Cisco Cisco MDS 9000 XRC Acceleration White Paper
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FCIP is a technology where the Fibre Channel frames are encapsulated into an IP packet and sent over to the other site on a dedicated TCP/IP
tunnel created for this purpose. This technology can very effectively deliver high throughput over long distances, but the latency of the I/O
depends on the distance.
depends on the distance.
These replication technologies can use either synchronous replication or asynchronous replication methods.
Synchronous replication, also referred to as real-time replication, implies that an I/O is not complete until both sides acknowledge its completion.
To maintain consistency of data, every write to the primary storage is written over to the secondary storage in the same sequence before the
application writing is considered successful. This method helps ensure that data stored at both sites is always consistent, and the only data that
could be lost in the event of a failure is the data that was not committed or was being transmitted at the time of the failure. The disadvantage of
performing synchronous replication is that Fibre Channel protocol (FCP) defines four steps, or two round trips, before every I/O can complete.
The speed of light becomes the limiting factor because a latency of 1 millisecond (ms) is introduced for every 50 km of distance between the data
centers, even with the use of dark fiber.
application writing is considered successful. This method helps ensure that data stored at both sites is always consistent, and the only data that
could be lost in the event of a failure is the data that was not committed or was being transmitted at the time of the failure. The disadvantage of
performing synchronous replication is that Fibre Channel protocol (FCP) defines four steps, or two round trips, before every I/O can complete.
The speed of light becomes the limiting factor because a latency of 1 millisecond (ms) is introduced for every 50 km of distance between the data
centers, even with the use of dark fiber.
Asynchronous replication, often referred to as “store-and-forward” replication, stores changes to the data at the primary storage site for a
predetermined amount of time and then forwards the changes to the secondary storage system for updates. This way the secondary storage system
lags behind the primary storage system by a fixed amount of time. This technology allows the data at the secondary storage system to be
synchronized with the primary storage system in a given amount of time. Some data changes may not be recoverable if those changes were still
stored on the primary storage system and were not yet forwarded at the time of the outage.
lags behind the primary storage system by a fixed amount of time. This technology allows the data at the secondary storage system to be
synchronized with the primary storage system in a given amount of time. Some data changes may not be recoverable if those changes were still
stored on the primary storage system and were not yet forwarded at the time of the outage.
Given the different technologies and methods of replication, the challenge for data-center managers is to ensure that the data is replicated in a timely
manner without affecting the performance of the applications using that data. The best possible solution for a data-center manager is to implement
synchronous replication on a wavelength division multiplexing solution. However the distance between the data centers, which, with the existing
technologies, cannot exceed 80 to 100 km without affecting the applications, limits this solution.
manner without affecting the performance of the applications using that data. The best possible solution for a data-center manager is to implement
synchronous replication on a wavelength division multiplexing solution. However the distance between the data centers, which, with the existing
technologies, cannot exceed 80 to 100 km without affecting the applications, limits this solution.
To mitigate this issue, Cisco Systems
®
introduced Fibre Channel Write Acceleration (FC-WA), an intelligent feature in the Cisco MDS 9000 Family
switches. FC-WA accelerates the writing process over long distances, thus helping perform synchronous replication over significantly longer
distances than what was previously possible.
distances than what was previously possible.
SOLUTION
Cisco MDS 9000 Family Storage Services Module
The Cisco MDS 9000 Family Storage Services Module provide the intelligent service of identifying the SCSI I/O flow for a given initiator-target
pair. This information is used to provide the FC-WA feature and the feature to gather advanced I/O statistics for a given initiator-target pair. The
FC-WA feature decreases the latency of an I/O over long distances and the advanced I/O statistics collected can be used to evaluate the storage
performance for the initiator-target pair.
pair. This information is used to provide the FC-WA feature and the feature to gather advanced I/O statistics for a given initiator-target pair. The
FC-WA feature decreases the latency of an I/O over long distances and the advanced I/O statistics collected can be used to evaluate the storage
performance for the initiator-target pair.
The Storage Services Module incorporates all the capabilities of the Cisco MDS 9000 Family 32-Port Fibre Channel Switching Module and also
provide scalable and intelligent storage services. The Storage Services Module features a highly distributed processing architecture and hardware-
based SCSI processing, capable of optimizing storage access. Data is processed in band by an application-specific processor called Data Path
Processor (DPP). The control commands and information are processed by a dedicated Control Path Processor (CPP). To deploy FC-WA in the
SAN, both the initiator and target devices must be directly attached to a Storage Services Module.
provide scalable and intelligent storage services. The Storage Services Module features a highly distributed processing architecture and hardware-
based SCSI processing, capable of optimizing storage access. Data is processed in band by an application-specific processor called Data Path
Processor (DPP). The control commands and information are processed by a dedicated Control Path Processor (CPP). To deploy FC-WA in the
SAN, both the initiator and target devices must be directly attached to a Storage Services Module.
Fibre Channel Write Acceleration
FC-WA minimizes storage latency and improves the number of application transactions per second over long distances. It increases the distance of
replication or reduces effective latency to improve performance during synchronous replication.
replication or reduces effective latency to improve performance during synchronous replication.
The improved performance results from a coordinated effort performed by the Storage Services Module local to the initiator and the Storage Services
Module local to the target. The initiator Storage Services Module, bearing the host-connected intelligent port (HI-port), allows the initiator to send
Module local to the target. The initiator Storage Services Module, bearing the host-connected intelligent port (HI-port), allows the initiator to send