ubifs: sync() causes writes even if nothing is changed

[Artem Bityutskiy]

On Wed, 2010-10-13 at 18:30 +0200, Hans J. Koch wrote:
> Running this command:
> 
> # while true ; do sync; sleep 1; done
> 
> causes two eraseblocks being erased every second, although there
> are no writes to the ubifs filesystem. I hacked some printks into
> my NAND driver that print page_address and column for each erase.
> With that, I get this output every second:
> 
> ...
> [   63.701765] erase p=0x0000ae40 c=0xffffffff
> [   63.706534] erase p=0xffffffff c=0xffffffff
> [   63.725492] erase p=0x0000ae80 c=0xffffffff
> [   63.730260] erase p=0xffffffff c=0xffffffff
> ...
> 
> From a quick glance at the ubifs code, this might come out of the
> garbage collector that is triggered on every sync() and writes
> something even if nothing has changed.
> 
> Is that really needed?

I briefly looked, and UBIFS is doing a lot of I/O in that case, which is
unnecessary and should not be done, I'll need to investigate this.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)