v Fiber-optic cables
If error codes sent you here, complete the following steps:
Procedure
1. Verify that the power is turned on to all switches and storage controllers that
the Storwize V7000 Unified system uses, and that they are not reporting any
hardware failures. If problems are found, resolve those problems before you
proceed further.
2. Verify that the Fibre Channel cables that connect the systems to the switches
are securely connected.
3. If you have a SAN management tool, use that tool to view the SAN topology
and isolate the failing component.
iSCSI performance analysis and tuning
This procedure provides a solution for Internet Small Computer Systems Interface
(iSCSI) host performance problems while connected to a Storwize V7000 Unified
system and its connectivity to the network switch.
About this task
Some of the attributes and host parameters that might affect iSCSI performance:
v Transmission Control Protocol (TCP) Delayed ACK
v Ethernet jumbo frame
v Network bottleneck or oversubscription
v iSCSI session login balance
v Priority flow control (PFC) setting and bandwidth allocation for iSCSI on the
network
Procedure
1. Disable the TCP delayed acknowledgment feature.
To disable this feature, refer to OS/platform documentation.
v VMWare: http://kb.vmware.com/selfservice/microsites/microsite.do
v Windows: http://support.microsoft.com/kb/823764
The primary signature of this issue: read performance is significantly lower
than write performance. Transmission Control Protocol (TCP) delayed
acknowledgment is a technique that is used by some implementations of the
TCP in an effort to improve network performance. However, in this scenario
where the number of outstanding I/O is 1, the technique can significantly
reduce I/O performance.
In essence, several ACK responses can be combined together into a single
response, reducing protocol overhead. As described in RFC 1122, a host can
delay sending an ACK response by up to 500 ms. Additionally, with a stream of
full-sized incoming segments, ACK responses must be sent for every second
segment.
Important: The host must be rebooted for these settings to take effect. A few
platforms (for example, standard Linux distributions) do not provide a way to
disable this feature. However, the issue was resolved with the version 7.1
release, and no host configuration changes are required to manage
TcpDelayedAck behavior.
2. Enable jumbo frame for iSCSI.
376 Storwize V7000 Unified: Problem Determination Guide 2073-720
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