For example, if the LUN numbers of three P-VOLs are 1, 5, and 6; and
you set LUN numbers for the initial S-VOL (Base Secondary Volume)
at 2, the LUN numbers of the three S-VOLs will be set at 2, 6, and 7,
respectively.
• Because the contents of the P-VOL and S-VOL are identical, the S-VOL
can be considered a duplicate of the P-VOL. Because the host operating
system does not allow duplicate volumes, the host system administrator
must take precautions to prevent system problems related to duplicate
volumes. You must define the S-VOLs so they do not auto mount or come
online to the same host at the same time as the P-VOLs.
TrueCopy does not allow the S-VOL to be online (except when the pair is
split). If the S-VOL is online, the TrueCopy paircreate operation will fail.
Caution: When S-VOLs and P-VOLs are connected to the same hosts, it is
strongly recommended that you define the S-VOLs to remain offline. This
is because under this condition, the S-VOL is usually offline when a pair is
released. If the host is then restarted, the system administrator may be
offered both volumes and asked which volume should be left offline. This
can be confusing and is prone to error.
Allowing I/O to the S-VOL
The secondary system rejects write I/O to the S-VOL, unless the S-VOL-write
operation is enabled. When this option is enabled, read and write I/O is
allowed to the S-VOL while the pair is split. In this case, S-VOL and P-VOL
track maps keep track of differential data and are used to resynchronize the
pair. Enabling S-VOL-write is done during the pairsplit operation.
• You can write to the S-VOL when the split operation is performed from
the primary system.
• When you resync a pair with the S-VOL write operation enabled, the
secondary system sends S-VOL differential data to the primary system.
This data is merged with P-VOL differential data, and out-of sync tracks
are determined and updated on both systems, thus ensuring proper
resynchronization.
Allowing I/O to the P-VOL after a split—Fence Level options
You can specify whether the host is denied access or continues to access the
P-VOL when the pair is split due to an error. This is done with the Primary
Volume Fence Level setting. You specify one of the following Fence Level
options during the initial copy and resync operations. You can also change the
Fence Level option outside these operations.
• Data – the P-VOL is fenced if an update copy operation fails. This
prevents the host from writing to the P-VOL during a failure. This setting
should be considered for the most critical volumes for disaster recovery.
This setting reduces the amount of time required to analyze the
consistency of S-VOL data with the P-VOL during disaster recovery
efforts.
3-10
Planning for TrueCopy
Hitachi TrueCopy® User Guide for Hitachi Virtual Storage Platform Gx00 and Fx00 Models
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