> On 19 Jun 2024, at 8:54, Justin <just...@safe-mbox.com> wrote: > >> I've run strace and I see calls to fallocate with these flags: >> FALLOC_FL_KEEP_SIZE|FALLOC_FL_PUNCH_HOLE >> >> I've tried passing these options: discard=off,detect-zeroes=off but >> this does not help. This is the full set of relevant options I'm >> using: >> >> -drive file=/vms/vm0/drive,format=raw,if=virtio,discard=off,detect-zeroes= >> off >> >> You don't need to disable detect-zeros - in my tests it makes dd if=/dev/zero >> 5 times faster (770 MiB/s -> 3 GiB/s) since zero writes are converted to >> fallocate(FALLOC_FL_KEEP_SIZE|FALLOC_FL_ZERO_RANGE). >> >> The issue seems to be ignoring the discard option when opening the image, >> and is fixed by this: >> https://lists.nongnu.org/archive/html/qemu-block/2024-06/msg00198.html > > Thanks. When might this patch (or something similar) be merged?
The patch need more work, when the work is done and qemu maintainers are happy it is a good estimate :-) > >> I think the change needs more work to keep the default behavior >> since most users want sparse images, but it seems to do what you >> want - keeping images thick. > > It seems that this patch is making the code align more closely with > the documentation? To, me, it appeared fairly clear that discard=unmap > would punch holes, and thus the inverse setting would stop hole > punching. Punching holes is used both for discard (e.g. fstrim in the guest) and for writing zeros. I think that discard should work only when you set discard=unmap or disacard=on, but writing zeros should always punch holes unless you set discard=off. I don’t think this behavior is documented now but it should be, at least the intent to keep images sparse when possible. Nir
