HP XP P9500 Storage - Manual (Page 20)

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Continuous Access Synchronous supports LUN Expansion (LUSE). This allows you to configure
expanded LUs using two or more contiguous LDEVs (up to 36).
◦ If two LUSE volumes are paired with Continuous Access Synchronous, a LUSE P-VOL must
be paired with an S-VOL of the same size and structure. For example, if a LUSE P-VOL is
connected with volumes of 1GB, 2GB, and 3GB in this order, you must specify a LUSE
volume that has exactly the same size and the same connection order as the S-VOL.
◦ The number of LDEV pairs in a Continuous Access Synchronous LUSE pair equals the
number of LDEVs in each LUSE volume.
◦ When the status of all LDEV pairs within one Continuous Access Synchronous LUSE pair
is not the same (for example, one LDEV pair is suspended), the pair status of the Continuous
Access Synchronous LUSE pair changes to PDUB.
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Continuous Access Synchronous also supports Virtual LVI/LUN. This allows you to configure
LUs that are smaller than standard LUs. When custom-size LUs are assigned to a Continuous
Access Synchronous pair, the S-VOL must have the same capacity as the P-VOL. Remote Web
Console displays the size of P-VOLs and S-VOL.
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When creating multiple pairs concurrently, make sure that you set up S-VOL LUNs in a way
that allows the system to correctly match them to your selected P-VOLs.
Even though you select multiple volumes as P-VOLs in the Remote Web Console Paircreate
procedure, only one of them displays in the dialog box—the one with the lowest LUN. Because
of this, you are able to specify only one S-VOL. The system automatically assigns LUs on the
secondary system as S-VOLs for the other selected P-VOLs according to LUN.
For example, three LUs have been set up on the secondary system to become an S-VOL, named
LUN001, LUN002, and LUN003. During the Paircreate operation, you select three volumes
as P-VOLs, and then select LUN001 as the S-VOL. The system will then assign LUN002 and
LUN003 as the S-VOLs for the other two P-VOLs. Therefore, make sure that you assign LUNs
to your S-VOL that will be selected in the proper order.
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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 (OS) 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.
Continuous Access Synchronous does not allow the S-VOL to be online (except when the pair
is split). If the S-VOL is online, the Continuous Access Synchronous 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.
Preparations for allowing I/O to the S-VOL
The secondary system rejects write I/O to the S-VOL, unless the S-VOL-write option is enabled.
Then, read and write I/O is allowed to the S-VOL while the pair is split. In this instance, S-VOL
20 Planning for Continuous Access Synchronous