HP StorageWorks XP24000 Continuous Access Software User and Reference Guide, v01 (T5278-96002, June 2007)

* Since the record set has not yet arrived a t the RCU, S2 does not have a time stamp now.
Figure 3 Selec ting and Settling Continuous Access Async hron ous Recordsets at the RCU
TypesofRecordsets
In addition to host update recordsets, the MCU passes control information to the RCU in special
non-update recordsets. These special recordsets indicate when pair status changes and when an MCU
power-off sequence is initiated, and also maintain sequenc e numbers in periods of low host activities.
Inflow Control of Recordsets
As descr
ibed in the previous sections, both the MCU and RCU create sidefiles for storing Continuous
Access
(and
TrueCopy for Ma inframe) Asynchronous recordsets. Since the sidefiles occupy exclusive
space
in cache, b oth the MCU and RCU p erform inflow control to prevent an overload of the storage
system’s cache resources. The storage systems use the following parameters for Continuous Access
Asynchronous cache inflow control, and the Continuous Access Asynchronous O ption dialog box (see
“Setting th e Asynchronous Copy Options” on page 84) allows you to modif y these parameters:
• Pending Update D ata Rate (%):sidefile (Continuous Access) threshold = maximum cache %
available for use by Continuous Access Asynchronous sidefiles.
• I/O D
elay Start (%): the threshold, which is the maximum amount of cache for starting the I/O
inflo
w
control.
• I/O Delay Increase (%): the threshold, which is the maximum amount of cache for starting to
strengthen control over the I/O inflow.
Inflow Control by MCU. When the amount of MCU sidefile cachereaches theuser-specified threshold
(I/O delay star t), the storage system’s I/O response is delayed. When the amount of MCU sidefile
cache reaches the user-specified threshold (I/ O delay increase), the storage system’s I/O delay starts
to
increase. If the MCU is not able to send a recordset to the RCU within the user-speci fied offloading
timer value due to the condition of the amount of MCU sidefile cacheexceeding thesidefile threshold
continuously, the MCU suspends all affected Continuous Access Asynchronous pairs and resets the
S
CP
condition to avoid hanging up the system.
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