1cdc3239f1
This sequence works efficiently if FALLOC_FL_ZERO_RANGE is not supported. Unfortunately, FALLOC_FL_ZERO_RANGE is supported on really modern systems and only for a couple of filesystems. FALLOC_FL_PUNCH_HOLE is much more mature. The sequence of 2 operations FALLOC_FL_PUNCH_HOLE and 0 is necessary due to the following reasons: - FALLOC_FL_PUNCH_HOLE creates a hole in the file, the file becomes sparse. In order to retain original functionality we must allocate disk space afterwards. This is done using fallocate(0) call - fallocate(0) without preceeding FALLOC_FL_PUNCH_HOLE will do nothing if called above already allocated areas of the file, i.e. the content will not be zeroed This should increase the performance a bit for not-so-modern kernels. CC: Max Reitz <mreitz@redhat.com> CC: Kevin Wolf <kwolf@redhat.com> CC: Stefan Hajnoczi <stefanha@redhat.com> CC: Peter Lieven <pl@kamp.de> CC: Fam Zheng <famz@redhat.com> Signed-off-by: Denis V. Lunev <den@openvz.org> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Read the documentation in qemu-doc.html or on http://wiki.qemu-project.org - QEMU team