Table 32 Rec Data Transfer Mode - Fujitsu ETERNUS DX100 S4 Design Manual

2. Basic Functions
Backup (Advanced Copy)
The REC data transfer mode has two modes: the synchronous transfer mode and the asynchronous
transfer mode. These modes can be selected according to whether importance is placed upon I/O
response time or complete backup of the data is performed until the point when a disaster occurs.

Table 32 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 comple-
tion 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
data that is written to the copy source guarantees the contents of the copy source and copy desti-
nation 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.
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 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 de-
vice in the same order as the writes occurred. Even if a problem occurs with the data transfer or-
der 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 databa-
ses 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 transferred or to resume transfers.
Fujitsu Storage ETERNUS DX100 S4/DX200 S4, ETERNUS DX100 S3/DX200 S3 Hybrid Storage Systems Design Guide (Basic)
I/O response
Affected by transmission delay
Not affected by transmission de-
lay
92
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Updated log status in the case of disaster
Data is completely backed up until the point
when a disaster occurs.
Data is backed up until a few seconds before
a disaster occurs.
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