On mounted NFS filesystem, ftruncate is much much slower than doing a zero write. Changing this significantly speeds up cluster allocation.
Comparing by converting a cirros image (296M) to VMDK on an NFS mount point, over 1Gbe LAN: $ time qemu-img convert cirros-0.3.1.img /mnt/a.raw -O vmdk Before: real 0m26.464s user 0m0.133s sys 0m0.527s After: real 0m2.120s user 0m0.080s sys 0m0.197s Signed-off-by: Fam Zheng <f...@redhat.com> --- block/vmdk.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/block/vmdk.c b/block/vmdk.c index b69988d..b829265 100644 --- a/block/vmdk.c +++ b/block/vmdk.c @@ -1036,6 +1036,7 @@ static int get_cluster_offset(BlockDriverState *bs, int min_index, i, j; uint32_t min_count, *l2_table; bool zeroed = false; + int ret; if (m_data) { m_data->valid = 0; @@ -1110,11 +1111,15 @@ static int get_cluster_offset(BlockDriverState *bs, /* Avoid the L2 tables update for the images that have snapshots. */ *cluster_offset = bdrv_getlength(extent->file); + assert(0 == (*cluster_offset & (extent->cluster_sectors - 1))); if (!extent->compressed) { - bdrv_truncate( - extent->file, - *cluster_offset + (extent->cluster_sectors << 9) - ); + ret = bdrv_write_zeroes(extent->file, + *cluster_offset >> BDRV_SECTOR_BITS, + extent->cluster_sectors, + 0); + if (ret) { + return VMDK_ERROR; + } } *cluster_offset >>= 9; -- 1.9.2