How Do I Plan Reserved Space for Immediately Available Clones?
Question
What is the purpose of reserving space in the storage pool for immediately available clones? How do I plan the reserved space size?
Answer
The reserved space is used to store snapshots generated during the synchronization implemented by an immediately available clone. Storage space is reserved because:
- During the synchronization implemented by an immediately available clone, COW is performed on the I/Os newly written by the host. As a result, the space of the primary storage pool is occupied.
- If the storage pool space is insufficient, COW of host I/Os will fail due to the failure to request space. In this case, the immediately available clone will enter the stopped state.
When configuring an immediately available clone, consider the reserved space of the primary storage pool. Ensure that the space is sufficient for COW of new host I/Os during the synchronization implemented by the immediately available clone. The space that needs to be reserved in the primary storage pool varies greatly depending on service scenarios and configurations. The maximum reserved space does not exceed the capacity of the primary LUN in the immediately available clone. In project planning, you are advised to plan the reserved space size based on the actual scenario and configuration to prevent resource waste or insufficiency.
The factors that affect the reserved space include the synchronization duration, host write bandwidth, and host I/O size. The following example describes how to plan reserved space:
An immediately available clone synchronizes data once a day. The duration of each synchronization is 1 hour, and the peak write bandwidth of the host is 5 MB/s. The amount of data written by the host in the one-hour synchronization period N = 1 x 60 x 60 x 5 = 18000 MB.
So, the reserved space at the primary end is at least 18000 MB.
- When planning the reserved space, also consider the size of data blocks written by the host. The host I/O COW granularity used by the storage system is 64 KB. If the data block size written by the host is smaller than 64 KB, the reserved space occupied by COW is larger than the amount of data delivered by the host in the synchronization period.
- Assume that a manual split or an external fault causes an immediately available clone to be disconnected. When incremental synchronization is performed again, the COW space required by the primary end is the size of service data delivered to the primary LUN during this period. The maximum COW space may be the same as the capacity of the primary LUN.
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