'\" t
.\"     Title: sd_readahead
.\"    Author: Lennart Poettering <lennart@poettering.net>
.\" Generator: DocBook XSL Stylesheets v1.76.1 <http://docbook.sf.net/>
.\"      Date: 10/07/2013
.\"    Manual: sd_readahead
.\"    Source: systemd
.\"  Language: English
.\"
.TH "SD_READAHEAD" "3" "10/07/2013" "systemd" "sd_readahead"
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.SH "NAME"
sd_readahead \- Control ongoing disk boot\-time read\-ahead operations
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include "sd\-readahead\&.h"
.fi
.ft
.HP \w'int\ sd_readahead('u
.BI "int sd_readahead(const\ char\ *" "action" ");"
.SH "DESCRIPTION"
.PP
\fBsd_readahead()\fR
may be called by programs involved with early boot\-up to control ongoing boot\-time disk read\-ahead operations\&. It may be used to terminate read\-ahead operations in case an uncommon disk access pattern is to be expected and hence read\-ahead replay or collection is unlikely to have the desired speed\-up effect on the current or future boot\-ups\&.
.PP
The
\fIaction\fR
should be one of the following strings:
.PP
cancel
.RS 4
Terminates read\-ahead data collection, and drops all read\-ahead data collected during this boot\-up\&.
.RE
.PP
done
.RS 4
Terminates read\-ahead data collection, but keeps all read\-ahead data collected during this boot\-up around for use during subsequent boot\-ups\&.
.RE
.PP
noreplay
.RS 4
Terminates read\-ahead replay\&.
.RE
.SH "RETURN VALUE"
.PP
On failure, these calls return a negative errno\-style error code\&. It is generally recommended to ignore the return value of this call\&.
.SH "NOTES"
.PP
This function is provided by the reference implementation of APIs for controlling boot\-time read\-ahead and distributed with the systemd package\&. The algorithm it implements is simple, and can easily be reimplemented in daemons if it is important to support this interface without using the reference implementation\&.
.PP
Internally, this function creates a file in
/run/systemd/readahead/
which is then used as flag file to notify the read\-ahead subsystem\&.
.PP
For details about the algorithm check the liberally licensed reference implementation sources:
\m[blue]\fB\%http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c\fR\m[]
resp\&.
\m[blue]\fB\%http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h\fR\m[]
.PP
\fBsd_readahead()\fR
is implemented in the reference implementation\*(Aqs drop\-in
sd\-readahead\&.c
and
sd\-readahead\&.h
files\&. It is recommended that applications consuming this API copy the implementation into their source tree\&. For more details about the reference implementation see
\fBsd-readahead\fR(7)
.PP
If \-DDISABLE_SYSTEMD is set during compilation this function will always return 0 and otherwise become a NOP\&.
.SH "EXAMPLES"
.PP
\fBExample\ \&1.\ \&Cancelling all read-ahead operations\fR
.PP
During boots where SELinux has to relabel the file system hierarchy, it will create a large amount of disk accesses that are not necessary during normal boots\&. Hence it is a good idea to disable both read\-ahead replay and read\-ahead collection\&.
.sp
.if n \{\
.RS 4
.\}
.nf
sd_readahead("cancel");
sd_readahead("noreplay");
.fi
.if n \{\
.RE
.\}
.SH "SEE ALSO"
.PP

\fBsystemd\fR(1),
\fBsd-readahead\fR(7),
\fBdaemon\fR(7)
.SH "AUTHOR"
.PP
\fBLennart Poettering\fR <\&lennart@poettering\&.net\&>
.RS 4
Developer
.RE