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Data Center High Availability Clusters Design Guide
OL-12518-01
Chapter 4 FCIP over IP/MPLS Core
Using FCIP Tape Acceleration
Write Acceleration
Write Acceleration is a configurable feature introduced in SAN-OS 1.3 that enhances FCIP SAN
extension with the IP Storage Services Module. Write Acceleration is a SCSI protocol spoofing
mechanism that improves application performance by reducing the overall service time for SCSI write
input/output (I/O) operations and replicated write I/Os over distance. Most SCSI Fibre Channel Protocol
(FCP) write I/O exchanges consist of two or more round trips between the host initiator and the target
array or tape. Write Acceleration reduces the number of FCIP WAN round trips per SCSI FCP write I/O
to one.
Write Acceleration is helpful in the following FCIP SAN extension scenarios:
• Distance and latency between data centers inhibits synchronous replication performance and
impacts overall application performance.
• Upper layer protocol chattiness inhibits replication throughput, and the underlying FCIP and IP
transport is not optimally utilized.
• Distance and latency severely reduces tape write performance during remote tape backup because
tapes typically allow only a single outstanding I/O. Write Acceleration can effectively double the
supported distance or double the transfer rate in this scenario.
• Shared data clusters are stretched between data centers and one host must write to a remote storage
array.
The performance improvement from Write Acceleration typically approaches 2 to 1, but depends upon
the specific situation.
Write Acceleration increases replication or write I/O throughput and reduces I/O response time in most
situations, particularly as the FCIP Round Trip Time (RTT) increases. Each FCIP link can be filled with
a number of concurrent or outstanding I/Os. These I/Os can originate from a single replication source or
a number of replication sources. The FCIP link is filled when the number of outstanding I/Os reaches a
certain ceiling. The ceiling is mostly determined by the RTT, write size, and available FCIP bandwidth.
If the maximum number of outstanding I/Os aggregated across all replication sessions (unidirectional)
is less than this ceiling, then the FCIP link is underutilized and thus benefits from Write Acceleration.
Using FCIP Tape Acceleration
FCIP Tape Acceleration is a new feature introduced in SAN-OS 2.0 to improve remote tape backup
performance by minimizing the effect of network latency or distance on remote tape applications. With
FCIP Tape Acceleration, the local Cisco MDS 9000 IPS or MPS module proxies as a tape library. The
remote MDS 9000, where the tape library is located, proxies as a backup server.
Similar to Write Acceleration, the MDS 9000 recognizes and proxies elements of the upper level SCSI
protocol to minimize the number of end-to-end round trips required to transfer a unit of data and to
optimally use the available network bandwidth. FCIP Write Acceleration achieves this by proxying the
SCSI Transfer Ready and Status responses (in contrast, Write Acceleration proxies the Transfer Ready
only). Write Filemarks and other non-write operations are not proxied and are passed directly to the
remote tape library. The Write Filemarks operation corresponds to a checkpoint within the tape backup
application. This is typically a tunable parameter but may default to 100 or 200 records depending upon
the tape backup product.