Compression; Fastwrite - Brocade Communications Systems A7990A - StorageWorks SAN Director 4/16 Blade Switch Administrator's Manual

Compression

The compression feature is a per-tunnel feature that allows the Fibre Channel data frames to be
compressed before they sent over the tunnel as FCIP frames. The compression can be enabled or
disabled using the portCfg command. The configuration of this feature is optional and by default the
feature is disabled.
The Fibre Channel command frames and all the frames that are generated from or destined for the
control processor are not compressed even when compression is enabled on a tunnel. The uncompressed
and compressed packets and bytes are counted and maintained on a per tunnel basis. The compression
ratio can be calculated for a given polling interval by reading the number of compressed and
uncompressed bytes. Compression is performed on the Fibre Channel data frames in the ingress
direction before they are encapsulated as FCIP frames.

Fastwrite

Fastwrite reduces the number of round-trip times required to complete a SCSI write I/O. This results in
both reducing I/O completion latency and increasing FCIP ISL bandwidth utilization. The FCIP port
intercepts the SCSI write command and immediately responds to the host with a Transfer Ready for the
entire SCSI write command data length. The SCSI write command is forwarded across the FCIP link
toward the target. The host can immediately begin to send data to the target.
At the other end of the FCIP link, the FCIP port intercepts Transfer Readies from the target. This port
buffers the SCSI write data received from host via the FCIP tunnel and forwards it to the target as
permitted by the target's Transfer Readies. The Status response for the SCSI write I/O is sent from the
target to the host. The status response is intercepted by both FCIP ports to allow for cleanup of any state
and resources held for the SCSI write I/O.
Note
Switches at both end of the tunnels must enable fastwrite; it is supported only on switches running
Fabric OS 5.2.0 or higher.
FCIP fastwrite reduces the number of round trip "waits" for SCSI write I/Os and increases the
performance. This reduction of SCSI write I/O completion latency and resulting improvement in FCIP
link utilization depends on the number of outstanding I/Os between hosts and targets utilizing the link.
Fastwrite is not supported when there are two or more tunnels with the same cost path going between
two SilkWorm 7500s and FR4-18i blades. (For more information on cost paths, see the
Configuring FCR Router Port Cost"
take any of these equal cost paths (tunnels) and I/O would fail. When you enable fastwrite, only one
FCIP tunnel with a low cost metric can exist between the host and target. The other tunnels have higher
cost metrics, otherwise, the result will be I/O failure.
Web Tools Administrator's Guide
Publication Number: 53-0000194-01
on page 11-8.) Under these conditions routing of I/O frames could
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