Fujitsu Eternus DX200F, User Manual

Page: 809 / 1082

Appendix A Parameter List
A.3 Volume Management
ETERNUS Web GUI User’s Guide (Settings)
Copyright 2015 FUJITSU LIMITED P2X0-1270-13ENZ0
809
• Prefetch Limit (PL)
• Force Prefetch Mode (FP)
• Multi Writeback Count (MWC)
Description Specify the prefetch limit of the volume.
The default PL value is "8". If "0" is specified, data prefetch is not performed. It is not
necessary to change the default value for normal use.
If multiple volumes with different settings are selected and [Modify Cache Parameters] is
clicked, the field is blank.
When specifying larger PL, the amount of data that is read from a drive for a single read
process is increased. As a result, the number of times that data needs to be read
(Staging) from the drive is reduced. When a large "Prefetch Limit" value is specified, the
amount of data that is read from a drive for a single read process is increased. As a
result, the amount of data that is read (Staging) from the drive 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.
Input condition/
Display contents 0 - 64
Description Select whether to enable (ON) or disable (OFF) the Force Prefetch Mode to forcibly fetch
the cache in advance.
The default FP value 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
may regard as random accesses occurs. By selecting "ON" for "Force Prefetch Mode",
prefetching is forcibly performed in such a case.
For random read access, unnecessary data is read from the drive and random read access
performance may be reduced.
Input condition/
Display contents
•
ON
•
OFF
Description Specify the value of the Multi Writeback Count.
It is not necessary to change the default value for normal use. The Multi Writeback Count
cannot be specified for volumes that are concatenated by the LUN concatenation
function.
If multiple volumes with different settings are selected and [Modify Cache Parameters] is
clicked, the field is blank.
When specifying larger MWC, sequential write access performance is improved. Note that
this is not effective when a number of random write accesses occur.
Depending on the ratio of read access and write access, read access performance may be
reduced.
Input condition/
Display contents 0 - 16
The setting values available for Multi Writeback Count vary depending on the RAID level and drive
configuration. Refer to
"F.2 Input Conditions for MWC" (page 1066) for details.