Re: when are buffers/caches flushed?

[Tejun Heo <htejun@gmail.com>]

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