Fujitsu ETERNUS DX500 S4/DX600 S4 Design Manual page 94

2. Basic Functions
Backup (Advanced Copy)
■
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.
The Stack mode and the Consistency mode are available in the Asynchronous transmission 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 inde-
pendent 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 device 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 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 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 transfer-
red or to resume transfers.
FUJITSU Storage ETERNUS DX500 S4/DX600 S4, ETERNUS DX500 S3/DX600 S3 Hybrid Storage Systems Design Guide (Basic)
94
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