Dell PowerVault MD3420 Bedienungsanleitung PDF herunterladen (Seite 103)

write operation to copy the affected data blocks to the secondary virtual disk at the secondary storage

array.

The Remote Replication (legacy) 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.

When using Asynchronous write mode, write requests are not guaranteed to be completed in the same

order on the secondary virtual disk as they are on the primary virtual disk. If the order of write requests is

not retained, data on the secondary virtual disk might become inconsistent with the data on the primary

virtual disk. This event could jeopardize any attempt to recover data if a disaster occurs on the primary

storage array.

Write Consistency Mode

When multiple replication relationships exist on a single storage array and have been configured to use

Asynchronous write mode and to preserve consistent write order, they are considered to be an

interdependent group that is in the Write consistency mode. The data on the secondary, remote storage

array cannot be considered fully synchronized until all of the remote replications that are in the Write

consistency mode are synchronized.

If one replication relationship in the group becomes unsynchronized, all of the replication relationships in

the group become unsynchronized. Any write activity to the remote, secondary storage arrays is

prevented to protect the consistency of the remote data set.

Link Interruptions Or Secondary Virtual Disk Errors

When processing write requests, the primary RAID controller module might be able to write to the

primary virtual disk, but a link interruption might prevent communication with the remote (secondary)

RAID controller module.

In this case, the remote write operation cannot be completed to the secondary virtual disk, and the

primary virtual disk and the secondary virtual disk are no longer correctly replicated. The primary RAID

controller module transitions the replicated pair into an Unsynchronized state and sends an I/O

completion to the primary host. The primary host can continue to write to the primary virtual disk, but

remote writes do not take place.

When communication is restored between the RAID controller module owner of the primary virtual disk

and the RAID controller module owner of the secondary virtual disk, a resynchronization takes place. This

resynchronization happens automatically, or it must be started manually, depending on which write

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