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Chapter 5 Volume Management
5.2 Functions in the Action Area for Volume
ETERNUS Web GUI User’s Guide
Copyright 2013 FUJITSU LIMITED P2X0-1090-10ENZ0
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When "-" (hyphen) is selected, the cache capacity is not limited. It is not necessary to change the
default setting for normal use. If multiple volumes with different settings are selected and [Modify
Cache Parameters] is clicked, a "-" (hyphen) is displayed for the field.
Data in low I/O load volumes may not be stored in the cache memory if volumes with a high I/O load
exist. This reduces the cache hit ratio for the low I/O load volumes. By limiting the cache capacity
that is used by high I/O load volumes, a reduction in performance when low I/O load volumes are
accessed can be prevented. However, the cache hit ratio for high I/O load volumes is lowered and
the access performance for the volumes may reduced.
• Prefetch Limit (PL)
Specify the Prefetch Limit of the volume between "0" and "64". The default value is "8".
If "0" is specified for the "Prefetch Limit (PL)", data prefetch is not performed. It is not necessary to
change the default setting for normal use. If multiple volumes with different settings are selected
and [Modify Cache Parameters] is clicked, the field is blank.
When specifying a larger "Prefetch Limit", the amount of data that is read from a disk for a single
read process is increased. As a result, the number of times that data needs to be read (Staging)
from the disk is reduced. However, the amount of data that is read from the disk for a single read
process increases and requested data may not be read before the next read or write request from
the host is issued. In this case, the waiting time for Staging may increase.
• Force Prefetch Mode (FP)
Select whether to enable ("ON") or disable ("OFF") the Force Prefetch Mode to forcibly read to the
cache. The default setting is "OFF". It is not necessary to change the default setting for normal use. If
multiple volumes with different settings are selected and [Modify Cache Parameters] is clicked,
"OFF" is selected for the field.
This setting is required to improve the sequential read access performance.
When a number of sequential accesses are performed for one volume, the ETERNUS DX Disk storage
system may determine that random accesses have occurred. By selecting "ON" for "Force Prefetch
Mode", prefetching is forcibly performed in such a case.
However, for random read access, unnecessary data is read from the disk and random read access
performance may be reduced.
• Note that this parameter is not used to specify the capacity of dedicated cache for the relevant
volume. This parameter limits the cache capacity that can be used for the relevant volume.
Although a volume with this parameter cannot use a cache memory that exceeds the specified
value, other volumes can use the entire cache, which includes the specified capacity.
• The cache page capacity must be changed when the I/O load of the volume is low.
• When selecting volumes that cannot use this parameter as the target of this function, the
"Cache Page Capacity" setting is not displayed on the screen.
• If write data exceeds the specified cache page capacity for a volume that is stored in the cache
memory, the cache page capacity cannot be changed.
• The cache page capacity cannot be changed for concatenated volumes that are created by the
LUN concatenation function.
• For the ETERNUS DX80 S2, "2080.00 MB" cannot be selected as the cache page capacity.
• The specified cache page capacity includes the mirror area. Note that the maximum cache
capacity used by read I/O is a half the specified value.
• When the cache page capacity is limited, the capacity cannot be expanded for the relevant
volume. Reset the cache page capacity to the default value before expanding the volume
capacity.