trusty (3) sd_readahead.3.gz

NAME

       sd_readahead - Control ongoing disk boot-time read-ahead operations

SYNOPSIS

       #include "sd-readahead.h"

       int sd_readahead(const char *action);

DESCRIPTION

       sd_readahead() 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.

       The action should be one of the following strings:

       cancel
           Terminates read-ahead data collection, and drops all read-ahead data collected during this boot-up.

       done
           Terminates read-ahead data collection, but keeps all read-ahead data collected during this boot-up
           around for use during subsequent boot-ups.

       noreplay
           Terminates read-ahead replay.

RETURN VALUE

       On failure, these calls return a negative errno-style error code. It is generally recommended to ignore
       the return value of this call.

NOTES

       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.

       Internally, this function creates a file in /run/systemd/readahead/ which is then used as flag file to
       notify the read-ahead subsystem.

       For details about the algorithm check the liberally licensed reference implementation sources:
       http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c and
       http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h

       sd_readahead() is implemented in the reference implementation's 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 sd-readahead(3)

       If -DDISABLE_SYSTEMD is set during compilation this function will always return 0 and otherwise become a
       NOP.

EXAMPLES

       Example 1. Cancelling all read-ahead operations

       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.

           sd_readahead("cancel");
           sd_readahead("noreplay");

SEE ALSO

       systemd(1), sd-readahead(3), daemon(7)