07-03-2012
Why is read-ahead a problem since above is only a 512bytes read, all it takes is normally positioning the drive head .. Then if the cache is the reason, it should affect the file as well, but I am getting the same result for all reps?
More importantly, my experiment is really about time/vs blocksize, and I am trying to make it more accurate :
- Shall I only use regular files?
- position the disk head somewhere else after each test?
- is it possible to flush all caches programmatically ?
Sorry for all those questions ..
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LEARN ABOUT CENTOS
sd_readahead
SD_READAHEAD(3) sd_readahead SD_READAHEAD(3)
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)
systemd 208 SD_READAHEAD(3)