Steven Scholz wrote:
Tejun Heo wrote:
Steven Scholz wrote:
Hi there,
According to an (old) man page of sync(2)
According to the standard specification (e.g., SVID),
sync() schedules the writes, but may return before the
actual writing is done. However, since version 1.3.20
Linux does actually wait. (This still does not guarantee
data integrity: modern disks have large caches.)
How about recent kernels? Does sync() block until buffers are flushed?
How can I find out if the disk caches are actually flushed?
I want to make sure that all data is flushed to my disk drive before
powering down the system.
Thanks.
All disk caches are flushed before shutdown via the following path.
kernel/sys.c::sys_reboot()
kernel/drivers/base/power/shutdown.c::device_shutdown()
driver specific ->shutdown callback, for ide disks, the path is
drivers/ide/ide-disk.c::ide_device_shutdown()
drivers/ide/ide-disk.c::ide_cacheflush_p()
drivers/ide/ide-disk.c::do_idedisk_flushcache()
And, AFAIK, sync() doesn't flush disk caches.
How about umount?
[CC'ing Bartlomiej (Hi!)]
Hmmm, umount doesn't. I think maybe adding cache flushing to sync and
umount can be helpful.
And one more thing, ide-disk doesn't flush cache when shutting down.
It flushes only when rebooting.
from ide_device_shutdown()...
#ifdef CONFIG_ALPHA
/* On Alpha, halt(8) doesn't actually turn the machine off,
it puts you into the sort of firmware monitor. Typically,
it's used to boot another kernel image, so it's not much
different from reboot(8). Therefore, we don't need to
spin down the disk in this case, especially since Alpha
firmware doesn't handle disks in standby mode properly.
On the other hand, it's reasonably safe to turn the power
off when the shutdown process reaches the firmware prompt,
as the firmware initialization takes rather long time -
at least 10 seconds, which should be sufficient for
the disk to expire its write cache. */
if (system_state != SYSTEM_POWER_OFF) {
#else
if (system_state == SYSTEM_RESTART) {
#endif
ide_cacheflush_p(drive);
return;
}
It seems that the if clause are there due to some historic thing. The
comment suggests that the body in the if clause spun down the drive
previously, and we needed the if to avoid spinning down unnecessarily,
and the if is left there after we changed the body to flush cache.
Bartlomiej, am I missing something?
--
tejun
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