sd_readahead — Control ongoing disk boot-time read-ahead operations
|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
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
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
implemented in the reference implementation's drop-in
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
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.