Re: [Qemu-devel] [Qemu-block] [PATCH v3 14/16] block/qcow2: falloc/full preallocating growth

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Fri, May 26, 2017 at 06:55:16PM +0200, Max Reitz wrote:
> @@ -2677,6 +2679,102 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
>          }
>          break;
>  
> +    case PREALLOC_MODE_FALLOC:
> +    case PREALLOC_MODE_FULL:
> +    {
> +        int64_t allocation_start, host_offset, guest_offset;
> +        int64_t clusters_allocated;
> +        int64_t old_file_size, new_file_size;
> +        uint64_t nb_new_data_clusters, nb_new_l2_tables;
> +
> +        old_file_size = bdrv_getlength(bs->file->bs);
> +        if (old_file_size < 0) {
> +            error_setg_errno(errp, -old_file_size,
> +                             "Failed to inquire current file length");
> +            return ret;
> +        }
> +
> +        nb_new_data_clusters = DIV_ROUND_UP(offset - old_length,
> +                                            s->cluster_size);
> +
> +        /* This is an overestimation; we will not actually allocate space for
> +         * these in the file but just make sure the new refcount structures are
> +         * able to cover them so we will not have to allocate new refblocks
> +         * while entering the data blocks in the potentially new L2 tables.
> +         * (We do not actually care where the L2 tables are placed. Maybe they
> +         *  are already allocated or they can be placed somewhere before
> +         *  @old_file_size. It does not matter because they will be fully
> +         *  allocated automatically, so they do not need to be covered by the
> +         *  preallocation. All that matters is that we will not have to allocate
> +         *  new refcount structures for them.) */
> +        nb_new_l2_tables = DIV_ROUND_UP(nb_new_data_clusters,
> +                                        s->cluster_size / sizeof(uint64_t));
> +        /* The cluster range may not be aligned to L2 boundaries, so add one L2
> +         * table for a potential head/tail */
> +        nb_new_l2_tables++;
> +
> +        allocation_start = qcow2_refcount_area(bs, old_file_size,
> +                                               nb_new_data_clusters +
> +                                               nb_new_l2_tables,
> +                                               true, 0, 0);
> +        if (allocation_start < 0) {
> +            error_setg_errno(errp, -allocation_start,
> +                             "Failed to resize refcount structures");
> +            return -allocation_start;
> +        }
> +
> +        clusters_allocated = qcow2_alloc_clusters_at(bs, allocation_start,
> +                                                     nb_new_data_clusters);
> +        if (clusters_allocated < 0) {
> +            error_setg_errno(errp, -clusters_allocated,
> +                             "Failed to allocate data clusters");
> +            return -clusters_allocated;
> +        }
> +
> +        assert(clusters_allocated == nb_new_data_clusters);
> +
> +        /* Allocate the data area */
> +        new_file_size = allocation_start +
> +                        nb_new_data_clusters * s->cluster_size;
> +        ret = bdrv_truncate(bs->file, new_file_size, prealloc, errp);
> +        if (ret < 0) {
> +            error_prepend(errp, "Failed to resize underlying file: ");
> +            qcow2_free_clusters(bs, allocation_start,
> +                                nb_new_data_clusters * s->cluster_size,
> +                                QCOW2_DISCARD_OTHER);
> +            return ret;
> +        }
> +
> +        /* Create the necessary L2 entries */
> +        host_offset = allocation_start;
> +        guest_offset = old_length;
> +        while (nb_new_data_clusters) {
> +            int64_t guest_cluster = guest_offset >> s->cluster_bits;
> +            int64_t nb_clusters = MIN(nb_new_data_clusters,
> +                                      s->l2_size - guest_cluster % s->l2_size);
> +            QCowL2Meta allocation = {
> +                .offset       = guest_offset,
> +                .alloc_offset = host_offset,
> +                .nb_clusters  = nb_clusters,
> +            };
> +            qemu_co_queue_init(&allocation.dependent_requests);
> +
> +            ret = qcow2_alloc_cluster_link_l2(bs, &allocation);
> +            if (ret < 0) {
> +                error_setg_errno(errp, -ret, "Failed to update L2 tables");
> +                qcow2_free_clusters(bs, host_offset,
> +                                    nb_new_data_clusters * s->cluster_size,
> +                                    QCOW2_DISCARD_OTHER);
> +                return ret;
> +            }
> +
> +            guest_offset += nb_clusters * s->cluster_size;
> +            host_offset += nb_clusters * s->cluster_size;
> +            nb_new_data_clusters -= nb_clusters;
> +        }
> +        break;
> +    }

Flush L1/L2 tables?

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