Re: [Libguestfs] [PATCH 0/7 v2] Fix and workaround for qcow2 issues in qemu causing data corruption.

https://bugzilla.redhat.com/show_bug.cgi?id=836710 https://bugzilla.redhat.com/show_bug.cgi?id=836913 There are at least two related bugs going on: (1) Linux sync(2) system call doesn't send a write barrier to the disk, so in effect it doesn't force the hard disk to flush its cache. libguestfs used sync(2) to force changes to disk.

We didn't expect that qemu was caching anything because we used 'cache=none' for all writable disks, but it turns out that qemu creates a writeback cache anyway when you do this (you need to use 'cache=directsync' when you don't want a cache at all).

(2) qemu's qcow2 disk cache code is buggy. If there are I/Os in flight when qemu shuts down, then qemu segfaults or assert fails. This can result in unwritten data. Unfortunately libguestfs ignored the result of waitpid(2) so we didn't see this problem happening. Patch 1/7 fixes the first problem by issuing fsync(2) on each whole block device when we sync. Patches 2/7 - 7/7 are needed to fix the second problem. We add a new API (guestfs_shutdown) so that we can actually catch the case where qemu is segfaulting instead of just ignoring it. Since qemu itself isn't likely to be fixed any time soon, patch 7/7 adds a crude but effective workaround to virt-resize.