SRDF write operations
This section describes SRDF write operations.
Write operations in synchronous mode
In synchronous mode, data must be successfully written to cache at the secondary
site before a positive command completion status is returned to the host that issued
the write command.
The following image shows the steps in a synchronous write operation:
1. The local host sends a write command to the local array.
The host emulations write data to cache and create a write request.
2. SRDF emulations frame updated data in cache according to the SRDF protocol,
and transmit it across the SRDF links.
3. The SRDF emulations in the remote array receive data from the SRDF links, write
it to cache and return an acknowledgment to SRDF emulations in the local array.
4. The SRDF emulations in the local array forward the acknowledgment to host
emulations.
Figure 30 Write I/O flow: simple synchronous SRDF
SRDF/S
Host
Drive
emulations
Cache
R2
Drive
emulations
Cache
R1
1
2
34
Write operations in asynchronous mode
In asynchronous mode (SRDF/A), host write I/Os are collected into delta sets on the
primary array and transferred in cycles to the secondary array.
SRDF/A sessions behave differently depending on:
l
Whether they are managed individually (Single Session Consistency (SSC)) or as a
consistency group (Multi Session Consistency (MSC)).
n
In Single Session Consistency (SSC) mode, the SRDF group is managed
individually, with cycle switching controlled by Enginuity or HYPERMAX OS.
SRDF/A cycles are switched independently of any other SRDF groups on any
array in the solution. Cycle switching in asynchronous mode on page 129
provides additional details.
n
In Multi Session Consistency (MSC) mode, the SRDF group is part of a
consistency group spanning all associated SRDF/A sessions. Cycle switching is
coordinated to provide dependent-write consistency across multiple sessions,
which may also span arrays. Cycle switching controlled by SRDF host software.
SRDF/A cycles are switched for all SRDF groups in the consistency group at
the same time. SRDF/A MSC cycle switching on page 130 provides additional
details.
Remote replication solutions
SRDF write operations 127
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