Resynchronization - Dell PowerVault MD3620f Command Line Interface Manual

the secondary virtual disk. After the resynchronization starts, the replicated
pair transitions from an Unsynchronized status to a Synchronization in
Progress status.
The primary RAID controller module also marks the replicated pair as
unsynchronized when a virtual disk error on the secondary side prevents the
remote write from completing. For example, an offline secondary virtual disk
or a failed secondary virtual disk can cause the remote replication to become
unsynchronized. When the virtual disk error is corrected (the secondary
virtual disk is placed online or recovered to an Optimal status), then
synchronization is required. The replicated pair then transitions to a
Synchronization in Progress status.

Resynchronization

Data replication between the primary virtual disk and the secondary virtual
disk in a replication relationship is managed by the RAID controller modules
and is transparent to host machines and applications. 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. The RAID controller module
then writes the data to the primary virtual disk. The RAID controller module
then initiates a write operation to copy the affected data to the secondary
virtual disk on the remote storage array.
If a link interruption or a virtual disk error prevents communication with the
secondary storage array, the RAID controller module owner of the primary
virtual disk transitions the replicated pair into an Unsynchronized status. The
RAID controller module owner then sends an I/O completion to the host
sending the write request. The host can continue to issue write requests to
the primary virtual disk, but remote writes to the secondary virtual disk do not
take place.
When connectivity is restored between the RAID controller module owner of
the primary virtual disk and the RAID controller module owner of the
secondary virtual disk, the virtual disks must be resynchronized by copying
the blocks of data that changed during the interruption to the secondary
virtual disk. Only the blocks of data that have changed on the primary virtual
disk during the link interruption are copied to the secondary virtual disk.
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