Software User Manual

Continuous Access Suspend Types” (page 34) for further information on suspended XP
Continuous Access Asynchronous pairs.
The RCU does not allow an XP Continuous Access S-VOL to be online (mounted) during normal XP
Continuous Access operations and rejects all host-requested write I/O operations for an S-VOL.
The special XP Continuous Access S-VOL write enable option enables write access to an S-VOL
while the pair is split (see “S-VOL Write Option” (page 19)). The S-VOL write option can only be
enabled when you split the pair from the MCU.
S-VOL Write Option
For additional flexibility, XP Continuous Access provides an S-VOL write option that enables write
access to the S-VOL of a split XP Continuous Access pair. The S-VOL write option can be selected
by the user during the pairsplit-r operation and applies only to the selected pair(s). The S-VOL write
option can be accessed only when you are connected to the MCU. When you resync a split XP
Continuous Access pair which has the S-VOL write option enabled, the RCU sends the S-VOL
differential data to the MCU, and the MCU merges the P-VOL and S-VOL differential data to
determine which tracks are out-of sync. This ensures proper resynchronization of the pair.
Difference Management
The differential data (updated by write I/Os during a split or suspension) between the P-VOL and
S-VOL is stored in each bitmap. When a split/suspended pair is resynchronized (pairresync), the
MCU merges the P-VOL and S-VOL bitmaps, and the differential data is copied to the S-VOL.
The maximum number of pairs you can create is restricted. Use the number of cylinders and bitmap
areas to calculate the maximum number of pairs that can be created in the storage system.
• The type of difference management (track or cylinder):
The differential data between P-VOL and S-VOL is stored in units of tracks or cylinders.
CAUTION: You may specify the type of difference management when you create a pair. If
an XP24000/XP20000 disk array is connected with an XP12000/XP10000 disk array, the
type of difference management is applied as it is when you specify the unit. If an
XP24000/XP20000 disk array is connected with an XP12000/XP10000 disk array, the
maximum number of pairs that you can create is the lesser number by comparing both numbers.
If the type of difference management is set to Auto, Track or Cylinder will be set automatically
according to the number of cylinders of the volume for which a pair will be created. If CVS
has been used, the number of cylinders that you set with the CVS software is applied. If the
volume with which you want to create a pair has 10,019 or more cylinders, Cylinder will be
set. If the volume has less than 10,019 cylinders, Track will be set.
If an XP24000/XP20000 disk array is connected with an XP12000/XP10000 disk array,
the type of difference management that you can specify depends on whether you use Remote
Web Console or RAID Manager:
◦ Remote Web Console: Auto, Track and Cylinder can be specified.
◦ RAID Manager: Track and Cylinder can be specified. If you specified nothing, the type
of difference management will be Track.
• The number of cylinders:
The number of cylinders depends on the capacity of the volumes (the number of cylinders set
by CVS, if you set the CVS feature) used for the pairs. Both the MCU and RCU are restricted.
To calculate the number of cylinders, use the following formulas:
◦ For OPEN-3, OPEN-8, OPEN-9, OPEN-E, OPEN-L, OPEN-K:
( ( ( (Max. LBA ÷ 96) ) ÷ 15) ) + 1
XP Continuous Access Remote Copy Operations 19