Triaging old bug tickets... can you still reproduce this issue with the
latest version of QEMU? Or could we close this ticket nowadays?

** Changed in: qemu
       Status: New => Incomplete

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  detect-zeroes=unmap fails to discard in some cases

Status in QEMU:

Bug description:
  Under certain circumstances, QEMU 2.1.2 fails to discard the
  underlaying block. The command to start QEMU is:

  qemu-system-x86_64 -machine pc,accel=kvm -m 2G -smp 2 -vga std -usb
  -usbdevice tablet -drive if=none,id=ff,aio=native,discard=unmap
  ,detect-zeroes=unmap,file=/tmp/test.qcow2 -device virtio-scsi-pci
  -device scsi-disk,drive=ff -cdrom
  /media/iso/archlinux-2014.06.01-dual.iso -vnc :1

  Steps to reproduce:

   0. qemu-img create -f qcow2 /tmp/test.qcow2 4G
   1. Boot in the guest (Arch Linux x86_64 with Linux 3.14.4)
   2. cat /dev/zero > /dev/sda. Observe a file of 4109M (size measured with `du 
   3. cat /dev/zero > /dev/sda
   4. blkdiscard /dev/sda
   5. Observe that test.qcow2 grows larger than 1M (13M in my testing).

  The results are more severe when you write other kind of data in step
  2, for instance `base64 /dev/zero > /dev/sda` and then continuing with
  step 3 and 4 will result in a file of 3642M, after blkdiscard.

  It makes not much of a difference if I create an ext4 filesystem on
  top of it and use fstrim (or rm).

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