The Question is:
The ECO ALPSHAD09_062 introduced the ability to control the rate of a volume
shadow merge by defining the system logical name SHAD$MERGE_DELAY_FACTOR.
If I change the value of this logical name during a merge, how long does it
take for the merge software to notice the change, and change the merge rate?
The Answer is :
Faster I/O subsystems including the HSZ50 and HSZ70 controllers were
taking longer to perform full shadowset merges than were some older
and slower I/O subsystems. Accordingly, changes were made to volume
shadowing to permit the system manager to adjust certain thresholds.
Two new logical names permit the system manager to vary the merge
multiplication factor used for each virtual unit, and to vary it on
a per-node basis.
The logical names must be defined in the system table, and should be
defined on each cluster member.
The valid range for a threshold is from 100 to 1000, or from 100 to
10000 on later versions. The default value is 200, and is used when
no value is set or when a value outside the supported range is set.
The current value is displayed at the start of a shadow set merge, in
the %SHADOW_SERVER-I-SSRVINIMRG message.
Excessively large values can cause application performance degradation
due to system load during shadowset merges. The value used is a tradeoff
between the shadowset merge speed and site-specific application
THe logical names are retranslated every one thousand I/Os, permitting
potential adjustments during merges.
The logical names are:
Where nnnn is the shadowset volume unit number to apply the specified
delay factor. The higher the delay factor, the faster the merge and
the higher the system overhead. If the consistency across the shadowset
members is less time-critical or if the shadowset merge is interfering
with the application I/O to the shadowset members, a lower delay factor
can be selected.
The system-wide default value can be established via the logical name:
After each one thousand I/Os, the delay factor logical name is translated.
The default delay factor value is 200, and the maximum value is 1000 or
(in later versions) 10000. With a minimal delay factor value set, there
exists the potential for multiple hours between re-translations of the
The shadowset full copy I/O transfer size can be established via the
following logical name:
DEFINE/SYSTEM SHAD$COPY_BUFFER_SIZE 31
This establishes the minimum buffer size value of 15872 bytes (512 bytes
per block times 31 blocks).