Table 39 Rec Data Transfer Mode - Fujitsu ETERNUS AF S3 Series Design Manual

2. Basic Functions
Backup (Advanced Copy)
Figure 49
SAN
ETERNUS AF/DX
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 39 REC Data Transfer Mode
Data transfer mode
Synchronous transfer mode
Asynchronous transfer mode
■
Synchronous Transfer Mode
Data that is updated in a copy source is immediately copied to the copy destination. Completion signals for write
requests from the server are sent only after data is written to the copy source and copied to the copy destination.
Synchronizing the data copy with the data that is written to the copy source guarantees the contents of the copy
source and copy destination at the time of completion.
■
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.
REC
Main site
Management
server
Operating
volume
I/O response
Affected by transmission delay
Not affected by transmission delay
Remote copy
(REC)
WAN
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.
96
Backup site
Management
server
SAN
Backup
volume
ETERNUS AF/DX
Design Guide