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OceanStor 18500, 18800, and 18800F Storage System V100R001C30 Remote Replication Feature Guide 09

This document describes the remote replication feature, in terms of the implementation principle, application scenario, configuration process, and reference information.
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Functions of Reserved Space for an Asynchronous Remote Replication Task

Functions of Reserved Space for an Asynchronous Remote Replication Task

Question

How do I plan reserved space for a storage pool when creating an asynchronous remote replication task? What are the functions of reserved space?

Answer

Reserved space stores snapshots generated during asynchronous remote replication. Storage space is reserved because:
  • Copy-on-write (COW) for I/Os newly written to the host during remote replication occupies the storage pool space of the primary storage array. In addition, COW for data synchronization I/Os of the secondary storage array occupies the storage pool space of the secondary storage array.
  • If the storage pool cannot provide sufficient space, COW for host I/Os or data synchronization I/Os of the secondary storage array fails due to insufficient space. As a result, the remote replication session enters the split state. You are advised to reserve 30% of LUN space in an asynchronous remote replication task for copy-on-write.
When configuring an asynchronous remote replication task, reserve certain space in the storage pool of the secondary storage array to ensure that sufficient space is provided for COW for host I/Os or data synchronization I/Os of the secondary storage array during remote replication.

The following example shows how to plan reserved space:

The scheduled synchronization period of an asynchronous remote replication task is 1 hour, the synchronization rate is medium (15 MB/s on average), and the host bandwidth is 5 MB/s.
  • Amount of data written into the host during 1 hour = 1 x 60 x 60 x 5 = 18000 MB
  • Time required for replicating data from the primary LUN to the secondary LUN = 18000/15 = 1200s
  • Amount of data written into the primary LUN when data is replicated from the primary LUN to the secondary LUN = 1200 x 5 = 6000 MB
Therefore, at least 6000 MB space must be reserved in the storage pool of the primary storage array and at least 18000 MB space must be reserved in the storage pool of the secondary storage array. If a primary/secondary switchover is required, at least 18000 MB space must be reserved in the storage pools of both the primary and secondary storage arrays.
NOTE:
  • The size of each data block written by the host must be considered during reserved space planning. If the COW granularity for host I/Os is 64 KB and the size of each data block written by the host is smaller than 64 KB, the reserved space for COW must be larger than the amount of data written by the host during the copy period.
  • If an incremental synchronization is implemented after the remote replication session is interrupted due to manual splitting or external faults, the space required for COW on the secondary storage array equals the amount of service data delivered by the primary storage array during the synchronization or the LUN capacity at most.
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Updated: 2019-01-28

Document ID: EDOC1000077758

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