diff options
| -rw-r--r-- | block/qcow2-cluster.c | 51 |
1 files changed, 38 insertions, 13 deletions
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index 3c9ace8..20fb531 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -777,20 +777,38 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m) Qcow2COWRegion *start = &m->cow_start; Qcow2COWRegion *end = &m->cow_end; unsigned buffer_size; + unsigned data_bytes = end->offset - (start->offset + start->nb_bytes); + bool merge_reads; uint8_t *start_buffer, *end_buffer; int ret; assert(start->nb_bytes <= UINT_MAX - end->nb_bytes); + assert(start->nb_bytes + end->nb_bytes <= UINT_MAX - data_bytes); + assert(start->offset + start->nb_bytes <= end->offset); if (start->nb_bytes == 0 && end->nb_bytes == 0) { return 0; } - /* Reserve a buffer large enough to store the data from both the - * start and end COW regions. Add some padding in the middle if - * necessary to make sure that the end region is optimally aligned */ - buffer_size = QEMU_ALIGN_UP(start->nb_bytes, bdrv_opt_mem_align(bs)) + - end->nb_bytes; + /* If we have to read both the start and end COW regions and the + * middle region is not too large then perform just one read + * operation */ + merge_reads = start->nb_bytes && end->nb_bytes && data_bytes <= 16384; + if (merge_reads) { + buffer_size = start->nb_bytes + data_bytes + end->nb_bytes; + } else { + /* If we have to do two reads, add some padding in the middle + * if necessary to make sure that the end region is optimally + * aligned. */ + size_t align = bdrv_opt_mem_align(bs); + assert(align > 0 && align <= UINT_MAX); + assert(QEMU_ALIGN_UP(start->nb_bytes, align) <= + UINT_MAX - end->nb_bytes); + buffer_size = QEMU_ALIGN_UP(start->nb_bytes, align) + end->nb_bytes; + } + + /* Reserve a buffer large enough to store all the data that we're + * going to read */ start_buffer = qemu_try_blockalign(bs, buffer_size); if (start_buffer == NULL) { return -ENOMEM; @@ -799,15 +817,22 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m) end_buffer = start_buffer + buffer_size - end->nb_bytes; qemu_co_mutex_unlock(&s->lock); - /* First we read the existing data from both COW regions */ - ret = do_perform_cow_read(bs, m->offset, start->offset, - start_buffer, start->nb_bytes); - if (ret < 0) { - goto fail; - } + /* First we read the existing data from both COW regions. We + * either read the whole region in one go, or the start and end + * regions separately. */ + if (merge_reads) { + ret = do_perform_cow_read(bs, m->offset, start->offset, + start_buffer, buffer_size); + } else { + ret = do_perform_cow_read(bs, m->offset, start->offset, + start_buffer, start->nb_bytes); + if (ret < 0) { + goto fail; + } - ret = do_perform_cow_read(bs, m->offset, end->offset, - end_buffer, end->nb_bytes); + ret = do_perform_cow_read(bs, m->offset, end->offset, + end_buffer, end->nb_bytes); + } if (ret < 0) { goto fail; } |