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Hitachi Command Control Interface (CCI) User and Reference Guide 25
2.4.2.1 Sidefile Cache for Hitachi TrueCopy 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 Hitachi 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 Hitachi
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 2.10).
Cache
(Total cache minus
FlashAccess)
Sidefile Area
Writing response are delayed.
Writing are waited until under threshold.
High water mark
30% of Cache
For TagmaStore USP
(0 to 70%) of Cache
the default is 40%
Sidefile Threshold =
30% Æ 70% of Cache
Figure 2.10 Sidefile Quantity Limit
Sidefile area: Sidefile area = 30% to 70% of cache as set on Storage Navigator (or SVP)
(default sidefile = 40% for TagmaStore USP/NSC, 50% for 9900V/9900).
Write I/O control at high-water mark (HWM): When the quantity of data in sidefile reaches
30% of cache, the Hitachi TrueCopy Async pair status is HWM of PAIR state, and the host
write I/Os receive 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 at sidefile threshold: When the quantity of data in sidefile reaches the
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 between 1 second
and 255 seconds (600 for seconds for Universal Replicator). The timeout value is defined on
Storage Navigator (or SVP) and specifies the maximum delay between the M-VOL update and
the corresponding R-VOL update. The default timeout value is 90 seconds, 60 seconds for
Universal Replicator. If the timeout occurs during this waiting 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.