It is an important building block of the DRBD. It reads and writes data to optional local backing devices. In synchronous mode it will signal completion of a write request after it receives completion events from the local backing storage device and from the peer s. Please note that the data path is very efficient.
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DRBD is a great thing to increase the availability of our data. It is a Linux-based opensource software component that facilitates the replacement of shared storage systems by networked mirroring. In short, we can say this as "Network based Raid 1 mirroring for the data". So it can be used to mirror the filesystems, VM Images and other block devices stuffs across the network.
One of the servers is commonly defined as the primary and the other one is secondary server. The DRBD software provides synchronization between the primary and secondary servers for user-based Read and Write operations as well as other synchronization operations.
The secondary node is promoted to primary if the clustering solution detects that the primary node is down. Write operations starts at primary node and are performed to the local storage and secondary storage simultaneously.
DRBD supports two modes for Write operations called fully synchronous and asynchronous. Protocol A - Asynchronous replication protocol. Local write operations on the primary node are considered completed as soon as the local disk write has finished, and the replication packet has been placed in the local TCP send buffer.
In the event of forced fail-over, data loss may occur. Protocol B - Memory synchronous semi-synchronous replication protocol. Local write operations on the primary node are considered completed as soon as the local disk write has occurred, and the replication packet has reached the peer node.
Normally, no writes are lost in case of forced fail-over. Protocol C - Synchronous replication protocol. Local write operations on the primary node are considered completed only after both the local and the remote disk write have been confirmed.
As a result, loss of a single node is guaranteed not to lead to any data loss. The kernel module implements a driver for a virtual block device which is replicated between a local disk and a remote disk across the network.
As a virtual disk, DRBD provides a flexible model that a variety of applications can use from file systems to other applications that can rely on a raw disk, such as a database. The DRBD module implements an interface not only to the underlying block driver as defined by the disk configuration item in drbd.
How to Install DRBD on CentOS Linux
Use the legacy Heartbeat v1 style drbddisk resource agent to move the Primary role. In this case, you must not let init load and configure DRBD, because this resource agent does that itself. This document describes the second option. The first resource agent to make full use of this functionality is the DRBD one. Complete status reflected in monitoring tools. Prerequisites DRBD must not be started by init.
How to install and setup DRBD on CentOS
What is DRBD? It is used to replicate the storage devices from one node to the other node over a network. It can provide assistance to deal with Disaster Recovery and Failovers. DRBD can be understood as a high availability for hardware and can be viewed as a replacement of network shared storages. How DRBD works? Those systems are defined as Primary node and Secondary node can switch Primary and Secndary nodes.
DRBD HowTo 1.0
What is DRBD, How DRBD works - DRBD Tutorial for Beginners