The Asynchronous Mirroring feature differs from the Synchronous Mirroring feature in one essential way: it captures the state of the source volume at a particular point in time and copies just the data that has changed since the last image capture.

With synchronous mirroring, the state of the primary volume is not captured at some point in time, but rather reflects all changes that were made on the primary volume to the secondary volume. The secondary volume is identical to the primary volume at every moment because, with this type of mirror, each time a write is done to the primary volume, a write is done to the secondary volume. The host does not receive an acknowledgment that the write was successful until the secondary volume is successfully updated with the changes that were made on the primary volume.

With asynchronous mirroring, the remote storage array is not fully synchronized with the local storage array, so if the application needs to transition to the remote storage array due to a loss of the local storage array, some transactions could be lost.

Comparison between mirroring features:

Mirroring is configured in the SANtricity Unified Manager interface.

To enable and configure mirroring between two arrays, verify the following:

For configuration instructions, see the following:

Follow these guidelines before you create a mirror consistency group.

You create a consistency group in Unified Manager in the Create Mirrored Pairs wizard.

Meet the following requirements for Unified Manager:

Also be sure to meet the following requirements for storage arrays:

You configure mirrored pairs in the SANtricity Unified Manager interface, and then manage the pairs in SANtricity System Manager.

Before creating a mirrored pair, follow these guidelines.

Typically, you should increase reserved capacity when you receive a warning that the reserved capacity for a mirrored pair is becoming full. You can increase reserved capacity only in increments of 8 GiB.

For asynchronous mirroring operations, reserved capacity is typically 20 percent of the base volume. Choose a larger capacity for reserved capacity if one or both of these conditions exist:

You can increase the reserved capacity for a mirrored pair by performing one of these actions:

You can increase reserved capacity only in increments of 4 GiB.

Review the following guidelines:

For asynchronous mirroring operations reserved capacity is typically 20 percent of the base volume. Use a higher percentage if you believe the base volume will undergo many changes or if the estimated life expectancy of a storage object's copy service operation will be very long.

Reserved capacity is typically 40 percent of the base volume for snapshot operations and 20 percent of the base volume for asynchronous mirroring operations.

Usually this capacity is sufficient. The capacity needed varies, depending on the frequency and size of I/O writes to the base volume and how long you intend to use the storage object's copy service operation.

In general, choose a larger percentage for reserved capacity if one or both of these conditions exist:

If there is more than one volume in a pool or volume group that meets the capacity percentage amount you selected for the storage object, you will see multiple candidates.

You can refresh the list of recommended candidates by changing the percentage of physical drive space that you want to reserve on the base volume for copy service operations. The best candidates are displayed based on your selection.

The table lists Not Available values when the data located on the remote storage array is not available to be displayed.

To display the remote storage array data, launch System Manager from Unified Manager.

When you create a secondary volume for the asynchronous mirrored pair, the system displays a list of all the eligible pools and volume groups for that asynchronous mirrored pair. Any pool or volume group that is not eligible to be used does not display in that list.

Pools or volume groups may not be eligible for any of the following reasons.

When you are selecting a primary volume for a mirrored pair, a list shows all the eligible volumes.

Any volumes that are not eligible to be used do not display in that list. Volumes may not be eligible for any of the following reasons:

When you are selecting a secondary volume on the remote storage array, a list shows all the eligible volumes for that mirrored pair.

Any volumes that are not eligible to be used, do not display in that list. Volumes might not be eligible for any of the following reasons:

You update your remote storage array's IP address when the IP address of an iSCSI port changes and the local storage array is unable to communicate with the remote storage array.

When establishing an asynchronous mirroring relationship with an iSCSI connection, both the local and the remote storage arrays store a record of the IP address of the remote storage array in the asynchronous mirroring configuration. If the IP address of an iSCSI port changes, the remote storage array that is attempting to use that port encounters a communication error.

The storage array with the changed IP address sends a message to each remote storage array associated with the mirror consistency groups that are configured to mirror over an iSCSI connection. Storage arrays that receive this message automatically update their remote-target IP address.

If the storage array with the changed IP address is unable to send its inter-array message to a remote storage array, the system sends you an alert of the connectivity issue. Use the Update Remote IP Address option to re-establish connection with the local storage array.