"Darrick J. Wong" <darrick.wong@xxxxxxxxxx> writes: > On Wed, Feb 03, 2016 at 02:43:02PM -0500, Jeff Moyer wrote: >> Hi, Christoph, >> >> Can you explain a bit what you mean by easing COW handling? Whenever I >> see COW referenced near DIO, my mind always turns to g_u_p vs. fork. > > Just to clarify, I'm talking about copy on write for disk blocks, not for > memory pages. > > Basically, XFS implements (disk block) copy on write for (perfectly block > aligned) directio writes by allocating a set of replacement blocks, mapping the > dio writes to the new blocks, and playing a punch/remap trick to map the new > blocks into the file at the appropriate offset. > > If the write fails then we don't want do the remap, so the dio_complete handler > has to know whether or not the IO succeeded, hence the new parameter. > Furthermore, if the write succeeds but the remap fails, we also want to be able > to report that to userspace, hence the change of return value from void to int. > > (If the dio write isn't block aligned, we fall back to the page cache.) > > Hope that helps, Eric said the magic word: reflink. I think I've got it now. Thanks! Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
