[Interest] QFile copy and sync/fsync

[Duane]

On 01/02/2013 04:45 PM, Nikos Chantziaras wrote:
> On 02/01/13 22:49, Duane wrote:
>> Qt 4.8 on Linux TinyCore or Fedora.
>>
>> I have a function to back up some files to a usb stick.  I've had
>> complaints from users that the files are sometime not copied.  It looks
>> like they're removing the usb stick before the standard sync cycle.  If
>> I call sync from my copy routine this works but it hangs the system
>> waiting for sync to complete.
>>
>> If I mount the usb stick with -o sync, this also solves my problem but
>> it also hangs the system.
>>
>> I have tried to use fsync on a per file basis but this doesn't solve my
>> problem.  I have a watchdog to maintain so I would need to use
>> qApp->processEvents() periodically in busy loops.
>>
>> Does fsync not force the write to disk?  Is there a way to sync only a
>> particular folder?
>
> Once QFile::flush() is called, the data is sent to the OS. From there,
> it's up to the system what to do with the data. Users need to unmount
> the stick before removing it. If they don't want to do that, then they
> need to set up their system such that it mounts the USB stick
> synchronously by default. It's either that or unmount first.
>
> This is not a QFile problem. When someone removes a live filesystem
> without unmounting it, loss of data is to be expected and should come as
> no surprise. If you try to work around it, then you'll be hurting
> performance for users who do know how to properly use their system
> (calling QFile::sync() all the time can mess up buffering.)

This device is a "controller" with a touch screen and no access to a 
terminal where the user can enter anything.  I would have to add 
something to the gui to unmount the device but I think this would be 
more or less the same as calling sync when the copying is done.