Cisco Systems OL-12518-01 manual Using Fcip Tape Acceleration, Write Acceleration

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.

Data Center High Availability Clusters Design Guide

OL-12518-01 4-7