If we go over the maximum number of iovecs support by syscall we get back EINVAL from the kernel which translate to I/O errors for the guest.
Signed-off-by: Christoph Hellwig <h...@lst.de> Index: qemu/block.c =================================================================== --- qemu.orig/block.c 2010-01-19 22:10:19.797003226 +0100 +++ qemu/block.c 2010-01-19 22:11:08.226005767 +0100 @@ -1711,6 +1711,10 @@ static int multiwrite_merge(BlockDriverS merge = bs->drv->bdrv_merge_requests(bs, &reqs[outidx], &reqs[i]); } + if (reqs[outidx].qiov->niov + reqs[i].qiov->niov + 1 > IOV_MAX) { + merge = 0; + } + if (merge) { size_t size; QEMUIOVector *qiov = qemu_mallocz(sizeof(*qiov));