HP-UX Reference (11i v2 03/08) - 5 Miscellaneous Topics, 7 Device (Special) Files, 9 General Information, Index (vol 9)

d
dbc_min_pct(5) dbc_min_pct(5)
(Tunable Kernel Parameters)
NAME
dbc_min_pct - minimum percentage of memory used for caching file I/O data and metadata
VALUES
Failsafe
5
Default
5
Allowed values
The minimum value allowed is 1 (1% of physical memory). The maximum value allowed is 70 (70% of
physical memory). The value is further constrained in that it must be equal to or less than
dbc_max_pct.
Specify a positive integer value.
Recommended values
The minimum cache size dictated by dbc_min_pct rarely needs to be modified from its default value.
However, users may need to change this tunable on systems with exceptionally large or small amount of
physical memory, or on systems with an exceptionally large or small number of system processes that
actively use disk I/O for file data.
To determine a reasonable (and conservative) value for the minimum cache size in megabytes use the fol-
lowing formula:
(number-of-system-processes
)*(largest-file-system-block-size)/1024
To determine the value for dbc_min_pct, divide the result by the number of megabytes of physical
memory installed in the computer and multiply that value by 100 to obtain the correct value in percent.
Only those processes that actively use file data should be included in the calculation. All others can be
excluded. Here are some examples of what processes should be included in or excluded from the calcula-
tion:
Include:
NFS daemons, text formatters such as
nroff, database management applications, text editors,
compilers, and so on, that access or use source and/or output files stored in one or more file systems
mounted on the system.
Exclude:
X-display applications,
hpterm, rlogin
, login shells, system daemons, telnet or uucp connec-
tions, and so on. These processes use very little, if any, disk I/O for file data.
DESCRIPTION
During file-system I/O operations, data is stored in a buffer cache, the size of which can be fixed or
dynamically allocated. When the parameters
dbc_min_pct and dbc_max_pct are different, the size
of the buffer cache grows or shrinks dynamically depending on competing requests for system memory.
The value of
dbc_min_pct specifies the minimum percentage of physical memory that is reserved for
use by the dynamic buffer cache.
By setting both
dbc_min_pct and dbc_max_pct to the same value the kernel creates a fixed size
buffer cache that uses exactly that percentage of physical memory and does not change in size.
Who Is Expected to Change This Tunable?
The minimum cache size dictated by
dbc_min_pct rarely needs to be modified from its default value.
However, users may need to change this value in systems with exceptionally large or small amount of
physical memory, or on systems with exceptionally large or small number of system processes that
actively use disk I/O for file data.
Restrictions on Changing
This tunable is dynamic (tuning will take effect immediately on the running system). Tuning of this
parameter will fail if there is not enough free physical memory to grow the file system buffer cache to the
requested new minimum value.
HP-UX 11i Version 2: August 2003 − 1 − Hewlett-Packard Company Section 5−−69