In the following section, we provide answers to questions that are most frequently asked about Distributed Replicated Block Device (DRBD).

Questions

Questions and Answers

26.14.1.1: What is DRBD?

DRBD is an acronym for Distributed Replicated Block Device. DRBD is an open source Linux kernel block device which leverages synchronous replication to achieve a consistent view of data between two systems, typically an Active and Passive system. DRBD currently supports all the major flavors of Linux and comes bundled in several major Linux distributions. The DRBD project is maintained by LINBIT.

26.14.1.2: What are “Block Devices”?

Block devices are the type of device used to represent storage in the Linux Kernel. All physical disk devices present a “block device” interface. Additionally, virtual disk systems like LVM or DRBD present a “block device” interface. In this way, the file system or other software that might want to access a disk device can be used with any number of real or virtual devices without having to know anything about their underlying implementation details.

26.14.1.3: How is DRBD licensed?

DRBD is licensed under the GPL.

26.14.1.4: Where can I download DRBD?

Please see http://www.drbd.org/download.html

26.14.1.5: If I find a bug in DRBD, to whom do I submit the issue?

Bug reports should be submitted to the DRBD mailing list. Please see: http://lists.linbit.com/ .

26.14.1.6: Where can I get more technical and business information concerning MySQL and DRBD?

Please visit: http://mysql.com/drbd/

In the following section, we provide answers to questions that are most frequently asked about Linux Heartbeat.

Questions

Questions and Answers

26.14.2.1: What is Linux Heartbeat?

The Linux-HA project (http://www.linux-ha.org/) offers a high availability solution commonly referred to as Linux Heartbeat. Linux Heartbeat ships as part of several Linux distributions, as well as within several embedded high availability systems. This solution can also be used for other applications besides databases servers, such as mail servers, web servers, file servers, and DNS servers.

Linux Heartbeat implements a heartbeat-protocol. A heartbeat-protocol means that messages are sent at regular intervals between two or more nodes. If a message is not received from a node within a given interval, then it is assumed the node has failed and some type of failover or recovery action is required. Linux Heartbeat is typically configured to send these heartbeat messages over standard Ethernet interfaces, but it does also support other methods, such as serial-line links.

26.14.2.2: How is Linux Heartbeat licensed?

Linux Heartbeat is licensed under the GPL.

26.14.2.3: Where can I download Linux Heartbeat?

Please see http://linux-ha.org/download/index.html.

26.14.2.4: If I find a bug with Linux Heartbeat, to whom do I submit the issue?

Bug reports should be submitted to http://www.linux-ha.org/ClusterResourceManager/BugReports.

In the following section, we provide answers to questions that are most frequently asked about DRBD Architecture.

Questions

Questions and Answers

26.14.3.1: Is an Active/Active option available for MySQL with DRBD?

Currently, MySQL does not support Active/Active configurations using DRBD “out of the box”.

26.14.3.2: What MySQL storage engines are supported with DRBD?

All of the MySQL transactional storage engines are supported by DRBD, including InnoDB and Falcon. For archived or read-only data, MyISAM or Archive can also be used.

26.14.3.3: How long does a failover take?

Failover time is dependent on many things, some of which are configurable. After activating the passive host, MySQL will have to start and run a normal recovery process. If the InnoDB log files have been configured to a large size and there was heavy write traffic, this may take a reasonably long period of time. However, under normal circumstances, failover tends to take less than a minute.

26.14.3.4: How long does it take to resynchronize data after a failure?

Resynchronization time depends on how long the two machines are out of communication and how much data was written during that period of time. Resynchronization time is a function of data to be synced, network speed and disk speed. DRBD maintains a bitmap of changed blocks on the primary machine, so only those blocks that have changed will need to be transferred.

In the following section, we provide answers to questions that are most frequently asked about MySQL Replication Scale-out.

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Questions and Answers

26.14.4.1: What is the difference between MySQL Cluster and DRBD?

Both MySQL Cluster and DRBD replicate data synchronously. MySQL Cluster leverages a shared-nothing storage architecture in which the cluster can be architected beyond an Active/Passive configuration. DRBD operates at a much lower level within the “stack”, at the disk I/O level. For a comparison of various high availability features between these two options, please refer to Figure A.1, “Availability comparison”.

Figure A.1. Availability comparison

26.14.4.2: What is the difference between MySQL Replication and DRBD?

MySQL Replication replicates data asynchronously while DRBD replicates data synchronously. For a comparison of various high availability features between these two options, please refer to the high availability comparison grid, Figure A.1, “Availability comparison”.

26.14.4.3: How can I combine MySQL Replication scale-out with DRBD?

MySQL Replication is typically deployed in a Master to many Slaves configuration. In this configuration, having many Slaves provides read scalability. DRBD is used to provide high-availability for the Master MySQL Server in an Active/Passive configuration. This provides for automatic failover, safeguards against data loss, and automatically synchronizes the failed MySQL Master after a failover.

The most likely scenario in which MySQL Replication scale-out can be leveraged with DRBD is in the form of attaching replicated MySQL “read-slaves” off of the Active-Master MySQL Server, shown in Figure A.2, “Active-Master MySQL server”. Since DRBD replicates an entire block device, master information such as the binary logs are also replicated. In this way, all of the slaves can attach to the Virtual IP Address managed by Linux Heartbeat. In the event of a failure, the asynchronous nature of MySQL Replication allows the slaves to continue with the new Active machine as their master with no intervention needed.

Figure A.2. Active-Master MySQL server

In the following section, we provide answers to questions that are most frequently asked about DRBD and file systems.

Questions

Questions and Answers

26.14.5.1: Can XFS be used with DRBD?

Yes. XFS uses dynamic block size, thus DRBD 0.7 or later is needed.

In the following section, we provide answers to questions that are most frequently asked about DRBD and LVM.

Questions

Questions and Answers

26.14.6.1: Can I use DRBD on top of LVM?

Yes, DRBD supports on-line resizing. If you enlarge your logical volume that acts as a backing device for DRBD, you can enlarge DRBD itself too, and of course your file system if it supports resizing.

26.14.6.2: Can I use LVM on top of DRBD?

Yes, you can use DRBD as a Physical Volume (PV) for LVM. Depending on the default LVM configuration shipped with your distribution, you may need to add the /dev/drbd* device files to the filter option in your lvm.conf so LVM scans your DRBDs for PV signatures.

26.14.6.3: Can I use DRBD on top of LVM while at the same time running LVM on top of that DRBD?

This requires careful tuning of your LVM configuration to avoid duplicate PV scans, but yes, it is possible.

In the following section, we provide answers to questions that are most frequently asked about DRBD and virtualization.

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Questions and Answers

26.14.7.1: Can I use DRBD with OpenVZ?

See http://wiki.openvz.org/HA_cluster_with_DRBD_and_Heartbeat.

26.14.7.2: Can I use DRBD with Xen and/or KVM?

Yes. If you are looking for professional consultancy or expert commercial support for Xen- or KVM-based virtualization clusters with DRBD, contact LINBIT (http://www.linbit.com).

In the following section, we provide answers to questions that are most frequently asked about DRBD and security.

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Questions and Answers

26.14.8.1: Can I encrypt/compress the exchanged data?

Yes. But there is no option within DRBD to allow for this. You’ll need to leverage a VPN and the network layer should do the rest.

26.14.8.2: Does DRBD do mutual node authentication?

Yes, starting with DRBD 8 shared-secret mutual node authentication is supported.

In the following section, we provide answers to questions that are most frequently asked about DRBD and System Requirements.

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Questions and Answers

26.14.9.1: What other packages besides DRBD are required?

When using pre-built binary packages, none except a matching kernel, plus packages for glibc and your favorite shell. When compiling DRBD from source additional prerequisite packages may be required. They include but are not limited to:

Pre-built x86 and x86_64 packages for specific kernel versions are available with a support subscription from LINBIT. Please note that if the kernel is upgraded, DRBD must be as well.

26.14.9.2: How many machines are required to set up DRBD?

Two machines are required to achieve the minimum degree of high availability. Although at any one given point in time one will be primary and one will be secondary, it is better to consider the machines as part of a mirrored pair without a “natural” primary machine.

26.14.9.3: Does DRBD only run on Linux?

DRBD is a Linux Kernel Module, and can work with many popular Linux distributions. DRBD is currently not available for non-Linux operating systems.

In the following section, we provide answers to questions that are most frequently asked about DRBD and resources.

Questions

Questions and Answers

26.14.10.1: Does MySQL offer professional consulting to help with designing a DRBD system?

Yes. MySQL offers consulting for the design, installation, configuration, and monitoring of high availability DRBD. For more information concerning a High Availability Jumpstart, please see: http://www.mysql.com/consulting/packaged/scaleout.html.

26.14.10.2: Does MySQL offer support for DRBD and Linux Heartbeat from MySQL?

Yes. Support for DRBD is available with an add-on subscription to MySQL Enterprise called “DRBD for MySQL”. For more information about support options for DRBD see: http://mysql.com/products/enterprise/features.html.

For the list of supported Linux distributions, please see: http://www.mysql.com/support/supportedplatforms/enterprise.html.

NOTE: DRBD is only available on Linux. DRBD is not available on Windows, MacOS, Solaris, HPUX, AIX, FreeBSD, or other non-Linux platforms.

26.14.10.3: Are pre-built binaries or RPMs available?

Yes. “DRBD for MySQL” is an add-on subscription to MySQL Enterprise, which provides pre-built binaries for DRBD. For more information see: http://mysql.com/products/enterprise/features.html.

26.14.10.4: Does MySQL have documentation to help me with the installation and configuration of DRBD and Linux Heartbeat?

Coming soon.

26.14.10.5: Is there a dedicated discussion forum for MySQL High-Availability?

Yes, http://forums.mysql.com/list.php?144.

26.14.10.6: Where can I get more information about MySQL for DRBD?

For more information about MySQL for DRBD, including a technical white paper please see: DRBD for MySQL High Availability.