svcadm(8)을 검색하려면 섹션에서 8 을 선택하고, 맨 페이지 이름에 svcadm을 입력하고 검색을 누른다.
directio(3c)
directio(3C) Standard C Library Functions directio(3C)
NAME
directio - provide advice to file system
SYNOPSIS
#include <sys/types.h>
#include <sys/fcntl.h>
int directio(int fildes, int advice);
DESCRIPTION
The directio() function provides advice to the system about the ex‐
pected behavior of the application when accessing the data in the file
associated with the open file descriptor fildes. The system uses this
information to help optimize accesses to the file's data. The direc‐
tio() function has no effect on the semantics of the other operations
on the data, though it may affect the performance of other operations.
The advice argument is kept per file; the last caller of directio()
sets the advice for all applications using the file associated with
fildes.
Values for advice are defined in <sys/fcntl.h>.
DIRECTIO_OFF Applications get the default system behavior when ac‐
cessing file data.
When an application reads data from a file, the data is
first cached in system memory and then copied into the
application's buffer (see read(2)). If the system de‐
tects that the application is reading sequentially from
a file, the system will asynchronously "read ahead"
from the file into system memory so the data is immedi‐
ately available for the next read(2) operation.
When an application writes data into a file, the data
is first cached in system memory and is written to the
device at a later time (see write(2)). When possible,
the system increases the performance of write(2) opera‐
tions by caching the data in memory pages. The data is
copied into system memory and the write(2) operation
returns immediately to the application. The data is
later written asynchronously to the device. When possi‐
ble, the cached data is "clustered" into large chunks
and written to the device in a single write operation.
The system behavior for DIRECTIO_OFF can change without
notice.
DIRECTIO_ON The system behaves as though the application is not go‐
ing to reuse the file data in the near future. In other
words, the file data is not cached in the system's mem‐
ory pages.
When possible, data is read or written directly between
the application's memory and the device when the data
is accessed with read(2) and write(2) operations. When
such transfers are not possible, the system switches
back to the default behavior, but just for that opera‐
tion. In general, the transfer is possible when the ap‐
plication's buffer is aligned on a two-byte (short)
boundary, the offset into the file is on a device sec‐
tor boundary, and the size of the operation is a multi‐
ple of device sectors.
This advisory is ignored while the file associated with
fildes is mapped (see mmap(2)).
The system behavior for DIRECTIO_ON can change without
notice.
RETURN VALUES
Upon successful completion, directio() returns 0. Otherwise, it returns
−1 and sets errno to indicate the error.
ERRORS
The directio() function will fail if:
EBADF The fildes argument is not a valid open file descriptor.
ENOTTY The fildes argument is not associated with a file system that
accepts advisory functions.
EINVAL The value in advice is invalid.
USAGE
Small sequential I/O generally performs best with DIRECTIO_OFF.
Large sequential I/O generally performs best with DIRECTIO_ON, except
when a file is sparse or is being extended and is opened with O_SYNC or
O_DSYNC (see open(2)).
The directio() function is supported for the NFS and UFS file system
types (see fstyp(8)).
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ MT-LevelMT-Safe
SEE ALSO
mmap(2), open(2), read(2), write(2), fcntl.h(3HEAD), attributes(7),
fstyp(8)
WARNINGS
Switching between DIRECTIO_OFF and DIRECTIO_ON can slow the system be‐
cause each switch to DIRECTIO_ON might entail flushing the file's data
from the system's memory.
Oracle Solaris 11.4 13 Nov 2020 directio(3C)