume in the resource, numbered zero 0. For multiple volumes in one resource, modify the syntax as follows assuming that the same lower-level storage block devices are used on both nodes :.
They may contain volume themselves, these values have precedence over inherited values. For compatibility with older releases of DRBD it supports also drbd The old version to specify the device, was to give a string containing the name of the resulting device file. This section is allowed only once in the configuration.
In a single-file configuration, it should go to the very top of the configuration file. Of the few options available in this section, only one is of relevance to most users:. This can be disabled by setting usage-count no;. The default is usage-count ask; which will prompt you every time you upgrade DRBD. This section provides a shorthand method to define configuration settings inherited by every resource.
You may define any option you can also define on a per-resource basis. Including a common section is not strictly required, but strongly recommended if you are using more than one resource. Otherwise, the configuration quickly becomes convoluted by repeatedly-used options. For other synchronization protocols available, see Replication modes. Any DRBD resource you define must be named by specifying a resource name in the configuration.
Every resource configuration must also have at least two on host sub-sections, one for every cluster node. In addition, options with equal values on all hosts can be specified directly in the resource section.
Thus, we can further condense our example configuration as follows:. Currently the communication links in DRBD 9 must build a full mesh, i. For the simple case of two hosts drbdadm will insert the single network connection by itself, for ease of use and backwards compatibility. The net effect of this is a quadratic number of network connections over hosts. If you have got enough network cards in your servers, you can create direct cross-over links between server pairs.
A single four-port ethernet card allows to have a single management interface, and to connect 3 other servers, to get a full mesh for 4 cluster nodes. The examples below will still be using two servers only; please see Example configuration for four nodes for a four-node example. DRBD allows configuring multiple paths per connection, by introducing multiple path sections in a connection.
Please see the following example:. Obviously the two endpoint hostname need to be equal in all paths of a connection. The TCP transport uses one path at a time. In case the backing TCP connections get dropped, or show timeouts, the TCP transport implementation tries to establish a connection over the next path. It goes over all paths in a round-robin fashion until a connection gets established. The RDMA transport uses all paths of a connection concurrently and it balances the network traffic between the paths evenly.
The tcp transport can be configured with the net options: sndbuf-size , rcvbuf-size , connect-int , sock-check-timeo , ping-timeo , timeout. The rdma transport is a zero-copy-receive transport.
In case one of the descriptor kinds becomes depleted you should increase sndbuf-size or rcvbuf-size. After you have completed initial resource configuration as outlined in the previous sections, you can bring up your resource. This step must be completed only on initial device creation. Please note that the number of bitmap slots that are allocated in the meta-data depends on the number of hosts for this resource; per default the hosts in the resource configuration are counted.
This step associates the resource with its backing device or devices, in case of a multi-volume resource , sets replication parameters, and connects the resource to its peer:. By now, DRBD has successfully allocated both disk and network resources and is ready for operation.
What it does not know yet is which of your nodes should be used as the source of the initial device synchronization. If you are dealing with newly-initialized, empty disks, this choice is entirely arbitrary. If one of your nodes already has valuable data that you need to preserve, however, it is of crucial importance that you select that node as your synchronization source.
If you do initial device synchronization in the wrong direction, you will lose that data. Exercise caution. This step must be performed on only one node, only on initial resource configuration, and only on the node you selected as the synchronization source. To perform this step, issue this command:. After issuing this command, the initial full synchronization will commence. You will be able to monitor its progress via drbdadm status. It may take some time depending on the size of the device.
By now, your DRBD device is fully operational, even before the initial synchronization has completed albeit with slightly reduced performance. If you started with empty disks you may now already create a filesystem on the device, use it as a raw block device, mount it, and perform any other operation you would with an accessible block device. You will now probably want to continue with Working with DRBD , which describes common administrative tasks to perform on your resource. Running drbdadm status now shows the disks as UpToDate even tough the backing devices might be out of sync.
You can now create a file system on the disk and start using it. In order to preseed a remote node with data which is then to be kept synchronized, and to skip the initial full device synchronization, follow these steps. This assumes that your local node has a configured, but disconnected DRBD resource in the Primary role. That is to say, device configuration is completed, identical drbd. You may do so, for example, by removing a hot-swappable drive from a RAID-1 mirror.
You would, of course, replace it with a fresh drive, and rebuild the RAID set, to ensure continued redundancy. But the removed drive is a verbatim copy that can now be shipped off site. If your local block device supports snapshot copies such as when using DRBD on top of LVM , you may also create a bitwise copy of that snapshot using dd.
Add the copies to the remote node. This may again be a matter of plugging a physical disk, or grafting a bitwise copy of your shipped data onto existing storage on the remote node. Be sure to restore or copy not only your replicated data, but also the associated DRBD metadata. If you fail to do so, the disk shipping process is moot.
On the new node we need to fix the node ID in the meta data, and exchange the peer-node info for the two nodes. Please see the following lines as example for changing node id from 2 to 1 on a resource r0 volume 0.
You need to edit the first 4 lines to match your needs. V is the resource name with the volume number. After the two peers connect, they will not initiate a full device synchronization. Instead, the automatic synchronization that now commences only covers those blocks that changed since the invocation of drbdadm --clear-bitmap new-current-uuid. Even if there were no changes whatsoever since then, there may still be a brief synchronization period due to areas covered by the Activity Log being rolled back on the new Secondary.
This may be mitigated by the use of checksum-based synchronization. You may use this same procedure regardless of whether the resource is a regular DRBD resource, or a stacked resource.
For stacked resources, simply add the -S or --stacked option to drbdadm. As another example, if the four nodes have enough interfaces to provide a complete mesh via direct links [ 2 ] , you can specify the IP addresses of the interfaces:. Please note the numbering scheme used for the IP addresses and ports. Another resource could use the same IP addresses, but ports 71 xy , the next one 72 xy , and so on.
It updates the state of DRBD resources in real time. It was used extensively up to DRBD 8. The first line, prefixed with version: , shows the DRBD version used on your system. The second line contains information about this specific build. Every few lines in this example form a block that is repeated for every node used in this resource, with small format exceptions for the local node — see below for more details.
The first line in each block shows the node-id for the current resource; a host can have different node-id s in different resources. Furthermore the role see Resource roles is shown. The next important line begins with the volume specification; normally these are numbered starting by zero, but the configuration may specify other IDs as well.
This line shows the connection state in the replication item see Connection states for details and the remote disk state in disk see Disk states. For the local node the first line shows the resource name, home , in our example. As the first block always describes the local node, there is no Connection or address information.
The other four lines in this example form a block that is repeated for every DRBD device configured, prefixed by the device minor number. This is a low-level mechanism to get information out of DRBD, suitable for use in automated tools, like monitoring. In its simplest invocation, showing only the current status, the output looks like this but, when running on a terminal, will include colors :. If you are interested in only a single connection of a resource, specify the connection name, too:.
No network configuration available. The resource has not yet been connected, or has been administratively disconnected using drbdadm disconnect , or has dropped its connection due to failed authentication or split brain.
Temporary state following a timeout in the communication with the peer. Next state: Unconnected. The volume is not replicated over this connection, since the connection is not Connected.
Full synchronization, initiated by the administrator, is just starting. Partial synchronization is just starting. Synchronization is about to begin. The local node is the source of an ongoing synchronization, but synchronization is currently paused. This may be due to a dependency on the completion of another synchronization process, or due to synchronization having been manually interrupted by drbdadm pause-sync.
The local node is the target of an ongoing synchronization, but synchronization is currently paused. On-line device verification is currently running, with the local node being the source of verification. On-line device verification is currently running, with the local node being the target of verification.
Data replication was suspended, since the link can not cope with the load. This state is enabled by the configuration on-congestion option see Configuring congestion policies and suspended replication. Data replication was suspended by the peer, since the link can not cope with the load. This state is enabled by the configuration on-congestion option on the peer node see Configuring congestion policies and suspended replication.
The resource is currently in the primary role, and may be read from and written to. This role only occurs on one of the two nodes, unless dual-primary mode is enabled. The resource is currently in the secondary role. It normally receives updates from its peer unless running in disconnected mode , but may neither be read from nor written to.
This role may occur on one or both nodes. The local resource role never has this status. No local block device has been assigned to the DRBD driver.
Next state: Diskless. The data is inconsistent. This status occurs immediately upon creation of a new resource, on both nodes before the initial full sync. Also, this status is found in one node the synchronization target during synchronization. Consistent data of a node without connection. When the connection is established, it is decided whether the data is UpToDate or Outdated.
Shows the network family, the local address and port that is used to accept connections from the peer. The command drbdsetup status --verbose --statistics can be used to show performance statistics.
These are also available in drbdsetup events2 --statistics , although there will not be a changed event for every change. The statistics include the following counters and gauges:. Typical causes are. Application data that is being written by the peer.
That is, DRBD has sent it to the peer and is waiting for the acknowledgement that it has been written. In sectors bytes. Resync data that is being written by the peer. That is, DRBD is SyncSource , has sent data to the peer as part of a resync and is waiting for the acknowledgement that it has been written. Number of requests received from the peer, but that have not yet been acknowledged by DRBD on this node.
Whether the resynchronization is currently suspended or not. Possible values are no , user , peer , dependency. Comma separated. If, however, you need to enable resources manually for any reason, you may do so by issuing the command.
As always, you are able to review the pending drbdsetup invocations by running drbdadm with the -d dry-run option. A resource configured to allow dual-primary mode can be switched to the primary role on two nodes; this is e.
Upgrading DRBD is a fairly simple process. This section will cover the process of upgrading from 8. DRBD is wire protocol compatible over minor versions. DRBD is protocol compatible within a major number.
All version 8. DRBD 9. Deconfigure resources, unload DRBD 8. Convert DRBD meta-data to format v09 , perhaps changing the number of bitmaps in the same step. Due to the number of changes between the 8. Perform this repository update on both servers. Before you begin make sure your resources are in sync. Now that you know the resources are in sync, start by upgrading the secondary node. Both processes are covered below.
Once the upgrade is finished will now have the latest DRBD 9. See Changes to the configuration syntax for a full list of changes. This will output both a new global config followed by the new resource config files. Take this output and make changes accordingly. When determining the number of possible peers please take setups like the DRBD client into account. Upgrading the DRBD metadata is as easy as running one command, and acknowledging the two questions:.
Of course, you can pass all for the resource names, too; and if you feel really lucky, you can avoid the questions via a command line like this here, too. Yes, the order is important. Now, the only thing left to do is to get the DRBD devices up and running again — a simple drbdadm up all should do the trick. If you are using a cluster manager follow its documentation. If you are already running 9. Dual-primary mode allows a resource to assume the primary role simultaneously on more than one node.
Doing so is possible on either a permanent or a temporary basis. Dual-primary mode requires that the resource is configured to replicate synchronously protocol C. Because of this it is latency sensitive, and ill suited for WAN environments. Additionally, as both resources are always primary, any interruption in the network between nodes will result in a split-brain.
To enable dual-primary mode, set the allow-two-primaries option to yes in the net section of your resource configuration:. After that, do not forget to synchronize the configuration between nodes. To temporarily enable dual-primary mode for a resource normally running in a single-primary configuration, issue the following command:.
On-line device verification for resources is not enabled by default. Normally, you should be able to choose at least from sha1 , md5 , and crc32c. If you make this change to an existing resource, as always, synchronize your drbd. After you have enabled on-line verification, you will be able to initiate a verification run using the following command:. Any applications using the device at that time can continue to do so unimpeded, and you may also switch resource roles at will.
If out-of-sync blocks were detected during the verification run, you may resynchronize them using the following commands after verification has completed. The first command will cause the local differences to be overwritten by the remote version. The second command does it in the opposite direction. Before drbd A way to do that is disconnecting from a primary and ensuring that the primary changes at least one block while the peer is away.
Most users will want to automate on-line device verification. This can be easily accomplished. Normally, one tries to ensure that background synchronization which makes the data on the synchronization target temporarily inconsistent completes as quickly as possible.
However, it is also necessary to keep background synchronization from hogging all bandwidth otherwise available for foreground replication, which would be detrimental to application performance. Likewise, and for the same reasons, it does not make sense to set a synchronization rate that is higher than the bandwidth available on the replication network.
So, in DRBD 8. In this mode, DRBD uses an automated control loop algorithm to determine, and adjust, the synchronization rate. It may be wise to engage professional consultancy in order to optimally configure this DRBD feature. An example configuration which assumes a deployment in conjunction with DRBD Proxy is provided below:.
Here a good starting value for c-fill-target would be 3MB. Please see the drbd. In a few, very restricted situations [ 5 ] , it might make sense to just use some fixed synchronization rate. In this case, first of all you need to turn the dynamic sync rate controller off, by using c-plan-ahead 0;. Then, the maximum bandwidth a resource uses for background re-synchronization is determined by the resync-rate option for a resource. Note that the rate setting is given in bytes , not bits per second; the default unit is Kibibyte , so a value of would be interpreted as 4MiB.
Whether you manage to reach that synchronization rate depends on your network and storage speed, network latency which might be highly variable for shared links , and application IO which not might be able to do anything about.
Checksum-based synchronization is not enabled for resources by default. In an environment where the replication bandwidth is highly variable as would be typical in WAN replication setups , the replication link may occasionally become congested. It is usually wise to set both congestion-fill and congestion-extents together with the pull-ahead option. This is the default and recommended option. On the primary node, it is reported to the mounted file system.
On the secondary node, it is ignored because the secondary has no upper layer to report to. Please login to server OEL and do the same steps from above:. Below are the steps Notice in above that the DRBD service did start not yet.
Now do below steps on server OEL only. This is to initialize who will be the initial primary server. Now, we've come to the final part which is testing of the DRBD service to ensure it meets the objective.
Now both DRBD servers are in secondary state. Once you've successfully mounted the DRBD partition to your new folder, you'll notice that the file that you've created in server OEL before automatically existed! This feature is only available to subscribers.
Get your subscription here. Log in or Sign up. On this page 1. Preliminary note 2. Installation Phase 3. Red Hat: How do I create a promotable clone resource in a Pacemaker cluster? If my articles on GoLinuxCloud has helped you, kindly consider buying me a coffee as a token of appreciation. For any other feedbacks or questions you can either use the comments section or contact me form. I tried the first method, but the auto-promote mecanism seems not working properly. I work with a 2-nodes cluster.
Any ideas? The problem is most likely with pacemaker cluster. You can check this article to configure 2 node cluster. I would start by looking at the cluster configuration and logs. Save my name and email in this browser for the next time I comment. Notify me via e-mail if anyone answers my comment. Table of Contents. HINT: pcs resource master is deprecated and with Pacemaker 2. For more details check: How do I create a promotable clone resource in a Pacemaker cluster? Related Posts.
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