block/backup.c

   1 /*
   2  * QEMU backup
   3  *
   4  * Copyright (C) 2013 Proxmox Server Solutions
   5  *
   6  * Authors:
   7  *  Dietmar Maurer (dietmar@proxmox.com)
   8  *
   9  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  10  * See the COPYING file in the top-level directory.
  11  *
  12  */
  13
  14 #include "qemu/osdep.h"
  15
  16 #include "trace.h"
  17 #include "block/block.h"
  18 #include "block/block_int.h"
  19 #include "block/blockjob_int.h"
  20 #include "block/block_backup.h"
  21 #include "qapi/error.h"
  22 #include "qapi/qmp/qerror.h"
  23 #include "qemu/ratelimit.h"
  24 #include "qemu/cutils.h"
  25 #include "sysemu/block-backend.h"
  26 #include "qemu/bitmap.h"
  27 #include "qemu/error-report.h"
  28
  29 #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)
  30
  31 typedef struct BackupBlockJob {
  32     BlockJob common;
  33     BlockBackend *target;
  34     /* bitmap for sync=incremental */
  35     BdrvDirtyBitmap *sync_bitmap;
  36     MirrorSyncMode sync_mode;
  37     BlockdevOnError on_source_error;
  38     BlockdevOnError on_target_error;
  39     CoRwlock flush_rwlock;
  40     uint64_t len;
  41     uint64_t bytes_read;
  42     int64_t cluster_size;
  43     bool compress;
  44     NotifierWithReturn before_write;
  45     QLIST_HEAD(, CowRequest) inflight_reqs;
  46
  47     HBitmap *copy_bitmap;
  48 } BackupBlockJob;
  49
  50 static const BlockJobDriver backup_job_driver;
  51
  52 /* See if in-flight requests overlap and wait for them to complete */
  53 static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
  54                                                        int64_t start,
  55                                                        int64_t end)
  56 {
  57     CowRequest *req;
  58     bool retry;
  59
  60     do {
  61         retry = false;
  62         QLIST_FOREACH(req, &job->inflight_reqs, list) {
  63             if (end > req->start_byte && start < req->end_byte) {
  64                 qemu_co_queue_wait(&req->wait_queue, NULL);
  65                 retry = true;
  66                 break;
  67             }
  68         }
  69     } while (retry);
  70 }
  71
  72 /* Keep track of an in-flight request */
  73 static void cow_request_begin(CowRequest *req, BackupBlockJob *job,
  74                               int64_t start, int64_t end)
  75 {
  76     req->start_byte = start;
  77     req->end_byte = end;
  78     qemu_co_queue_init(&req->wait_queue);
  79     QLIST_INSERT_HEAD(&job->inflight_reqs, req, list);
  80 }
  81
  82 /* Forget about a completed request */
  83 static void cow_request_end(CowRequest *req)
  84 {
  85     QLIST_REMOVE(req, list);
  86     qemu_co_queue_restart_all(&req->wait_queue);
  87 }
  88
  89 static int coroutine_fn backup_do_cow(BackupBlockJob *job,
  90                                       int64_t offset, uint64_t bytes,
  91                                       bool *error_is_read,
  92                                       bool is_write_notifier)
  93 {
  94     BlockBackend *blk = job->common.blk;
  95     CowRequest cow_request;
  96     struct iovec iov;
  97     QEMUIOVector bounce_qiov;
  98     void *bounce_buffer = NULL;
  99     int ret = 0;
 100     int64_t start, end; /* bytes */
 101     int n; /* bytes */
 102
 103     qemu_co_rwlock_rdlock(&job->flush_rwlock);
 104
 105     start = QEMU_ALIGN_DOWN(offset, job->cluster_size);
 106     end = QEMU_ALIGN_UP(bytes + offset, job->cluster_size);
 107
 108     trace_backup_do_cow_enter(job, start, offset, bytes);
 109
 110     wait_for_overlapping_requests(job, start, end);
 111     cow_request_begin(&cow_request, job, start, end);
 112
 113     for (; start < end; start += job->cluster_size) {
 114         if (!hbitmap_get(job->copy_bitmap, start / job->cluster_size)) {
 115             trace_backup_do_cow_skip(job, start);
 116             continue; /* already copied */
 117         }
 118         hbitmap_reset(job->copy_bitmap, start / job->cluster_size, 1);
 119
 120         trace_backup_do_cow_process(job, start);
 121
 122         n = MIN(job->cluster_size, job->len - start);
 123
 124         if (!bounce_buffer) {
 125             bounce_buffer = blk_blockalign(blk, job->cluster_size);
 126         }
 127         iov.iov_base = bounce_buffer;
 128         iov.iov_len = n;
 129         qemu_iovec_init_external(&bounce_qiov, &iov, 1);
 130
 131         ret = blk_co_preadv(blk, start, bounce_qiov.size, &bounce_qiov,
 132                             is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0);
 133         if (ret < 0) {
 134             trace_backup_do_cow_read_fail(job, start, ret);
 135             if (error_is_read) {
 136                 *error_is_read = true;
 137             }
 138             hbitmap_set(job->copy_bitmap, start / job->cluster_size, 1);
 139             goto out;
 140         }
 141
 142         if (buffer_is_zero(iov.iov_base, iov.iov_len)) {
 143             ret = blk_co_pwrite_zeroes(job->target, start,
 144                                        bounce_qiov.size, BDRV_REQ_MAY_UNMAP);
 145         } else {
 146             ret = blk_co_pwritev(job->target, start,
 147                                  bounce_qiov.size, &bounce_qiov,
 148                                  job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
 149         }
 150         if (ret < 0) {
 151             trace_backup_do_cow_write_fail(job, start, ret);
 152             if (error_is_read) {
 153                 *error_is_read = false;
 154             }
 155             hbitmap_set(job->copy_bitmap, start / job->cluster_size, 1);
 156             goto out;
 157         }
 158
 159         /* Publish progress, guest I/O counts as progress too.  Note that the
 160          * offset field is an opaque progress value, it is not a disk offset.
 161          */
 162         job->bytes_read += n;
 163         job_progress_update(&job->common.job, n);
 164     }
 165
 166 out:
 167     if (bounce_buffer) {
 168         qemu_vfree(bounce_buffer);
 169     }
 170
 171     cow_request_end(&cow_request);
 172
 173     trace_backup_do_cow_return(job, offset, bytes, ret);
 174
 175     qemu_co_rwlock_unlock(&job->flush_rwlock);
 176
 177     return ret;
 178 }
 179
 180 static int coroutine_fn backup_before_write_notify(
 181         NotifierWithReturn *notifier,
 182         void *opaque)
 183 {
 184     BackupBlockJob *job = container_of(notifier, BackupBlockJob, before_write);
 185     BdrvTrackedRequest *req = opaque;
 186
 187     assert(req->bs == blk_bs(job->common.blk));
 188     assert(QEMU_IS_ALIGNED(req->offset, BDRV_SECTOR_SIZE));
 189     assert(QEMU_IS_ALIGNED(req->bytes, BDRV_SECTOR_SIZE));
 190
 191     return backup_do_cow(job, req->offset, req->bytes, NULL, true);
 192 }
 193
 194 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 195 {
 196     BdrvDirtyBitmap *bm;
 197     BlockDriverState *bs = blk_bs(job->common.blk);
 198
 199     if (ret < 0) {
 200         /* Merge the successor back into the parent, delete nothing. */
 201         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
 202         assert(bm);
 203     } else {
 204         /* Everything is fine, delete this bitmap and install the backup. */
 205         bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
 206         assert(bm);
 207     }
 208 }
 209
 210 static void backup_commit(Job *job)
 211 {
 212     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 213     if (s->sync_bitmap) {
 214         backup_cleanup_sync_bitmap(s, 0);
 215     }
 216 }
 217
 218 static void backup_abort(Job *job)
 219 {
 220     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 221     if (s->sync_bitmap) {
 222         backup_cleanup_sync_bitmap(s, -1);
 223     }
 224 }
 225
 226 static void backup_clean(Job *job)
 227 {
 228     BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
 229     assert(s->target);
 230     blk_unref(s->target);
 231     s->target = NULL;
 232 }
 233
 234 static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
 235 {
 236     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 237
 238     blk_set_aio_context(s->target, aio_context);
 239 }
 240
 241 void backup_do_checkpoint(BlockJob *job, Error **errp)
 242 {
 243     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 244     int64_t len;
 245
 246     assert(block_job_driver(job) == &backup_job_driver);
 247
 248     if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
 249         error_setg(errp, "The backup job only supports block checkpoint in"
 250                    " sync=none mode");
 251         return;
 252     }
 253
 254     len = DIV_ROUND_UP(backup_job->len, backup_job->cluster_size);
 255     hbitmap_set(backup_job->copy_bitmap, 0, len);
 256 }
 257
 258 void backup_wait_for_overlapping_requests(BlockJob *job, int64_t offset,
 259                                           uint64_t bytes)
 260 {
 261     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 262     int64_t start, end;
 263
 264     assert(block_job_driver(job) == &backup_job_driver);
 265
 266     start = QEMU_ALIGN_DOWN(offset, backup_job->cluster_size);
 267     end = QEMU_ALIGN_UP(offset + bytes, backup_job->cluster_size);
 268     wait_for_overlapping_requests(backup_job, start, end);
 269 }
 270
 271 void backup_cow_request_begin(CowRequest *req, BlockJob *job,
 272                               int64_t offset, uint64_t bytes)
 273 {
 274     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 275     int64_t start, end;
 276
 277     assert(block_job_driver(job) == &backup_job_driver);
 278
 279     start = QEMU_ALIGN_DOWN(offset, backup_job->cluster_size);
 280     end = QEMU_ALIGN_UP(offset + bytes, backup_job->cluster_size);
 281     cow_request_begin(req, backup_job, start, end);
 282 }
 283
 284 void backup_cow_request_end(CowRequest *req)
 285 {
 286     cow_request_end(req);
 287 }
 288
 289 static void backup_drain(BlockJob *job)
 290 {
 291     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 292
 293     /* Need to keep a reference in case blk_drain triggers execution
 294      * of backup_complete...
 295      */
 296     if (s->target) {
 297         BlockBackend *target = s->target;
 298         blk_ref(target);
 299         blk_drain(target);
 300         blk_unref(target);
 301     }
 302 }
 303
 304 static BlockErrorAction backup_error_action(BackupBlockJob *job,
 305                                             bool read, int error)
 306 {
 307     if (read) {
 308         return block_job_error_action(&job->common, job->on_source_error,
 309                                       true, error);
 310     } else {
 311         return block_job_error_action(&job->common, job->on_target_error,
 312                                       false, error);
 313     }
 314 }
 315
 316 typedef struct {
 317     int ret;
 318 } BackupCompleteData;
 319
 320 static void backup_complete(Job *job, void *opaque)
 321 {
 322     BackupCompleteData *data = opaque;
 323
 324     job_completed(job, data->ret, NULL);
 325     g_free(data);
 326 }
 327
 328 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 329 {
 330     uint64_t delay_ns;
 331
 332     if (job_is_cancelled(&job->common.job)) {
 333         return true;
 334     }
 335
 336     /* We need to yield even for delay_ns = 0 so that bdrv_drain_all() can
 337      * return. Without a yield, the VM would not reboot. */
 338     delay_ns = block_job_ratelimit_get_delay(&job->common, job->bytes_read);
 339     job->bytes_read = 0;
 340     job_sleep_ns(&job->common.job, delay_ns);
 341
 342     if (job_is_cancelled(&job->common.job)) {
 343         return true;
 344     }
 345
 346     return false;
 347 }
 348
 349 static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
 350 {
 351     int ret;
 352     bool error_is_read;
 353     int64_t cluster;
 354     HBitmapIter hbi;
 355
 356     hbitmap_iter_init(&hbi, job->copy_bitmap, 0);
 357     while ((cluster = hbitmap_iter_next(&hbi)) != -1) {
 358         do {
 359             if (yield_and_check(job)) {
 360                 return 0;
 361             }
 362             ret = backup_do_cow(job, cluster * job->cluster_size,
 363                                 job->cluster_size, &error_is_read, false);
 364             if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
 365                            BLOCK_ERROR_ACTION_REPORT)
 366             {
 367                 return ret;
 368             }
 369         } while (ret < 0);
 370     }
 371
 372     return 0;
 373 }
 374
 375 /* init copy_bitmap from sync_bitmap */
 376 static void backup_incremental_init_copy_bitmap(BackupBlockJob *job)
 377 {
 378     BdrvDirtyBitmapIter *dbi;
 379     int64_t offset;
 380     int64_t end = DIV_ROUND_UP(bdrv_dirty_bitmap_size(job->sync_bitmap),
 381                                job->cluster_size);
 382
 383     dbi = bdrv_dirty_iter_new(job->sync_bitmap);
 384     while ((offset = bdrv_dirty_iter_next(dbi)) != -1) {
 385         int64_t cluster = offset / job->cluster_size;
 386         int64_t next_cluster;
 387
 388         offset += bdrv_dirty_bitmap_granularity(job->sync_bitmap);
 389         if (offset >= bdrv_dirty_bitmap_size(job->sync_bitmap)) {
 390             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 391             break;
 392         }
 393
 394         offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset);
 395         if (offset == -1) {
 396             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 397             break;
 398         }
 399
 400         next_cluster = DIV_ROUND_UP(offset, job->cluster_size);
 401         hbitmap_set(job->copy_bitmap, cluster, next_cluster - cluster);
 402         if (next_cluster >= end) {
 403             break;
 404         }
 405
 406         bdrv_set_dirty_iter(dbi, next_cluster * job->cluster_size);
 407     }
 408
 409     /* TODO job_progress_set_remaining() would make more sense */
 410     job_progress_update(&job->common.job,
 411         job->len - hbitmap_count(job->copy_bitmap) * job->cluster_size);
 412
 413     bdrv_dirty_iter_free(dbi);
 414 }
 415
 416 static void coroutine_fn backup_run(void *opaque)
 417 {
 418     BackupBlockJob *job = opaque;
 419     BackupCompleteData *data;
 420     BlockDriverState *bs = blk_bs(job->common.blk);
 421     int64_t offset, nb_clusters;
 422     int ret = 0;
 423
 424     QLIST_INIT(&job->inflight_reqs);
 425     qemu_co_rwlock_init(&job->flush_rwlock);
 426
 427     nb_clusters = DIV_ROUND_UP(job->len, job->cluster_size);
 428     job_progress_set_remaining(&job->common.job, job->len);
 429
 430     job->copy_bitmap = hbitmap_alloc(nb_clusters, 0);
 431     if (job->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 432         backup_incremental_init_copy_bitmap(job);
 433     } else {
 434         hbitmap_set(job->copy_bitmap, 0, nb_clusters);
 435     }
 436
 437
 438     job->before_write.notify = backup_before_write_notify;
 439     bdrv_add_before_write_notifier(bs, &job->before_write);
 440
 441     if (job->sync_mode == MIRROR_SYNC_MODE_NONE) {
 442         /* All bits are set in copy_bitmap to allow any cluster to be copied.
 443          * This does not actually require them to be copied. */
 444         while (!job_is_cancelled(&job->common.job)) {
 445             /* Yield until the job is cancelled.  We just let our before_write
 446              * notify callback service CoW requests. */
 447             job_yield(&job->common.job);
 448         }
 449     } else if (job->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 450         ret = backup_run_incremental(job);
 451     } else {
 452         /* Both FULL and TOP SYNC_MODE's require copying.. */
 453         for (offset = 0; offset < job->len;
 454              offset += job->cluster_size) {
 455             bool error_is_read;
 456             int alloced = 0;
 457
 458             if (yield_and_check(job)) {
 459                 break;
 460             }
 461
 462             if (job->sync_mode == MIRROR_SYNC_MODE_TOP) {
 463                 int i;
 464                 int64_t n;
 465
 466                 /* Check to see if these blocks are already in the
 467                  * backing file. */
 468
 469                 for (i = 0; i < job->cluster_size;) {
 470                     /* bdrv_is_allocated() only returns true/false based
 471                      * on the first set of sectors it comes across that
 472                      * are are all in the same state.
 473                      * For that reason we must verify each sector in the
 474                      * backup cluster length.  We end up copying more than
 475                      * needed but at some point that is always the case. */
 476                     alloced =
 477                         bdrv_is_allocated(bs, offset + i,
 478                                           job->cluster_size - i, &n);
 479                     i += n;
 480
 481                     if (alloced || n == 0) {
 482                         break;
 483                     }
 484                 }
 485
 486                 /* If the above loop never found any sectors that are in
 487                  * the topmost image, skip this backup. */
 488                 if (alloced == 0) {
 489                     continue;
 490                 }
 491             }
 492             /* FULL sync mode we copy the whole drive. */
 493             if (alloced < 0) {
 494                 ret = alloced;
 495             } else {
 496                 ret = backup_do_cow(job, offset, job->cluster_size,
 497                                     &error_is_read, false);
 498             }
 499             if (ret < 0) {
 500                 /* Depending on error action, fail now or retry cluster */
 501                 BlockErrorAction action =
 502                     backup_error_action(job, error_is_read, -ret);
 503                 if (action == BLOCK_ERROR_ACTION_REPORT) {
 504                     break;
 505                 } else {
 506                     offset -= job->cluster_size;
 507                     continue;
 508                 }
 509             }
 510         }
 511     }
 512
 513     notifier_with_return_remove(&job->before_write);
 514
 515     /* wait until pending backup_do_cow() calls have completed */
 516     qemu_co_rwlock_wrlock(&job->flush_rwlock);
 517     qemu_co_rwlock_unlock(&job->flush_rwlock);
 518     hbitmap_free(job->copy_bitmap);
 519
 520     data = g_malloc(sizeof(*data));
 521     data->ret = ret;
 522     job_defer_to_main_loop(&job->common.job, backup_complete, data);
 523 }
 524
 525 static const BlockJobDriver backup_job_driver = {
 526     .job_driver = {
 527         .instance_size          = sizeof(BackupBlockJob),
 528         .job_type               = JOB_TYPE_BACKUP,
 529         .free                   = block_job_free,
 530         .user_resume            = block_job_user_resume,
 531         .drain                  = block_job_drain,
 532         .start                  = backup_run,
 533         .commit                 = backup_commit,
 534         .abort                  = backup_abort,
 535         .clean                  = backup_clean,
 536     },
 537     .attached_aio_context   = backup_attached_aio_context,
 538     .drain                  = backup_drain,
 539 };
 540
 541 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
 542                   BlockDriverState *target, int64_t speed,
 543                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
 544                   bool compress,
 545                   BlockdevOnError on_source_error,
 546                   BlockdevOnError on_target_error,
 547                   int creation_flags,
 548                   BlockCompletionFunc *cb, void *opaque,
 549                   JobTxn *txn, Error **errp)
 550 {
 551     int64_t len;
 552     BlockDriverInfo bdi;
 553     BackupBlockJob *job = NULL;
 554     int ret;
 555
 556     assert(bs);
 557     assert(target);
 558
 559     if (bs == target) {
 560         error_setg(errp, "Source and target cannot be the same");
 561         return NULL;
 562     }
 563
 564     if (!bdrv_is_inserted(bs)) {
 565         error_setg(errp, "Device is not inserted: %s",
 566                    bdrv_get_device_name(bs));
 567         return NULL;
 568     }
 569
 570     if (!bdrv_is_inserted(target)) {
 571         error_setg(errp, "Device is not inserted: %s",
 572                    bdrv_get_device_name(target));
 573         return NULL;
 574     }
 575
 576     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
 577         error_setg(errp, "Compression is not supported for this drive %s",
 578                    bdrv_get_device_name(target));
 579         return NULL;
 580     }
 581
 582     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
 583         return NULL;
 584     }
 585
 586     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
 587         return NULL;
 588     }
 589
 590     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 591         if (!sync_bitmap) {
 592             error_setg(errp, "must provide a valid bitmap name for "
 593                              "\"incremental\" sync mode");
 594             return NULL;
 595         }
 596
 597         /* Create a new bitmap, and freeze/disable this one. */
 598         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
 599             return NULL;
 600         }
 601     } else if (sync_bitmap) {
 602         error_setg(errp,
 603                    "a sync_bitmap was provided to backup_run, "
 604                    "but received an incompatible sync_mode (%s)",
 605                    MirrorSyncMode_str(sync_mode));
 606         return NULL;
 607     }
 608
 609     len = bdrv_getlength(bs);
 610     if (len < 0) {
 611         error_setg_errno(errp, -len, "unable to get length for '%s'",
 612                          bdrv_get_device_name(bs));
 613         goto error;
 614     }
 615
 616     /* job->len is fixed, so we can't allow resize */
 617     job = block_job_create(job_id, &backup_job_driver, txn, bs,
 618                            BLK_PERM_CONSISTENT_READ,
 619                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 620                            BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD,
 621                            speed, creation_flags, cb, opaque, errp);
 622     if (!job) {
 623         goto error;
 624     }
 625
 626     /* The target must match the source in size, so no resize here either */
 627     job->target = blk_new(BLK_PERM_WRITE,
 628                           BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 629                           BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD);
 630     ret = blk_insert_bs(job->target, target, errp);
 631     if (ret < 0) {
 632         goto error;
 633     }
 634
 635     job->on_source_error = on_source_error;
 636     job->on_target_error = on_target_error;
 637     job->sync_mode = sync_mode;
 638     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
 639                        sync_bitmap : NULL;
 640     job->compress = compress;
 641
 642     /* If there is no backing file on the target, we cannot rely on COW if our
 643      * backup cluster size is smaller than the target cluster size. Even for
 644      * targets with a backing file, try to avoid COW if possible. */
 645     ret = bdrv_get_info(target, &bdi);
 646     if (ret == -ENOTSUP && !target->backing) {
 647         /* Cluster size is not defined */
 648         warn_report("The target block device doesn't provide "
 649                     "information about the block size and it doesn't have a "
 650                     "backing file. The default block size of %u bytes is "
 651                     "used. If the actual block size of the target exceeds "
 652                     "this default, the backup may be unusable",
 653                     BACKUP_CLUSTER_SIZE_DEFAULT);
 654         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 655     } else if (ret < 0 && !target->backing) {
 656         error_setg_errno(errp, -ret,
 657             "Couldn't determine the cluster size of the target image, "
 658             "which has no backing file");
 659         error_append_hint(errp,
 660             "Aborting, since this may create an unusable destination image\n");
 661         goto error;
 662     } else if (ret < 0 && target->backing) {
 663         /* Not fatal; just trudge on ahead. */
 664         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 665     } else {
 666         job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
 667     }
 668
 669     /* Required permissions are already taken with target's blk_new() */
 670     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
 671                        &error_abort);
 672     job->len = len;
 673
 674     return &job->common;
 675
 676  error:
 677     if (sync_bitmap) {
 678         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
 679     }
 680     if (job) {
 681         backup_clean(&job->common.job);
 682         job_early_fail(&job->common.job);
 683     }
 684
 685     return NULL;
 686 }