Figure 34 Write commands to R21 devices
SRDF/S
SRDF/A
or
Adaptive copy disk
Site C
Host
Site A Site B
R1
R2
Cache Cache
Cache
R21
When a write command arrives to cache in Site B:
l
The SRDF emulation at Site B sends a positive status back across the SRDF links
to Site A (synchronous operations), and
l
Creates a request for SRDF emulations at Site B to send data across the SRDF
links to Site C.
SRDF/A cache management
Unbalanced SRDF/A configurations or I/O spikes can cause SRDF/A solutions to use
large amounts of cache. Transient network outages can interrupt SRDF sessions. An
application may write to the same record repeatedly. This section describes the
SRDF/A features that address these common problems.
Tunable cache
You can set the SRDF/A maximum cache utilization threshold to a percentage of the
system write pending limit for an individual SRDF/A session in single session mode and
multiple SRDF/A sessions in single or MSC mode.
When the SRDF/A maximum cache utilization threshold or the system write pending
limit is exceeded, the array exhausts its cache.
By default, the SRDF/A session drops if array cache is exhausted. You can keep the
SRDF/A session running for a user-defined period. You can assign priorities to
sessions, keeping SRDF/A active for as long as cache resources allow. If the condition
is not resolved at the expiration of the user-defined period, the SRDF/A session still
drops.
Use the features described below to prevent SRDF/A from exceeding its maximum
cache utilization threshold.
SRDF/A cache data offloading
If the system approaches the maximum SRDF/A cache utilization threshold, DSE
offloads some or all of the delta set data. DSE can be configured/enabled/disabled
independently on the R1 and R2 sides.
EMC recommends that DSE be configured the same on both sides.
DSE works in tandem with group-level write pacing to prevent cache over-utilization
during spikes in I/O or network slowdowns.
Resources to support offloading vary depending on the version of Enginuity running on
the array.
Remote replication solutions
132 Product Guide VMAX 100K, VMAX 200K, VMAX 400K with HYPERMAX OS
To Next Page
Loading...
