OpenStack Storage Design Mode

Table 5.1. OpenStack Storage

	Ephemeral storage	Block storage	Object storage
Used to…	Run operating system and scratch space	Add additional persistent storage to a virtual machine (VM)	Store data, including VM images
Accessed through…	A file system	A block device that can be partitioned, formatted and mounted (such as, /dev/vdc)	REST API
Accessible from…	Within a VM	Within a VM	Anywhere
Managed by…	OpenStack Compute (Nova)	OpenStack Block Storage (Cinder)	OpenStack Object Storage (Swift)
Persists until…	VM is terminated	Deleted by user	Deleted by user
Sizing determined by…	Administrator configures size settings, known as flavors	Specified by user in initial request	Amount of available physical storage
Example of typical usage…	10 GB first disk, 30GB second disk	1 TB disk	10s of TBs of dataset storage

If you only deploy the OpenStack Compute Service (nova), your users do not have access to any form of persistent storage by default. The disks associated with VMs are "ephemeral", meaning that (from the user's point of view) they effectively disappear when a virtual machine is terminated. You must identify what type of persistent storage you want to support for your users.

Today, OpenStack clouds explicitly support two types of persistent storage: object storage and block storage.

 Object Storage

With object storage, users access binary objects through a REST API. You may be familiar with Amazon S3, which is a well-known example of an object storage system. If your intended users need to archive or manage large datasets, you want to provide them with object storage. In addition, OpenStack can store your virtual machine (VM) images inside of an object storage system, as an alternative to storing the images on a file system.

Object-Storage vs Block-Storage Comparison in OpenStack Folsom version

Object Storage Capabilities

• OpenStack provides redundant, scalable object storage using clusters of standardized servers capable of storing petabytes of data
• Object Storage is not a traditional file system, but rather a distributed storage system for static data such as virtual machine images, photo storage, email storage, backups and archives. Having no central "brain" or master point of control provides greater scalability, redundancy and durability.
• Objects and files are written to multiple disk drives spread throughout servers in the data center, with the OpenStack software responsible for ensuring data replication and integrity across the cluster.
• Storage clusters scale horizontally simply by adding new servers. Should a server or hard drive fail, OpenStack replicates its content from other active nodes to new locations in the cluster. Because OpenStack uses software logic to ensure data replication and distribution across different devices, inexpensive commodity hard drives and servers can be used in lieu of more expensive equipment.

Block Storage Capabilities

• OpenStack provides persistent block level storage devices for use with OpenStack compute instances.
• The block storage system manages the creation / attaching / detaching of the block devices to servers. Block storage volumes are fully integrated into OpenStack Compute and the Dashboard allowing for cloud users to manage their own storage needs.
• In addition to using simple Linux server storage, it has unified storage support for numerous storage platforms including Ceph, NetApp, Nexenta, SolidFire, and Zadara.
• Block storage is appropriate for performance sensitive scenarios such as database storage, expandable file systems, or providing a server with access to raw block level storage.
• Snapshot management provides powerful functionality for backing up data stored on block storage volumes. Snapshots can be restored or used to create a new block storage volume.

而Rackspace所提供的Cloud Block Storage服務，就是利用OpenStack去打造的。

Reference:

[1] Amazon Elastic Block Storage, http://aws.amazon.com/ebs/
[2] Rackspace  Block Store, http://www.rackspace.com/cloud/block-storage/

Object Storage Device

An Object-based Storage Device (OSD) is a computer storage device, similar to disk storage but working at a higher level. Instead of providing a block-oriented interface that reads and writes fixed sized blocks of data, an OSD organizes data into flexible-sized data containers, called objects. Each object has both data (an uninterpreted sequence of bytes) and metadata (an extensible set of attributes describing the object).


Reference:
[1] http://en.wikipedia.org/wiki/Object_storage_device
[2] http://lab.howie.tw/2013/04/Object-based-storage-vs-bolck-device-storage.htm
[3] https://researchweb.watson.ibm.com/haifa/projects/storage/objectstore/papers/PositionOSD.pdf
[4] http://www.ascc.sinica.edu.tw/sysinfo/storage/conference/object-sun.pdf