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Resumen del manual
Creating
DBEFiles
DBEFiles
are
the
ph
ysical
rep ositories
of
ALLBASE/SQL
data
on
the
op erating
system.
Since
y
ou
ha
v
e
great
exibilit
y
in
dening
DBEFile
t
yp es,
sizes,
and
assignment
to
DBEFileSets,
y
our
c
hoices
often
aect
the
o
v
erall
p erformance
of
y
our
system.
Here
are
some
suggestions
for
creating
DBEFiles:
Create
DBEFiles
in
sizes
that
are
m
ultiples
of
253
pages
to
minimize
the
space
used
b
y
page
table
pages.
Eac
h
253-page
segmen
t
con
tains
one
page
table
page
plus
252
pages
of
data
follo wing
the
page
table
page.
Create
separate
T
ABLE
and
INDEX
DBEFiles,
esp ecially
for
large
tables,
so
that
they
can
b e
placed
on
separate
disks
to
improv
e
I/O
p erformance.
Create
a
MIXED
DBEFile
in
its
o
wn
DBEFileSet
for
a
small
table
(less
than
1000
pages)
with
a
single
index.
(In
this
case,
data
and
index
rows
will
b e
on
alternating
pages,
whic
h
will
result
in
faster
pro cessing
and
b etter
space
utilization).
Use
dynamic
DBEFile
expansion
to
a
v
oid
running
out
of
DBEFile
space
at
run
time.
In
the
CREA
TE
DBEFILE
statement,
sp ecify
the
initial
size,
the
maximum
size,
and
the
incremen
t
size.
Use
pseudo-mapped
I/O
to
shorten
the
I/O
path
length
and
eliminate
op erating
system
buering
of
database
pages.
This
feature
is
appropriate
only
for
large
DBEFiles
con
taining
large
tables
accessed
randomly
.
Standard
I/O
is
more
appropriate
for
small
DBEFiles
or
DBEFiles
that
are
frequen
tly
accessed
sequen
tially
.
When
appropriate,
use
pseudo-mapped
I/O
with
memory-residen
t
data
buers.
File
system
prefetc
hing
is
not
a
v
ailable
when
using
pseudo-mapp
ed
I/O.
Restrictions
on
pseudo-mapped
les:
They
cannot
b e
dynamically
expanded.
Y
ou
cannot
use
concurren
t
bac
kup
(that
is,
SQLUtil
STOREONLINE
do es
not
w
ork
with
pseudo-mapped
les).
F
or
additional
information
on
pseudo-mapped
les,
refer
to
the
app endix
\Using
Pseudo-Mapp ed
DBEFiles"
in
the
the
ALLBASE/SQL
Datab
ase
A
dministr
ation
Guide
.
Av
oiding
Extra
DBEFile
Space
When
sequen
tial
scans
are
made
frequen
tly
for
queries
in
v
olving
tables
in
a
particular
DBEFileSet,
extra
space
should
b e
minimized.
When
sequen
tial
scans
are
made,
ev
ery
page
table
page
in
the
DBEFileSet
is
read
to
determine
if
pages
might
qualify
for
the
query
.
When
a
large
amoun
t
of
extra
space
exists,
then
extra
I/O
is
required
to
fetc
h
the
page
table
pages
for
empt
y
DBEFile
space.
One
w
a
y
to
minimize
the
extra
space
in
y
our
DBEFiles
is
b
y
sp ecifying
dynamic
space
expansion
in
the
CREA
TE
DBEFILE
statement.
Guidelines
on
Logical
and
Physical
Design
2-13
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