Dell PowerVault MD3620f Command Line Interface Manual page 96

between the storage arrays that are participating in Remote Replication. This
section also describes the actions taken by the RAID controller module owner
of the primary virtual disk if a link interruption occurs between storage arrays.
Write Modes
When the RAID controller module owner of the primary virtual disk receives
a write request from a host, the RAID controller module first logs information
about the write to a replication repository virtual disk, and then writes the
data to the primary virtual disk. The RAID controller module then initiates a
remote write operation to copy the affected data blocks to the secondary
virtual disk at the secondary storage array.
The Remote Replication premium feature provides two write mode options
that affect when the I/O completion indication is sent back to the host:
Synchronous and Asynchronous.
Synchronous Write Mode
Synchronous write mode provides the highest level security for full data
recovery from the secondary storage array in the event of a disaster.
Synchronous write mode does reduce host I/O performance. When this write
mode is selected, host write requests are written to the primary virtual disk
and then copied to the secondary virtual disk. After the host write request has
been written to the primary virtual disk and the data has been successfully
copied to the secondary virtual disk, the RAID controller module removes the
log record on the replication repository virtual disk. The RAID controller
module then sends an I/O completion indication back to the host system.
Synchronous write mode is selected as the default value and is the
recommended write mode.
Asynchronous Write Mode
Asynchronous write mode offers faster host I/O performance but does not
guarantee that a copy operation has successfully completed before processing
the next write request. When you use Asynchronous write mode, host write
requests are written to the primary virtual disk. The RAID controller module
then sends an "I/O complete" indication back to the host system, without
acknowledging that the data has been successfully copied to the secondary
(remote) storage array.
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Using the Remote Replication Premium Feature