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29 PS Series Asynchronous Replication Best Practices and Sizing Guide | BP1012
The system defaults are set conservatively so that replication will continue in all cases, even if the volume
contents change completely from one replication to the next.
When using thin provisioning, be aware that percentages are based on the internally allocated size of the
volume, rather than based on the size of the volume as reported to the server.
The local reserve default allows for the system to continue replicating even if 100 percent of the previously-
written data was changed before the new replica can be completed. If fast failback is enabled for the volume,
then the default setting provides an additional 100 percent for keeping a snapshot of the data that was
previously replicated, even if the data was changed 100 percent between each point in time.
Similarly, the default replica reserve is set to 200 percent so that a complete copy can be stored on the
remote site even as another completely changed copy is replicated. In this case, even if the volume changes
100 percent, there would never be a case where replication cannot proceed, and regardless of how the
system is configured there will always be at least one replica copy stored on the remote site.
Dell EMC recommends monitoring the use of these reserves through Group Manager and SAN Headquarters
to determine the optimal settings for your environment. After monitoring, make adjustments as needed to
ensure the number of retained replicas meets RPO and RTO requirements for the business, and the
replication reserve disk space is being used most efficiently.
Dell EMC also strongly recommends that overall free pool space does not fall below the following limit
(whichever is smaller for the pool):
 5 percent of the total pool space
 100 GB multiplied by the number of pool members
If the utilization exceeds these levels, steps should be taken promptly to make more space available. This will
ensure multi-member page balancing, performance balancing, vacate, snapshot, replication, and thin
provisioning operations perform optimally. If choosing to take advantage of the Borrow from Free Space
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replication option, this requires at least 10 percent free pool space on the primary side or the borrow option
will be disabled. If this option is selected, free pool space will be temporarily used if there is not enough free
local replica reserve space to complete a replication.
Although some applications perform a consistent number of volume writes, others have a workloads that
change daily. Therefore, one replication operation might transfer little data and complete quickly, while
another replication might transfer a large amount of data and take a long time.
In some cases, a volume might appear to have few changes, but the transferred data is relatively large.
Random writes to a volume can result in a large amount of transferred data, even if the actual data changes
are small. Some disk operations, such as defragmenting a disk or reorganizing a database can also increase
the amount of transferred data because these operations modify the source volume.
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This setting allows temporary borrowing of free pool space if the local reserve size is inadequate. At least 10% free pool
space must be available for this setting to take effect.