Storage architecture

Storage architecture

There are many different storage architectures available when designing an OpenStack cloud. The convergence of orchestration and automation within the OpenStack platform enables rapid storage provisioning without the hassle of the traditional manual processes like volume creation and attachment.

However, before choosing a storage architecture, a few generic questions should be answered:

  • Will the storage architecture scale linearly as the cloud grows and what are its limits?
  • What is the desired attachment method: NFS, iSCSI, FC, or other?
  • Is the storage proven with the OpenStack platform?
  • What is the level of support provided by the vendor within the community?
  • What OpenStack features and enhancements does the cinder driver enable?
  • Does it include tools to help troubleshoot and resolve performance issues?
  • Is it interoperable with all of the projects you are planning on using in your cloud?

Choosing storage back ends

Users will indicate different needs for their cloud architecture. Some may need fast access to many objects that do not change often, or want to set a time-to-live (TTL) value on a file. Others may access only storage that is mounted with the file system itself, but want it to be replicated instantly when starting a new instance. For other systems, ephemeral storage is the preferred choice. When you select storage back ends, consider the following questions from user’s perspective:

First and foremost:

  • Do I need block storage?
  • Do I need object storage?
  • Do I need file-based storage?

Next answer the following:

  • Do I need to support live migration?
  • Should my persistent storage drives be contained in my compute nodes, or should I use external storage?
  • What type of performance do I need in regards to IOPS? Total IOPS and IOPS per instance? Do I have applications with IOPS SLAs?
  • Are my storage needs mostly read, or write, or mixed?
  • Which storage choices result in the best cost-performance scenario I am aiming for?
  • How do I manage the storage operationally?
  • How redundant and distributed is the storage? What happens if a storage node fails? To what extent can it mitigate my data-loss disaster scenarios?
  • What is my company currently using and can I use it with OpenStack?
  • Do I need more than one storage choice? Do I need tiered performance storage?

While this is not a definitive list of all the questions possible, the list above will hopefully help narrow the list of possible storage choices down.

A wide variety of use case requirements dictate the nature of the storage back end. Examples of such requirements are as follows:

  • Public, private, or a hybrid cloud (performance profiles, shared storage, replication options)
  • Storage-intensive use cases like HPC and Big Data clouds
  • Web-scale or development clouds where storage is typically ephemeral in nature

Data security recommendations:

  • We recommend that data be encrypted both in transit and at-rest. To this end, carefully select disks, appliances, and software. Do not assume these features are included with all storage solutions.
  • Determine the security policy of your organization and understand the data sovereignty of your cloud geography and plan accordingly.

If you plan to use live migration, we highly recommend a shared storage configuration. This allows the operating system and application volumes for instances to reside outside of the compute nodes and adds significant performance increases when live migrating.

To deploy your storage by using only commodity hardware, you can use a number of open-source packages, as described in Persistent file-based storage support.

Persistent file-based storage support
  Object Block File-level
Swift ../_images/Check_mark_23x20_02.png    
LVM   ../_images/Check_mark_23x20_02.png  
Ceph ../_images/Check_mark_23x20_02.png ../_images/Check_mark_23x20_02.png Experimental
Gluster ../_images/Check_mark_23x20_02.png ../_images/Check_mark_23x20_02.png ../_images/Check_mark_23x20_02.png
NFS   ../_images/Check_mark_23x20_02.png ../_images/Check_mark_23x20_02.png
ZFS   ../_images/Check_mark_23x20_02.png  
Sheepdog ../_images/Check_mark_23x20_02.png ../_images/Check_mark_23x20_02.png  

This list of open source file-level shared storage solutions is not exhaustive. Your organization may already have deployed a file-level shared storage solution that you can use.


Storage driver support

In addition to the open source technologies, there are a number of proprietary solutions that are officially supported by OpenStack Block Storage. You can find a matrix of the functionality provided by all of the supported Block Storage drivers on the CinderSupportMatrix wiki.

Also, you need to decide whether you want to support object storage in your cloud. The two common use cases for providing object storage in a compute cloud are to provide:

  • Users with a persistent storage mechanism for objects like images and video.
  • A scalable, reliable data store for OpenStack virtual machine images.
  • An API driven S3 compatible object store for application use.

Selecting storage hardware

Storage hardware architecture is determined by selecting specific storage architecture. Determine the selection of storage architecture by evaluating possible solutions against the critical factors, the user requirements, technical considerations, and operational considerations. Consider the following factors when selecting storage hardware:

Storage can be a significant portion of the overall system cost. For an organization that is concerned with vendor support, a commercial storage solution is advisable, although it comes with a higher price tag. If initial capital expenditure requires minimization, designing a system based on commodity hardware would apply. The trade-off is potentially higher support costs and a greater risk of incompatibility and interoperability issues.
Performance of block based storage is typically measured in the maximum read and write operations to non-contiguous storage locations per second. This measurement typically applies to SAN, hard drives, and solid state drives. While IOPS can be broadly measured and is not an official benchmark, many vectors like to be used by vendors to communicate performance levels. Since there are no real standards for measuring IOPS, vendor test results may vary, sometimes wildly. However, along with transfer rate which measures the speed that data can be transferred to contiguous storage locations, IOPS can be used in a performance evaluation. Typically, transfer rate is represented by a bytes per second calculation but IOPS is measured by an integer.
To calculate IOPS for a single drive you could use:
IOPS = 1 / (AverageLatency + AverageSeekTime) For example: Average Latency for Single Disk = 2.99ms or .00299 seconds Average Seek Time for Single Disk = 4.7ms or .0047 seconds IOPS = 1/(.00299 + .0047) IOPS = 130
To calculate maximum IOPS for a disk array:
Maximum Read IOPS: In order to accurately calculate maximum read IOPS for a disk array, multiply the IOPS for each disk by the maximum read or write IOPS per disk. maxReadIOPS = nDisks * diskMaxIOPS For example, 15 10K Spinning Disks would be measured the following way: maxReadIOPS = 15 * 130 maxReadIOPS = 1950
Maximum write IOPS per array:

Determining the maximum write IOPS is a little different because most administrators configure disk replication using RAID and since the RAID controller requires IOPS itself, there is a write penalty. The severity of the write penalty is determined by the type of RAID used.

Raid Type Penalty
1 2
5 4
10 2


Raid 5 has the worst penalty (has the most cross disk writes.) Therefore, when using the above examples, a 15 disk array using RAID 5 is capable of 1950 read IOPS however, we need to add the penalty when determining the write IOPS:

maxWriteIOPS = 1950 / 4
maxWriteIOPS = 487.5

A RAID 5 array only has 25% of the write IOPS of the read IOPS while a RAID 1 array in this case would produce a maximum of 975 IOPS.

What about SSD? DRAM SSD?

In an HDD, data transfer is sequential. The actual read/write head “seeks” a point in the hard drive to execute the operation. Seek time is significant. Transfer rate can also be influenced by file system fragmentation and the layout. Finally, the mechanical nature of hard disks also has certain performance limitations.

In an SSD, data transfer is not sequential; it is random so it is faster. There is consistent read performance because the physical location of data is irrelevant because SSDs have no read/write heads and thus no delays due to head motion (seeking).


Some basic benchmarks for small read/writes:

  • HDDs: Small reads – 175 IOPs, Small writes – 280 IOPs
  • Flash SSDs: Small reads – 1075 IOPs (6x), Small writes – 21 IOPs (0.1x)
  • DRAM SSDs: Small reads – 4091 IOPs (23x), Small writes – 4184 IOPs (14x)
Scalability, along with expandability, is a major consideration in a general purpose OpenStack cloud. It might be difficult to predict the final intended size of the implementation as there are no established usage patterns for a general purpose cloud. It might become necessary to expand the initial deployment in order to accommodate growth and user demand. Many vendors have implemented their own solutions to this problem. Some use clustered file systems that span multiple appliances, while others have similar technologies to allow block storage to scale past a fixed capacity. Ceph, a distributed storage solution that offers block storage, was designed to solve this scale issue and does not have the same limitations on domains, clusters, or scale issues of other appliance driven models.
Expandability is a major architecture factor for storage solutions with general purpose OpenStack cloud. A storage solution that expands to 50 PB is considered more expandable than a solution that only scales to 10 PB. This meter is related to scalability, which is the measure of a solution’s performance as it expands.

Implementing Block Storage

Configure Block Storage resource nodes with advanced RAID controllers and high-performance disks to provide fault tolerance at the hardware level.

We recommend deploying high performing storage solutions such as SSD drives or flash storage systems for applications requiring additional performance out of Block Storage devices.

In environments that place substantial demands on Block Storage, we recommend using multiple storage pools. In this case, each pool of devices should have a similar hardware design and disk configuration across all hardware nodes in that pool. This allows for a design that provides applications with access to a wide variety of Block Storage pools, each with their own redundancy, availability, and performance characteristics. When deploying multiple pools of storage, it is also important to consider the impact on the Block Storage scheduler which is responsible for provisioning storage across resource nodes. Ideally, ensure that applications can schedule volumes in multiple regions, each with their own network, power, and cooling infrastructure. This will give tenants the option of building fault-tolerant applications that are distributed across multiple availability zones.

In addition to the Block Storage resource nodes, it is important to design for high availability and redundancy of the APIs, and related services that are responsible for provisioning and providing access to storage. We recommend designing a layer of hardware or software load balancers in order to achieve high availability of the appropriate REST API services to provide uninterrupted service. In some cases, it may also be necessary to deploy an additional layer of load balancing to provide access to back-end database services responsible for servicing and storing the state of Block Storage volumes. It is imperative that a highly available database cluster is used to store the Block Storage metadata.

In a cloud with significant demands on Block Storage, the network architecture should take into account the amount of East-West bandwidth required for instances to make use of the available storage resources. The selected network devices should support jumbo frames for transferring large blocks of data, and utilize a dedicated network for providing connectivity between instances and Block Storage.

Implementing Object Storage

While consistency and partition tolerance are both inherent features of the Object Storage service, it is important to design the overall storage architecture to ensure that the implemented system meets those goals. The OpenStack Object Storage service places a specific number of data replicas as objects on resource nodes. Replicas are distributed throughout the cluster, based on a consistent hash ring also stored on each node in the cluster.

When designing your cluster, you must consider durability and availability which is dependent on the spread and placement of your data, rather than the reliability of the hardware.

Consider the default value of the number of replicas, which is three. This means that before an object is marked as having been written, at least two copies exist in case a single server fails to write, the third copy may or may not yet exist when the write operation initially returns. Altering this number increases the robustness of your data, but reduces the amount of storage you have available. Look at the placement of your servers. Consider spreading them widely throughout your data center’s network and power-failure zones. Is a zone a rack, a server, or a disk?

Consider these main traffic flows for an Object Storage network:

Object Storage frequent communicates among servers hosting data. Even a small cluster generates megabytes per second of traffic.

Consider the scenario where an entire server fails and 24 TB of data needs to be transferred “immediately” to remain at three copies — this can put significant load on the network.

Another consideration is when a new file is being uploaded, the proxy server must write out as many streams as there are replicas, multiplying network traffic. For a three-replica cluster, 10 Gbps in means 30 Gbps out. Combining this with the previous high bandwidth bandwidth private versus public network recommendations demands of replication is what results in the recommendation that your private network be of significantly higher bandwidth than your public network requires. OpenStack Object Storage communicates internally with unencrypted, unauthenticated rsync for performance, so the private network is required.

The remaining point on bandwidth is the public-facing portion. The swift-proxy service is stateless, which means that you can easily add more and use HTTP load-balancing methods to share bandwidth and availability between them. More proxies means more bandwidth.

You should consider designing the Object Storage system with a sufficient number of zones to provide quorum for the number of replicas defined. For example, with three replicas configured in the swift cluster, the recommended number of zones to configure within the Object Storage cluster in order to achieve quorum is five. While it is possible to deploy a solution with fewer zones, the implied risk of doing so is that some data may not be available and API requests to certain objects stored in the cluster might fail. For this reason, ensure you properly account for the number of zones in the Object Storage cluster.

Each Object Storage zone should be self-contained within its own availability zone. Each availability zone should have independent access to network, power, and cooling infrastructure to ensure uninterrupted access to data. In addition, a pool of Object Storage proxy servers providing access to data stored on the object nodes should service each availability zone. Object proxies in each region should leverage local read and write affinity so that local storage resources facilitate access to objects wherever possible. We recommend deploying upstream load balancing to ensure that proxy services are distributed across the multiple zones and, in some cases, it may be necessary to make use of third-party solutions to aid with geographical distribution of services.

A zone within an Object Storage cluster is a logical division. Any of the following may represent a zone:

  • A disk within a single node
  • One zone per node
  • Zone per collection of nodes
  • Multiple racks
  • Multiple data centers

Selecting the proper zone design is crucial for allowing the Object Storage cluster to scale while providing an available and redundant storage system. It may be necessary to configure storage policies that have different requirements with regards to replicas, retention, and other factors that could heavily affect the design of storage in a specific zone.

Planning and scaling storage capacity

An important consideration in running a cloud over time is projecting growth and utilization trends in order to plan capital expenditures for the short and long term. Gather utilization meters for compute, network, and storage, along with historical records of these meters. While securing major anchor tenants can lead to rapid jumps in the utilization of resources, the average rate of adoption of cloud services through normal usage also needs to be carefully monitored.

Scaling Block Storage

You can upgrade Block Storage pools to add storage capacity without interrupting the overall Block Storage service. Add nodes to the pool by installing and configuring the appropriate hardware and software and then allowing that node to report in to the proper storage pool through the message bus. Block Storage nodes generally report into the scheduler service advertising their availability. As a result, after the node is online and available, tenants can make use of those storage resources instantly.

In some cases, the demand on Block Storage may exhaust the available network bandwidth. As a result, design network infrastructure that services Block Storage resources in such a way that you can add capacity and bandwidth easily. This often involves the use of dynamic routing protocols or advanced networking solutions to add capacity to downstream devices easily. Both the front-end and back-end storage network designs should encompass the ability to quickly and easily add capacity and bandwidth.


Sufficient monitoring and data collection should be in-place from the start, such that timely decisions regarding capacity, input/output metrics (IOPS) or storage-associated bandwidth can be made.

Scaling Object Storage

Adding back-end storage capacity to an Object Storage cluster requires careful planning and forethought. In the design phase, it is important to determine the maximum partition power required by the Object Storage service, which determines the maximum number of partitions which can exist. Object Storage distributes data among all available storage, but a partition cannot span more than one disk, so the maximum number of partitions can only be as high as the number of disks.

For example, a system that starts with a single disk and a partition power of 3 can have 8 (2^3) partitions. Adding a second disk means that each has 4 partitions. The one-disk-per-partition limit means that this system can never have more than 8 disks, limiting its scalability. However, a system that starts with a single disk and a partition power of 10 can have up to 1024 (2^10) disks.

As you add back-end storage capacity to the system, the partition maps redistribute data amongst the storage nodes. In some cases, this involves replication of extremely large data sets. In these cases, we recommend using back-end replication links that do not contend with tenants’ access to data.

As more tenants begin to access data within the cluster and their data sets grow, it is necessary to add front-end bandwidth to service data access requests. Adding front-end bandwidth to an Object Storage cluster requires careful planning and design of the Object Storage proxies that tenants use to gain access to the data, along with the high availability solutions that enable easy scaling of the proxy layer. We recommend designing a front-end load balancing layer that tenants and consumers use to gain access to data stored within the cluster. This load balancing layer may be distributed across zones, regions or even across geographic boundaries, which may also require that the design encompass geo-location solutions.

In some cases, you must add bandwidth and capacity to the network resources servicing requests between proxy servers and storage nodes. For this reason, the network architecture used for access to storage nodes and proxy servers should make use of a design which is scalable.


When making swift more redundant, one approach is to add additional proxy servers and load balancing. HAProxy is one method of providing load balancing and high availability and is often combined with keepalived or pacemaker to ensure the HAProxy service maintains a stable VIP. Sample HAProxy configurations can be found in the OpenStack HA Guide..


Replicas in Object Storage function independently, and clients only require a majority of nodes to respond to a request in order for an operation to be considered successful. Thus, transient failures like network partitions can quickly cause replicas to diverge. Fix These differences are eventually reconciled by asynchronous, peer-to-peer replicator processes. The replicator processes traverse their local filesystems, concurrently performing operations in a manner that balances load across physical disks.

Replication uses a push model, with records and files generally only being copied from local to remote replicas. This is important because data on the node may not belong there (as in the case of handoffs and ring changes), and a replicator can not know what data exists elsewhere in the cluster that it should pull in. It is the duty of any node that contains data to ensure that data gets to where it belongs. Replica placement is handled by the ring.

Every deleted record or file in the system is marked by a tombstone, so that deletions can be replicated alongside creations. The replication process cleans up tombstones after a time period known as the consistency window. The consistency window encompasses replication duration and the length of time a transient failure can remove a node from the cluster. Tombstone cleanup must be tied to replication to reach replica convergence.

If a replicator detects that a remote drive has failed, the replicator uses the get_more_nodes interface for the ring to choose an alternative node with which to synchronize. The replicator can maintain desired levels of replication in the face of disk failures, though some replicas may not be in an immediately usable location.


The replicator does not maintain desired levels of replication when other failures occur, such as entire node failures, because most failures are transient.

Replication is an area of active development, andimplementation details are likely to change over time.

There are two major classes of replicator: the db replicator, which replicates accounts and containers, and the object replicator, which replicates object data.

For more information, please see the Swift replication page.

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