Implementing vSphere Metro Storage Cluster With ActiveCluster: Overview and Introduction

Components

Purity ActiveCluster is composed of three core components: The Pure1 Mediator, active/active clustered array pairs, and stretched storage containers.

The Pure1 Cloud Mediator - A required component of the solution that is used to determine which array will continue data services should an outage occur in the environment.

Active/Active Clustered FlashArrays - Utilize synchronous replication to maintain a copy of data on each array and present those as one consistent copy to hosts that are attached to either, or both, arrays.

Stretched Storage Containers - Management containers that collect storage objects such as volumes into groups that are stretched between two arrays.

Administration

ActiveCluster introduces a new management object: Pods. A pod is a stretched storage container that defines a set of objects that are synchronously replicated together and which arrays they are replicated between. An array can support multiple pods. Pods can exist on just one array or on two arrays simultaneously with synchronous replication. Pods that are synchronously replicated between two arrays are said to be stretched between arrays.      

 

Pods can contain volumes, protection groups (for snapshot scheduling and asynchronous replication) and other configuration information such as which volumes are connected to which hosts. The pod acts as a consistency group, ensuring that multiple volumes within the same pod remain write order consistent.

Pods also provide volume namespaces, that is different volumes may have the same volume name if they are in different pods. In the image above the volumes in pod3 and pod4 are different volumes than those in pod1, a stretched active/active pod. This allows migration of workloads between arrays or consolidation of workloads from multiple arrays to one, without volume name conflicts.

Mediator

Transparent failover between arrays in ActiveCluster is automatic and requires no intervention from the storage administrator. Failover occurs within standard host I/O timeouts similar to the way failover occurs between two controllers in one array during non-disruptive hardware or software upgrades.

ActiveCluster is designed to provide maximum availability across symmetric active/active storage arrays while preventing a split-brain condition from occurring; split-brain is the case where two arrays might serve I/O to the same volume, without keeping the data in sync between the two arrays.

Any active/active synchronous replication solution designed to provide continuous availability across two different sites requires a component referred to as a witness or voter to mediate failovers while preventing split-brain. ActiveCluster includes a simple to use, lightweight, and automatic way for applications to transparently failover, or simply move, between sites in the event of a failure without user intervention: The Pure1 Cloud Mediator.

The Pure1 Cloud Mediator is responsible for ensuring that only one array is allowed to stay active for each pod when there is a loss of communication between the arrays.

In the event that the arrays can no longer communicate with each other over the replication interconnect, both arrays will pause I/O and reach out to the mediator to determine which array can stay active for each sync replicated pod. This is called a race to the mediator. The first array to reach the mediator is allowed to keep its synchronously replicated pods online. The second array to reach the mediator must stop servicing I/O to its synchronously replicated volumes, in order to prevent split brain. The entire operation occurs within standard host I/O timeouts to ensure that applications experience no more than a pause and resume of I/O.

The Pure1® Cloud Mediator

A failover mediator must be located in a 3rd site that is in a separate failure domain from either site where the arrays are located. Each array site must have independent network connectivity to the mediator such that a single network outage does not prevent both arrays from accessing the mediator. A mediator should also provide a very lightweight and easy to administer component of the solution. The Pure Storage solution provides this automatically by utilizing an integrated cloud based mediator. The Pure1 Cloud Mediator provides two main functions:

• Prevent a split-brain condition from occurring where both arrays are independently allowing access to data without synchronization between arrays.
• Determine which array will continue to service IO to synchronously replicated volumes in the event of an array failure, replication link outage, or site outage.

The Pure1 Cloud Mediator has the following advantages over a typical non-Pure heavy handed voter or witness component:

• SaaS operational benefits - As with any SaaS solution, the operational maintenance complexity is removed: nothing to install onsite, no hardware or software to maintain, nothing to configure and support for HA, no security patch updates and more.
• Automatically a 3rd site - The Pure1 Cloud Mediator is inherently in a separate failure domain from either of the two arrays.
• Automatic configuration - Arrays configured for synchronous replication will automatically connect to and use the Pure1 Cloud Mediator.
• No misconfiguration - With automatic and default configuration there is no risk that the mediator could be incorrectly configured.
• No human intervention - A significant number of issues in non-Pure active/active synchronous replication solutions, particularly those related to accidental split-brain, are related to human error. Pure’s automated non-human mediator eliminates operator error from the equation.
• Passive mediation - Continuous access to the mediator is not required for normal operations. The arrays will maintain a heartbeat with the mediator, however if the arrays lose connection to the mediator they will continue to synchronously replicate and serve data as long as the replication link is active.

On-Premises Failover Mediator

Failover mediation for ActiveCluster can also be provided using an on-premises mediator distributed as an OVF file and deployed as a VM. Failover behaviors are exactly the same as described above. The on-premises mediator simply replaces the role of the Pure1 Cloud Mediator during failover events.

The on-premises mediator has the following basic requirements:

• The on-premises mediator can only be deployed as a VM on virtualized hardware. It is not installable as a stand-alone application.
• High Availability for the mediator must be provided by the hosts on which the mediator is deployed. For example, using VMware HA, or Microsoft Hyper-V HA Clustering.
• Storage for the mediator must not allow the configuration of the mediator to be rolled back to previous versions. This applies to situations such as storage snapshot restores, or cases where the mediator might be stored on mirrored storage.
• The arrays must be configured to use the on-premises mediator rather than the Pure1 Cloud Mediator.
• The mediator must be deployed in a third site, in a separate failure domain that will not be affected by any failures in either of the sites where the arrays are installed.
• Both array sites must have independent network connections to the mediator such that a failure of one network connection does not prevent both arrays from accessing the mediator.

Pre-Election

Pre-Election is an ActiveCluster feature that automatically engages when both FlashArrays lose connectivity to the Mediator to ensure stretched pod volumes will remain online should a subsequent loss of replication links occur.  

What are the requirements of Pre-Election? 

Enabled by default in Purity 5.3.x and higher. 

What does Pre-Election do? 

After both FlashArrays detect that the Mediator is unavailable, Pre-Election will:

• ensure the pre-elected array in the pod will stay online if the replication network fails 

• ensure the pre-elected array in the pod will stay online if the non-elected array fails

• designate the winning FlashArray for each pod based on the pod failover preference, if set, otherwise it picks a FlashArray

After one (or both) FlashArray detect that the Mediator is available, Pre-Election will:

• disengage and return to standard ActiveCluster transparent failover behavior

What does Pre-Election not do?

Pre-Election does not:

• override normal behavior: if one array still has mediator access it must race to mediator to stay online.

• provide a hard failover preference.

• force the non-elected side online if the pre-elected array fails.

• keep the pod online if both the mediator network and replication network fail at the same time.

Uniform and Non-Uniform Access

Hosts can be configured to see just the array that is local to it, or to both arrays. The former option is called non-uniform vMSC, the latter is referred to as uniform vMSC.

A uniform storage access model can be used in environments where there is host-to-array connectivity of either FC or ethernet (for iSCSI), and array-to-array ethernet connectivity, between the two sites. When deployed in this way, a host has access to the same volume through both the local array and the remote array. The solution supports connecting arrays with up to 11ms of round trip time (RTT) latency between the arrays.

The image above represents the logical paths that exist between the hosts and arrays, and the replication connection between the two arrays in a uniform access model. Because a uniform storage access model allows all hosts, regardless of site location, to access both arrays, there will be paths with different latency characteristics. Paths from hosts to the local array will have lower latency; paths from each local host to the remote array will have higher latency.

For the best performance in an active/active synchronous replication environments, hosts should be prevented from using paths that access the remote array unless necessary. For example, in the image below if VM 2A were to perform a write to volume A over the host side connection to array A, that write would incur 2X the latency of the inter-site link, 1X for each traverse of the network. The write would experience 11ms of latency for the trip from host B to array A and experience another 11ms of latency while array A synchronously sends the write back to array B.

With Pure Storage Purity ActiveCluster, there are no such management headaches. ActiveCluster does make use of ALUA to expose paths to local hosts as active/optimized paths and expose paths to remote hosts as active/non-optimized. However, there are two advantages in the ActiveCluster implementation.

• In ActiveCluster volumes in stretched pods are read/write on both arrays. There is no such thing as a passive volume that cannot service both reads and writes.
• The optimized path is defined on a per host-to-volume connection basis using a preferred-array option; this ensures that regardless of what host a VM or application is running on, it will have a local optimized path to that volume.

ActiveCluster enables truly active/active datacenters and removes the concern around what site or host a VM runs on; the VM will always have the same performance regardless of site. While a VM 1A is running on host A accessing volume A, it will use only the local optimized paths as shown in the next image.
 

If the VM or application is switched to a host in the other site, with the data left in place, only local paths will be used in the other site as shown in the next image. There is no need to adjust path priorities or migrate the data to a different volume to ensure local optimized access.

Non-Uniform Access

A non-uniform storage access model is used in environments where there is host-to-array connectivity of either FC or ethernet (for iSCSI), only locally within the same site. Ethernet connectivity for the array-to-array replication interconnect must still exist between the two sites. When deployed in this way, each host has access to a volume only through the local array and not the remote array. The solution supports connecting arrays with up to 11ms of round trip time (RTT) latency between the arrays.

Hosts will distribute I/Os across all paths to the storage only, because only the local Active/Optimized paths are available.