> > > > > If there is no backing file or snapshot you still need to fill
> > > > > the cluster with zeroes, and that's going to be slower with > > > > > larger clusters. > > > > If not fill zeroes and only write guest data ,what`s wrong could > > > > happen ? > > > The following could happen: > > > 1) Guest reads at offset [0, 4k] -> there's only zeroes > > > 2) Guest writes at offset [8k, 16k] > > > 3) Guest reads at offset [0, 4k] -> there's something else now > > Why could guest read the area at offset [0, 4k] has not be writen > > yet ? > Jakob already gave you some answers, but here's a simple one: because > it might have already been written. > If the guest wrote zeroes to [0, 1M] you can't generally assume that > there's an allocated 1MB cluster on the qcow2 file filled with zeroes. > - QEMU can detect that the guest tried to write zeroes and decide to > leave the cluster unallocated (see for example the "detect_zeroes" > option, or the "WRITE SAME" SCSI command). > - The qcow2 file could have been converted at some point, and > zero-filled clusters could have been deallocated for efficiency. > Berto Thank for every replay,and I understand completely . Then I look forward to using the “subcluster allocation ”feature as early as possible. Yang.bin
