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.
