SD_READAHEAD(3)sd_readaheadSD_READAHEAD(3)NAMEsd_readahead - Control ongoing disk boot-time read-ahead operations
SYNOPSIS
#include "sd-readahead.h"
int sd_readahead(const char *action);
DESCRIPTIONsd_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 ALSOsystemd(1), sd-readahead(3), daemon(7)systemd 212SD_READAHEAD(3)
