Storage Provisioning and Networking - Deploying and Managing a Cloud Infrastructure - page 262

Further, there are two kinds of replication: passive and active.
Passive Replication In the passive replication model, the primary storage takes the respon-
sibility of immediately serving the client and recording any updates. The system would then
perform lazy updates to the backup storage. If the primary node fails, one of the backups
would take over from the last consistent state of data.
Active Replication The active replication scheme depends on the nodes updating each
other almost instantly as the record update is being processed. This makes such a model a
completely decentralized one where the system does not have any hierarchical structure (i.e.,
no predefined primary or backups) but rather a collaborative system. The advantage of such
a system is that this could support models with high-frequency transaction requirements.
However, such systems are operated only for specialized purposes in controlled environments
such as a stock exchange or in mission-critical environments such as a launch sequence of a
space rocket.

Amazon Elastic Block Store (EBS)

The Elastic Block Store (EBS) ( http://aws.amazon.com/ebs/ ) is Amazon's block-level storage
technology for data volumes in use with Amazon Elastic Compute Cloud (EC2). These vol-
umes are networked together and are independent of the lifetime of an instance. EBS is multi-
purpose storage that provides a user with the capability to use it as a database or a file system,
or even as block-level storage for deploying their own virtual cloud. EBS provides high avail-
ability and high reliability and can be plugged dynamically to a running EC2 instance.

Features
Amazon EBS offers many features with regard to provisioning, usage and performance.
A few relevant ones are listed here:

Volumes behave like raw, unformatted block devices with a block device interface.
Users can use the block storage like a disk volume or create a file system on top of
the EBS.

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The latest (December 2013) EBS technology allows provisioning storage volume of
sizes ranging from 1 GB to 1 TB. These volumes can then be mounted as drives in EC2
instances. The biggest advantage of such a system is the ability to add or remove vol-
umes at will.

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Provisioned I/O operations per second (IOPS) ( http://aws.amazon.com/ebs/piops/ )
provides the capability to provision specific levels of I/O performance. This is particu-
larly important for high-transaction systems such as databases. The user can specify an
IOPS rate during provisioning of volumes.

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Snapshots provide backup capability as well as the ability to use a snapshot as a start-
ing point for new Amazon EBS volumes. Snapshots can also be used to instantiate
more volumes and can be copied across AWS (geographical) regions. This helps in
future expansion of data stores, migration, and disaster recovery.

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