Table 6.3 Rec Data Transfer Mode - Fujitsu ETERNUS DX80 S2 Overview

Chapter 6 Optional Functions
6.2 Backup (Advanced Copy)
The REC data transfer mode has two modes: the synchronous transmission mode and the asynchronous
transmission mode. These modes can be selected according to the intended use of REC.
Table 6.3 REC data transfer mode
Data transfer mode
Synchronous
transmission mode
Asynchronous
transmission mode
■ Synchronous transmission mode
Data that is updated in a copy source is immediately copied to the copy destination. Write completion signals
to write requests for the server are only returned after both the write to the copy source and the copy to the
copy destination have been done. Synchronizing the data copy with the copy source write guarantees the
contents of the copy source and copy destination at the time of completion.
■ Asynchronous transmission 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. Therefore, transmission delays in the WAN do not effect REC operation.
Asynchronous transmission mode has two sub-modes, the Stack mode and the Consistency mode. Selection of
mode will depend on usage pattern. There is also Through mode that is used to finalize data transfer for the
other two modes.
• 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. Actual transfer of the recorded blocks can be performed by an
independent transfer process.
This mode can be used even when the available 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 device in the
same order as the writes occurred. Even if a problem occurs with the data transfer order due to a transmis-
sion delay in the WAN, the update order in the copy destination is maintained.
This is useful when mirroring databases and copies of multiple connected areas.
This mode uses part of the cache 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.
• Through mode
After an I/O response is returned, this mode copies updated data as an extension of the process.
This 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 all data that has not been
transferred or to resume transfers.
I/O response
Affected by transmission delay
Not affected by transmission
delay
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ETERNUS DX80 S2/DX90 S2 Disk storage system Overview
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