From 351c8efff9ad809c822d55620df54d575d536f68 Mon Sep 17 00:00:00 2001 From: Max Reitz Date: Tue, 1 May 2018 18:57:49 +0200 Subject: [PATCH] qemu-img: Special post-backing convert handling Currently, qemu-img convert writes zeroes when it reads zeroes. Sometimes it does not because the target is initialized to zeroes anyway, so we do not need to overwrite (and thus potentially allocate) it. This is never the case for targets with backing files, though. But even they may have an area that is initialized to zeroes, and that is the area past the end of the backing file (if that is shorter than the overlay). So if the target format's unallocated blocks are zero and there is a gap between the target's backing file's end and the target's end, we do not have to explicitly write zeroes there. Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1527898 Signed-off-by: Max Reitz Message-id: 20180501165750.19242-2-mreitz@redhat.com Reviewed-by: Eric Blake Signed-off-by: Max Reitz --- qemu-img.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-) diff --git a/qemu-img.c b/qemu-img.c index ebe1b866da..ae4acb655b 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -1556,7 +1556,9 @@ typedef struct ImgConvertState { BlockBackend *target; bool has_zero_init; bool compressed; + bool unallocated_blocks_are_zero; bool target_has_backing; + int64_t target_backing_sectors; /* negative if unknown */ bool wr_in_order; bool copy_range; int min_sparse; @@ -1586,12 +1588,23 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num) { int64_t src_cur_offset; int ret, n, src_cur; + bool post_backing_zero = false; convert_select_part(s, sector_num, &src_cur, &src_cur_offset); assert(s->total_sectors > sector_num); n = MIN(s->total_sectors - sector_num, BDRV_REQUEST_MAX_SECTORS); + if (s->target_backing_sectors >= 0) { + if (sector_num >= s->target_backing_sectors) { + post_backing_zero = s->unallocated_blocks_are_zero; + } else if (sector_num + n > s->target_backing_sectors) { + /* Split requests around target_backing_sectors (because + * starting from there, zeros are handled differently) */ + n = s->target_backing_sectors - sector_num; + } + } + if (s->sector_next_status <= sector_num) { int64_t count = n * BDRV_SECTOR_SIZE; @@ -1613,7 +1626,7 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num) n = DIV_ROUND_UP(count, BDRV_SECTOR_SIZE); if (ret & BDRV_BLOCK_ZERO) { - s->status = BLK_ZERO; + s->status = post_backing_zero ? BLK_BACKING_FILE : BLK_ZERO; } else if (ret & BDRV_BLOCK_DATA) { s->status = BLK_DATA; } else { @@ -2379,6 +2392,16 @@ static int img_convert(int argc, char **argv) } } + if (s.target_has_backing) { + /* Errors are treated as "backing length unknown" (which means + * s.target_backing_sectors has to be negative, which it will + * be automatically). The backing file length is used only + * for optimizations, so such a case is not fatal. */ + s.target_backing_sectors = bdrv_nb_sectors(out_bs->backing->bs); + } else { + s.target_backing_sectors = -1; + } + ret = bdrv_get_info(out_bs, &bdi); if (ret < 0) { if (s.compressed) { @@ -2388,6 +2411,7 @@ static int img_convert(int argc, char **argv) } else { s.compressed = s.compressed || bdi.needs_compressed_writes; s.cluster_sectors = bdi.cluster_size / BDRV_SECTOR_SIZE; + s.unallocated_blocks_are_zero = bdi.unallocated_blocks_are_zero; } ret = convert_do_copy(&s);