HP XP P9500 Storage - Manual (Page 135)

Continuous Access Asynchronous and Continuous Access Journal volume characteristics
Continuous Access Asynchronous/Continuous Access Journal volumes have the following
characteristics:
•
PAIR state: A Continuous Access Asynchronous pair changes to the PAIR status as soon as all
pending recordsets have been placed in the queue at the primary volume, without waiting for
the updates to complete at the secondary volume.
•
Pair splitting:When a Continuous Access Asynchronous pair is split or deleted, all pending
recordsets at the primary volume are sent to the secondary volume, then the pair status changes
to PSUS or SMPL. With the pairsplit command only, updates for the primary volume that
occur during and after the pairsplit operation are marked on the bitmap of the primary volume.
•
Pair resynchronization: The pairresync command resynchronizes the secondary volume
based on the primary volume. This resynchronization does not guarantee the sequenced data
transfer.
•
Error suspending: Pending recordsets that have not yet been sent to the secondary volume are
marked on the bitmap of the primary volume, then deleted from the queue, and then the pair
status changes to PSUE.
•
Group operations: Continuous Access Asynchronous automatically registers the CTGIDs with
the storage system when paired volumes are created using the paircreate command, and
groups in the configuration file are mapped to their corresponding CTGIDs. If more than one
group, defined in the configuration definition file, is assigned to the same CTGID, then pair
operations on the group specificity apply to the entire consistency group.
Sidefile cache for Continuous Access Asynchronous
The first-in-first-out (FIFO) queue of each CT group is placed in an area of cache called the sidefile.
The sidefile is used for transferring Continuous Access Asynchronous recordsets to the RCU. The
sidefile is not a fixed area in cache but has variable capacity for write I/Os for the primary volume.
If the host write I/O rate is high and the MCU cannot transfer the Continuous Access Asynchronous
recordsets to the RCU fast enough, then the sidefile capacity expands gradually. The sidefile has
a threshold to control the quantity of data transfer of host side write I/O. Host side write I/Os are
controlled by delaying response when the sidefile exceeds the constant quantity limit on cache in
the storage system (see
Figure 55 (page 135) ).
Figure 55 Sidefile quantity limit
Sidefile area: Sidefile area = 30% to 70% of cache as set on Remote Web Console (default sidefile
= 50% for XP24000/XP20000 Disk Array, XP1024/XP128 Disk Array; 40% for XP12000 Disk
Array/XP10000 Disk Array).
Write I/O control via the high-water mark (HWM): When the quantity of data in the sidefile reaches
30% of cache, the Continuous Access Asynchronous pair status is HWM of PAIR state, and the
host write I/Os receive a delayed response in the range of 0.5 seconds to 4 seconds. Following
is an arithmetic expression of the HWM at 100% of a sidefile space:
HWM(%) = High water mark(%) / Sidefile threshold (30 to 70) * 100
Write I/O control via the sidefile threshold: When the quantity of data in the sidefile occupies the
maximum defined sidefile area, host write I/Os are delayed until there is enough sidefile space
Continuous Access Synchronous, Business Copy, and Continuous Access Journal operations 135