HP Hitachi Dynamic Link Manager Software - Manual (Page 34)

When an intermittent error #2 occurs in a path, automatic failback repeatedly
places the path offline and then online again, which can cause I/O
performance to drop. In this case, we recommend that you configure
intermittent error monitoring to exclude paths with intermittent errors from
the automatic failback process.
Automatic failback is implemented by functionality provided by VMware
vSphere ESXi, and cannot be disabled.
Intermittent error monitoring can be specified in the HDLM command's set
operation. For details on the set operation, see set (Sets Up the Operating
Environment) on page 6-15 .
#1
This storage system applies when the dynamic I/O path control function is
disabled.
#2
An intermittent error means an error that occurs irregularly because of
some reason such as a loose cable connection.
Manual Path Switching
You can switch the status of a path by manually placing the path online or
offline. Manually switching a path is useful, for example, when system
maintenance needs to be done.
You can manually place a path online or offline by doing the following:
• Execute the dlnkmgr command's online or offline operation.
For details on the online operation, see
online (Places Paths Online) on
page 6-11 . For details on the offline operation, see offline (Places
Paths Offline) on page 6-7 .
However, if there is only one online path for a particular LU, that path cannot
be manually switched offline. Also, a path with an error that has not been
recovered from yet cannot be switched online.
HDLM uses the same algorithms to select the path that will be used next,
regardless of whether automatic or manual path switching is used.
When using the Hitachi AMS/WMS series, HDLM will select the next path to be
used first from among the online owner paths, and then from the online non-
owner paths. When the Universal Storage Platform V/VM series, Virtual
Storage Platform series, Hitachi AMS2000 series
# , Hitachi SMS series # ,
HUS100 series # , or HUS VM is being used, all the paths that access the same
LU as the path that is currently being used are candidates for the switching
destination path.
Executing the online operation places the offline path online. For details on
the online operation, see online (Places Paths Online) on page 6-11 . After
the path status is changed to online, HDLM selects the path to use in the
same way as for automatic path switching. When the Hitachi AMS/WMS series
is being used, HDLM selects the path to use from online owner paths, and
2-16
HDLM Functions
Hitachi Dynamic Link Manager User Guide for VMware ®