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Sidefile cache for TrueCopy Async
The first-in-first-out (FIFO) queue of each consistency group is placed in an
area of cache called the sidefile. The sidefile is used for transferring TrueCopy
Async 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 TrueCopy Async 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 6-10 Sidefile quantity limit on page 6-27 ).
Figure 6-10 Sidefile quantity limit
Sidefile area: Sidefile area = 30% to 70% of cache as set on Storage
Navigator (default sidefile = 50% for USP V/VM, 9900V; 40% for TagmaStore
USP/TagmaStore NSC).
Write I/O control via the high-water mark (HWM): When the quantity of data
in the sidefile reaches 30% of cache, the TrueCopy Async 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 to store the next new write data.
The copy pending timeout group option is defined using Storage Navigator
and specifies the maximum delay between the M-VOL update and the
corresponding R-VOL update. The range for the copy pending timeout option
is 1-255 seconds (600 seconds for Universal Replicator), and default value is
90 seconds (60 seconds for UR). If the timeout occurs during this wait state,
the pair status changes from PAIR to PSUS (sidefile full), and host write I/Os
continue with updates being managed by the cylinder bitmap. Important:
The copy pending timeout value should be less than the I/O timeout value of
the host system.
TrueCopy Async transition states and sidefile control
TrueCopy Async volumes have special states for sidefile control during status
transitions. Table 6-8 State table for TrueCopy vs. TrueCopy Async on page
6-28 shows the transition states for TrueCopy and TrueCopy Async volumes.
Data replication operations with CCI
6-27
Command Control Interface User and Reference Guide