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Comparing Microsoft SQL Server and SC Series availability features
9 SQL Server High Availability and Disaster Recovery with Dell EMC SC Series | CML1119
2 Comparing Microsoft SQL Server and SC Series availability
features
2.1 High availability features
When selecting an architecture for high availability, it may be beneficial to compare SQL Server features to
SC Series array features. High Availability Solutions (SQL Server) recommended by Microsoft include Always
On Failover Cluster Instances (discussed above), Always On Availability Groups and log shipping. While
Microsoft SQL Server does offer various types of data replication they are not listed in this section, as they
are not typically used for to satisfy high availability requirements.
Log shipping automates the process of sending transaction log backups and restoring them on the destination
server. The RPO for log shipping is typically measured in minutes based on the backup frequency and
backup file availability.
The Always On Availability Groups feature provides database protection by replicating all database changes
for a set of primary databases to one or more sets of secondary databases. Availability groups can be
configured to replicate asynchronously or synchronously.
2.1.1 Asynchronous replication or log shipping
Asynchronous replication offers a great amount of flexibility due to the various options that it supports. When
the Replicate Active Snapshot option is not used, replication takes place whenever a snapshot is taken. In
this mode, it operates somewhat like SQL Server log shipping in that changes are sent in batch on a schedule
that is defined by the backup/snapshot frequency. When a snapshot is taken (preferably with Replay
Manager), those changes are forwarded to the volumes at the destination. For example, configuring SQL
Server log shipping with an hourly backup schedule would provide a similar RPO as asynchronous replication
with a Replay Manager Job on an hourly schedule.
2.1.2 Asynchronous replication or Always On Availability Groups
When using asynchronous replication with the Replicate Active Snapshot option, writes are queued and
replicated as soon as they are written on the source. Assuming the storage network and destination array can
keep up, this provides a near zero data loss solution. Although there are design factors to take into account
when comparing the two, asynchronous replication could provide a similar RPO to Always On Availability
Groups in asynchronous mode.
2.1.3 Synchronous replication or Always On Availability Groups
For situations where zero data loss is required, synchronous replication is a great solution. The key difference
between synchronous and asynchronous replication, is that with synchronous replication, any writes to the
source volume must also be written to the destination volume before the write is acknowledged to the host. In
High Consistency mode of synchronous replication if the destination volume is unreachable, then the source
volume will not accept writes until the issue is resolved. This prevents data loss in the event that the
destination volume is unreachable for any reason. High Availability mode of synchronous replication allows
the primary volume to remain online in the event the secondary volume is unavailable. The tradeoff is that
higher availability is offered at the risk of data loss, if the primary volume were to fail before the issue with the
secondary volume could be resolved.