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39 PS Series Asynchronous Replication Best Practices and Sizing Guide | BP1012
Replication borrowing was introduced in firmware v8 and allows for temporary use of available space. For
instance, replication may need to use beyond the local reserve by borrowing the space from one of the
previous replicas or local snapshots. After the replication is complete the borrowed space will be freed up if no
longer needed.
Navigation tip: Group Manager GUI (primary) > Volumes > (select volume) > Status (tab)
Borrowed space is free after the pending changes are replicated
7.5.2 Secondary group considerations
Secondary space considerations are typically the most important piece to sizing for replication, since an
existing array may need to be used or a new PS Series array will need to be acquired. For this exercise, the
assumption is that the secondary group will only be used as a replication target. The secondary group will
need to be sized to accommodate all the volume replications as well as the number of replicas to keep.
The main consideration on the target will be the amount of delegated space that will be needed. Delegated
space will hold all replicas and their changes between replication cycles. Typically, to size to this, you would
need to know the total amount of primary data to replicate and some understanding of the change rate along
with the number of replicas (snapshots) to keep. If you have a single 200 GB volume to replicate, then 400
GB should be allocated to delegated space to account for the possibility of 100 percent changes. In addition if
multiple replicas are to be kept, then the total changes those snapshots contain should also be added into the
calculation. For a 200 GB volume with two replicas (each with 25 percent changes kept), the delegated space
would need to add another 100 GB for a total of 500 GB ((200% x 200 GB) + 100 GB of changes).
The delegated space is defined during the replication partner configuration step. For PS Series firmware v8
and higher, multiple pools may participate in the allocation of delegated space. For this example, we will
allocate 200 percent of the reported space. For the two 420 GB volumes on the primary, this works out to
1.64 TB.