Re: when are buffers/caches flushed?

[Tejun Heo <htejun@gmail.com>]

Steven Scholz wrote:
> Tejun,
> 
>>>>> 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.
>>>>
>>>>
>>>>  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.
> 
> 
> Ehm. So you're saying umount does _not_ flush the disk caches? BUT it 
> will flush the the fs buffers, right?

  Yeap, but it seems that currently ide-disk seems to flush only on 
reboot not on halt.  As soon as Bartlomiej confirms it, I'll submit a 
patch or he'll fix it.

> 
>>  And one more thing, ide-disk doesn't flush cache when shutting down. 
>> It flushes only when rebooting.
> 
> 
> How about ide-cs then? When I unregister a drive?
> 

  ide-cs is low-level driver, it just registers ide device with upper 
ide midlayer and ide-disk handles all the rest, so it's all the same there.

-- 
tejun