Asynchronous Transfer Mode

Data that is updated in a copy source is copied to the copy destination after a completion signal to the write request is returned.

The Stack mode and the Consistency mode are available in the Asynchronous transfer mode. Selection of the mode depends on the usage pattern of the remote copy. The Through mode is used to stop data transfer by the Stack mode or the Consistency mode.

  • Stack mode

    Only updated block positions are recorded before returning the completion signal to the server, so waiting-for-response affects on the server are small. Data transfer of the recorded blocks can be performed by an independent transfer process.

    The Stack mode can be used for a copy even when the line bandwidth is small. Therefore, this mode is mainly used for remote backup.

  • Consistency mode

    This mode guarantees the sequential transmission of updates to the remote copy destination in the same order as the writes occurred. Even if a problem occurs with the data transfer order due to a transmission delay in the WAN, the update order in the copy destination is controlled to be maintained.

    The Consistency mode is used to perform mirroring for data with multiple areas such as databases in order to maintain the transfer order for copy sessions.

    This mode uses part of the system memory as a buffer (REC Buffer). A copy via the REC Buffer stores multiple REC session I/Os in the REC Buffer for a certain period of time. Data for these I/Os is copied in blocks.

    When a capacity shortage for the REC Buffer occurs, the REC Disk Buffer can also be used. A REC Disk Buffer is used as a temporary destination to save copy data.

    Specification

    Number of CMs

    ETERNUS DX600 S6

    ETERNUS DX900 S6

    ETERNUS DX8900 S6

    Number of REC disk buffers allocated (per REC buffer for transmission)

    2CM

    1 or 2

    1 or 2

    1 or 2

    4CM

    —

    1, 2, or 4

    1, 2, or 4

    6CM

    —

    —

    1, 2, 4, or 6

    Note that when multiple REC Disk Buffers are allocated to a REC Buffer, REC Disk Buffers that have different numbers of member drives cannot be specified.

    Note that in the following cases, REC buffer is not used. Until the session changes to the equivalence state, the data is transferred with a method equivalent to the Through mode.

    • When the power is turned on after a power off or power outage while there is untransferred data

    • When the copy paths are recovered after an error occurs in all the copy paths

    • When sessions are restarted

  • Through mode

    After an I/O response is returned, this mode copies the data that has not been transferred as an extension of the process.

    The Through mode is not used for normal transfers. When stopping or suspending the Stack mode or the Consistency mode, this mode is used to change the transfer mode to transfer data that has not been transferred or to resume transfer.

Caution

  • When an REC is performed over a WAN, a bandwidth that supports the amount of updates from the server must be secured. Regardless of the amount of updates from the server, a bandwidth of at least 50Mbit/s is required for the Synchronous mode and a bandwidth of at least 2Mbit/s for the Consistency mode (when data is not being compressed by network devices).

  • When an REC is performed over a WAN, the round-trip time for data transmissions must be 100ms or less. A setup in which the round-trip time is 10ms or less is recommended for the synchronous transmission mode because the effect upon the I/O response is significant.

  • For REC, the data in the copy destination cannot be referenced or updated until the copy session is suspended. If the monitoring software (ServerView Agents) performs I/O access to the data in the copy destination, an I/O access error message is output to the server log message and other destinations. To prevent error messages from being output, consider using other monitoring methods.

  • When a firmware update is performed, copy sessions must be suspended.

Note

REC Disk Buffers can only be used if the ETERNUS DX set as the REC Buffer (for transmission) and the ETERNUS DX set for the REC Buffer (for reception) both support REC Disk Buffers. The following models, which can execute remote copies with the ETERNUS DX S6 series, support REC Disk Buffers.

  • ETERNUS AF series

  • ETERNUS DX S6 series (*1)

  • ETERNUS DX S5 series (*2)

  • ETERNUS DX S4 series (*3)

  • ETERNUS DX S3 series (*4)

*1 : 

The ETERNUS DX8100 S6 is not supported.
*2 : 

The ETERNUS DX60 S5 is not supported.
*3 : 

The ETERNUS DX60 S4 and the ETERNUS DX8100 S4 are not supported.
*4 : 

The ETERNUS DX60 S3 is not supported.