Reference Guide

optimum performance of workloads. Low bandwidth and high latency between sites can result in very poor performance on the

primary site in the case of both synchronous and asynchronous replication.

Synchronous replication involves data blocks being written to log files on both sites before being committed. In asynchronous

replication, the remote node accepts the block of replicated data and acknowledges back to the source copy. Application

performance is not affected unless the rate of change of data is faster than the bandwidth of the replica link between the sites

for large periods of time. This point is critical and must be taken into consideration when you are designing the solution.

The size of the log volume has no bearing on the performance of the solution. A larger log collects and retains more write I/Os

before they are wrapped out. This allows for an interruption in service between the two sites (such as a network outage or the

destination site being offline) to go on for a longer period.

Table 3. Disk writes

Scenario Writes in two-way mirrored volumes Writes in three-way mirrored

volumes

Standalone storage spaces 2x 3x

Replication to secondary site 4x 6x

NOTE: WAN latency and additional writes to log volumes on both sites causes higher write latency. Along with writes to

the log and data disks, the inter-site bandwidth and latency also play a role in limiting the IOPs in the environment. For this

reason, we highly recommend using all-flash configurations for stretched clusters.

NOTE: In a Storage Spaces Direct environment both data and log volumes eventually reside on the same SSD pool because

multiple storage pools per site are not supported.

The following figure illustrates the difference between synchronous and asynchronous replication:

Figure 2. Synchronous and asynchronous replication

Synchronous replication: A block of data written by an application to a volume on Site A (1) is written first to the

corresponding log volume on the same site (2), and is then replicated to Site B (2). At site B, the block of data is written

to the Replica log volume (3) before a commit is sent back to the application using the same route (4 and 5). The block is

subsequently pushed to the data volumes on both sites. For each block of data that the application writes, the commit is issued

only after data is written to the secondary site. Thus there is no data loss at file system level in the event of a site failure. This

results in a lower application write performance compared to a standalone deployment.

Asynchronous replication: A block of data written by an application to a volume on Site A (1) is written first to the

corresponding log volume on the same site (2). A commit is immediately sent back to the application. At the same time,

the block of data is replicated to Site B and written to the Replica log volume. In the case of a site failure, the cluster ensures

that no data is lost beyond the configured Recovery Point Objective (RPO). Application performance is not affected unless the

Solution deployment