sd_readahead — Control ongoing disk boot-time read-ahead operations
#include "sd-readahead.h"
| int sd_readahead( | const char *action ); | 
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:
On failure, these calls return a negative errno-style error code. It is generally recommended to ignore the return value of this call.
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.
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");