On Wed, Dec 21, 2011 at 05:50:32PM +0100, Christoph Hellwig wrote: > On Wed, Dec 21, 2011 at 04:00:36PM +0000, Stefan Hajnoczi wrote: > > The ability to zero regions of an image file is a useful primitive for > > higher-level features such as image streaming or zero write detection. > > > > Image formats may support an optimized metadata representation instead > > of writing zeroes into the image file. This allows zero writes to be > > potentially faster than regular write operations and also preserve > > sparseness of the image file. > > > > The .bdrv_co_write_zeroes() interface should be implemented by block > > drivers that wish to provide efficient zeroing. > > > > Note that this operation is different from the discard operation, which > > may leave the contents of the region indeterminate. That means > > discarded blocks are not guaranteed to contain zeroes and may contain > > junk data instead. > > Most real life discard operations zero the data, and both the ATA and SCSI > spec allow the device to set a bit which gurantees this behaviour. I think > we also should make these one interface, and if the caller needs it to > actually zero out the discarded blocks it should check if the discard > implementation guarantees that.
Okay, I see how that could work but still need to look into the details of how to combine the two and check the zero/indeterminate bit coming from ATA/SCSI. Stefan
