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V7000: Background copy bandwidth impact on foreground I/O latency


Background copy bandwidth impact on foreground I/O latency
The background copy bandwidth determines the rate at which the background copy for Metro Mirror or Global Mirror Copy Services are attempted.
The background copy bandwidth can affect foreground I/O latency in one of three ways:
•    If the background copy bandwidth is set too high for the intercluster link capacity, the following results can occur:
o    The intercluster link is not able to process the background copy I/Os fast enough, and the I/Os can back up (accumulate).
o    For Metro Mirror, there is a delay in the synchronous secondary writes of foreground I/Os.
o    For Global Mirror, the work is backlogged, which delays the processing of writes and causes the relationship to stop.
o    The foreground I/O latency increases as detected by applications.
•    If the background copy bandwidth is set too high for the storage at the primary site, background copy read I/Os overload the primary storage and delay foreground I/Os.
•    If the background copy bandwidth is set too high for the storage at the secondary site, background copy writes at the secondary overload the secondary storage and again delay the synchronous secondary writes of foreground I/Os.
o    For Global Mirror, the work is backlogged and again the relationship is stopped
To set the background copy bandwidth optimally, you must consider all three resources (the primary storage, the intercluster link bandwidth, and the secondary storage). Provision the most restrictive of these three resources between the background copy bandwidth and the peak foreground I/O workload. You must also consider concurrent host I/O because if other write operations arrive at the primary cluster for copy to the remote site, these write operations can be delayed by a high level of background copy and the hosts at the primary site receive poor write-operation response times.
The provisioning for optimal bandwidth for the background copy can also be calculated by determining how much background copy can be allowed before performance of host I/O becomes unacceptable. The background copy bandwidth can be decreased slightly to accommodate peaks in workload and provide a safety margin for host I/O.
Example
If the bandwidth setting at the primary site for the secondary cluster is set to 200 MBps (megabytes per second) and the relationships are not synchronized, the Storwize V7000 attempts to resynchronize the relationships at a maximum rate of 200 MBps with a 25 MBps restriction for each individual relationship. The Storwize V7000 cannot resynchronize the relationship if the throughput is restricted. The following can restrict throughput:
•    The read response time of backend storage at the primary cluster
•    The write response time of the backend storage at the secondary site
•    Intercluster link latency