HP StorageWorks XP Remote Web Console Software - Manual (Page 17)

NOTE: For further information on cylinder/track difference management, see “Difference
Management” (page 19) .
When the S-VOL write enable pairsplit option is used, the RCU keeps track of S-VOL updates and
sends the S-VOL differential data to the MCU when the split pair is resynchronized. In this case,
the MCU merges the P-VOL and S-VOL differential data to identify the out-of-sync tracks.
For additional flexibility, XP Continuous Access provides the following options for initial copy:
•
The number of tracks option allows you to specify how many tracks are copied simultaneously
by the XP Continuous Access initial copy operation when creating/resynchronizing an XP
Continuous Access pair. This option can only be specified using the XP Continuous Access
software (not RAID Manager).
•
The initial copy priority option allows you to specify the order in which the initial copy
operations are performed when creating/resynchronizing multiple XP Continuous Access
pairs. This option can only be specified using the XP Continuous Access software (not RAID
Manager).
•
The Maximum initial copy activity option allows you to specify the maximum number of
concurrent initial copy and resync operations that each MCU can perform (not pair-specific).
This option can only be specified using the XP Continuous Access software (not RAID Manager).
•
The Round Trip Time option is a reference value used for automatic control of the initial copy
pace and for avoiding the remote I/O response time to the update I/Os being affected by
initial copy while the copy operation is in progress. The value of Round Trip Time is compared
with the remote I/O response time, and then, the copy pace is controlled when the difference
between the two values is large.
If the difference between the remote I/O response time and the value of Round Trip Time is
large, for example, the remote I/O response time is 500 ms but the value of Round Trip Time
is 1 ms, the copy pace will slow or sometimes the initial copy will interrupt, so that the initial
copy process does not occupy the entire bandwidth of the lines.
On the other hand, if the difference between the remote I/O response time and the value of
Round Trip Time is small, for example, the remote I/O response time is 5 ms and the value of
Round Trip Time is 1 ms, the initial copy runs at a specified pace.
The default value of Round Trip Time is 1 ms. Specify an appropriate value at the Round Trip
Time when the distance between MCU and RCU is long or there is a delay caused by the line
equipment. If the initial copy operation is performed with the default value of Round Trip Time
instead of the appropriate value, it may take unexpectedly long time to complete the initial
copy operation.
Update Copy Operation
An update copy operation occurs when the host issues a write I/O operation to the XP Continuous
Access P-VOL pair. The update copy operation duplicates the P-VOL write I/O at the S-VOL to
keep the pair synchronized. XP Continuous Access provides two modes for update copy operations:
synchronous and asynchronous. The update copy mode is specified when you add an XP Continuous
Access pair and cannot be changed.
For the synchronous update copy mode, the MCU ensures that the P-VOL and S-VOL are
synchronized at all times. The MCU does not return a final ending status for the P-VOL write I/O
until both the P-VOL write and its associated update copy operation at the RCU are complete. For
XP Continuous Access Remote Copy Operations 17