Fujitsu ETERNUS DX S5 Series Design Manual page 111

2. Basic Functions
Backup (Advanced Copy)
• 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.
One or two REC Disk Buffers can be allocated to each transmission REC Buffer.
When only one CM is installed, one REC Disk Buffer is allocated.
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.
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 S5 series, support REC Disk Buffers.
• ETERNUS AF series
• ETERNUS DX S5 series (*1)
• ETERNUS DX S4 series (*2)
• ETERNUS DX S3 series (*3)
*1: The ETERNUS DX60 S5 is not supported.
*2: The ETERNUS DX60 S4 and the ETERNUS DX8100 S4 are not supported.
*3: The ETERNUS DX60 S3 is not supported.
111 Design Guide