[Interest] QFile copy and sync/fsync

[Duane]

On 01/03/2013 12:45 PM, Nikos Chantziaras wrote:
> On 03/01/13 15:51, Duane wrote:
>> 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.
>>>>   [...]
>>>
>>> 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 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.
>
> Keep in mind that if you're not mounting synchronously by default, then
> removing the stick without unmounting it can lead to filesystem
> corruption and inconsistencies.
>
> The fact that it's a USB stick doesn't matter.  The only thing that
> matters is that a non-synchronous filesystem is being removed while it's
> still live.

Yes, the first thing that I tried was to change the mount to 
synchronous.  It solves the problem but what it does is a bit strange. 
I have a loop that I copy N files.  After all N are copied, it gets 
synced which makes the system non-responsive for some amount of time.  I 
would prefer it to sync them on each copy.