[adding libvirt list] On 4/7/20 2:13 PM, Tim Haley wrote: It looks like https://bugzilla.redhat.com/show_bug.cgi?id=1814664 is a similar description of the same problem: namely, if qemu is not able to determine that the destination already reads as zero, then it forcefully zeroes the destination of a backup job. We may want to copy the fact that qemu 5.0 is adding 'qemu-img convert --target-is-zero' to add a similar knob to the QMP commands that trigger disk copying (blockdev-backup, blockdev-mirror, possibly others) as well as logic to avoid writing zeroes when the destination is already treated as zero (whether by a probe, or by the knob being set). ... For a local file, qemu is easily able to probe whether the destination starts as all zeroes (thanks to lseek(SEEK_DATA)); whereas for s3fs, it looks like qemu does not have access to a quick test to learn if the image starts all zero (POSIX does not provide a quick way for doing this on a generic block device, but if you are aware of an ioctl or otherwise that qemu could use, that might be helpful). Or maybe the s3fs really is random contents rather than all zero, in which case forcefully writing zeroes is the only correct behavior. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org