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 #define SLICE_TIME 100000000ULL /* ns */
  31
  32 typedef struct BackupBlockJob {
  33     BlockJob common;
  34     BlockBackend *target;
  35     /* bitmap for sync=incremental */
  36     BdrvDirtyBitmap *sync_bitmap;
  37     MirrorSyncMode sync_mode;
  38     RateLimit limit;
  39     BlockdevOnError on_source_error;
  40     BlockdevOnError on_target_error;
  41     CoRwlock flush_rwlock;
  42     uint64_t sectors_read;
  43     unsigned long *done_bitmap;
  44     int64_t cluster_size;
  45     bool compress;
  46     NotifierWithReturn before_write;
  47     QLIST_HEAD(, CowRequest) inflight_reqs;
  48 } BackupBlockJob;
  49
  50 /* Size of a cluster in sectors, instead of bytes. */
  51 static inline int64_t cluster_size_sectors(BackupBlockJob *job)
  52 {
  53   return job->cluster_size / BDRV_SECTOR_SIZE;
  54 }
  55
  56 /* See if in-flight requests overlap and wait for them to complete */
  57 static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
  58                                                        int64_t start,
  59                                                        int64_t end)
  60 {
  61     CowRequest *req;
  62     bool retry;
  63
  64     do {
  65         retry = false;
  66         QLIST_FOREACH(req, &job->inflight_reqs, list) {
  67             if (end > req->start && start < req->end) {
  68                 qemu_co_queue_wait(&req->wait_queue, NULL);
  69                 retry = true;
  70                 break;
  71             }
  72         }
  73     } while (retry);
  74 }
  75
  76 /* Keep track of an in-flight request */
  77 static void cow_request_begin(CowRequest *req, BackupBlockJob *job,
  78                                      int64_t start, int64_t end)
  79 {
  80     req->start = start;
  81     req->end = end;
  82     qemu_co_queue_init(&req->wait_queue);
  83     QLIST_INSERT_HEAD(&job->inflight_reqs, req, list);
  84 }
  85
  86 /* Forget about a completed request */
  87 static void cow_request_end(CowRequest *req)
  88 {
  89     QLIST_REMOVE(req, list);
  90     qemu_co_queue_restart_all(&req->wait_queue);
  91 }
  92
  93 static int coroutine_fn backup_do_cow(BackupBlockJob *job,
  94                                       int64_t sector_num, int nb_sectors,
  95                                       bool *error_is_read,
  96                                       bool is_write_notifier)
  97 {
  98     BlockBackend *blk = job->common.blk;
  99     CowRequest cow_request;
 100     struct iovec iov;
 101     QEMUIOVector bounce_qiov;
 102     void *bounce_buffer = NULL;
 103     int ret = 0;
 104     int64_t sectors_per_cluster = cluster_size_sectors(job);
 105     int64_t bytes_per_cluster = sectors_per_cluster * BDRV_SECTOR_SIZE;
 106     int64_t start, end;
 107     int n;
 108
 109     qemu_co_rwlock_rdlock(&job->flush_rwlock);
 110
 111     start = sector_num / sectors_per_cluster;
 112     end = DIV_ROUND_UP(sector_num + nb_sectors, sectors_per_cluster);
 113
 114     trace_backup_do_cow_enter(job, start * bytes_per_cluster,
 115                               sector_num * BDRV_SECTOR_SIZE,
 116                               nb_sectors * BDRV_SECTOR_SIZE);
 117
 118     wait_for_overlapping_requests(job, start, end);
 119     cow_request_begin(&cow_request, job, start, end);
 120
 121     for (; start < end; start++) {
 122         if (test_bit(start, job->done_bitmap)) {
 123             trace_backup_do_cow_skip(job, start * bytes_per_cluster);
 124             continue; /* already copied */
 125         }
 126
 127         trace_backup_do_cow_process(job, start * bytes_per_cluster);
 128
 129         n = MIN(sectors_per_cluster,
 130                 job->common.len / BDRV_SECTOR_SIZE -
 131                 start * sectors_per_cluster);
 132
 133         if (!bounce_buffer) {
 134             bounce_buffer = blk_blockalign(blk, job->cluster_size);
 135         }
 136         iov.iov_base = bounce_buffer;
 137         iov.iov_len = n * BDRV_SECTOR_SIZE;
 138         qemu_iovec_init_external(&bounce_qiov, &iov, 1);
 139
 140         ret = blk_co_preadv(blk, start * job->cluster_size,
 141                             bounce_qiov.size, &bounce_qiov,
 142                             is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0);
 143         if (ret < 0) {
 144             trace_backup_do_cow_read_fail(job, start * bytes_per_cluster, ret);
 145             if (error_is_read) {
 146                 *error_is_read = true;
 147             }
 148             goto out;
 149         }
 150
 151         if (buffer_is_zero(iov.iov_base, iov.iov_len)) {
 152             ret = blk_co_pwrite_zeroes(job->target, start * job->cluster_size,
 153                                        bounce_qiov.size, BDRV_REQ_MAY_UNMAP);
 154         } else {
 155             ret = blk_co_pwritev(job->target, start * job->cluster_size,
 156                                  bounce_qiov.size, &bounce_qiov,
 157                                  job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
 158         }
 159         if (ret < 0) {
 160             trace_backup_do_cow_write_fail(job, start * bytes_per_cluster, ret);
 161             if (error_is_read) {
 162                 *error_is_read = false;
 163             }
 164             goto out;
 165         }
 166
 167         set_bit(start, job->done_bitmap);
 168
 169         /* Publish progress, guest I/O counts as progress too.  Note that the
 170          * offset field is an opaque progress value, it is not a disk offset.
 171          */
 172         job->sectors_read += n;
 173         job->common.offset += n * BDRV_SECTOR_SIZE;
 174     }
 175
 176 out:
 177     if (bounce_buffer) {
 178         qemu_vfree(bounce_buffer);
 179     }
 180
 181     cow_request_end(&cow_request);
 182
 183     trace_backup_do_cow_return(job, sector_num * BDRV_SECTOR_SIZE,
 184                                nb_sectors * BDRV_SECTOR_SIZE, ret);
 185
 186     qemu_co_rwlock_unlock(&job->flush_rwlock);
 187
 188     return ret;
 189 }
 190
 191 static int coroutine_fn backup_before_write_notify(
 192         NotifierWithReturn *notifier,
 193         void *opaque)
 194 {
 195     BackupBlockJob *job = container_of(notifier, BackupBlockJob, before_write);
 196     BdrvTrackedRequest *req = opaque;
 197     int64_t sector_num = req->offset >> BDRV_SECTOR_BITS;
 198     int nb_sectors = req->bytes >> BDRV_SECTOR_BITS;
 199
 200     assert(req->bs == blk_bs(job->common.blk));
 201     assert((req->offset & (BDRV_SECTOR_SIZE - 1)) == 0);
 202     assert((req->bytes & (BDRV_SECTOR_SIZE - 1)) == 0);
 203
 204     return backup_do_cow(job, sector_num, nb_sectors, NULL, true);
 205 }
 206
 207 static void backup_set_speed(BlockJob *job, int64_t speed, Error **errp)
 208 {
 209     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 210
 211     if (speed < 0) {
 212         error_setg(errp, QERR_INVALID_PARAMETER, "speed");
 213         return;
 214     }
 215     ratelimit_set_speed(&s->limit, speed, SLICE_TIME);
 216 }
 217
 218 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 219 {
 220     BdrvDirtyBitmap *bm;
 221     BlockDriverState *bs = blk_bs(job->common.blk);
 222
 223     if (ret < 0 || block_job_is_cancelled(&job->common)) {
 224         /* Merge the successor back into the parent, delete nothing. */
 225         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
 226         assert(bm);
 227     } else {
 228         /* Everything is fine, delete this bitmap and install the backup. */
 229         bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
 230         assert(bm);
 231     }
 232 }
 233
 234 static void backup_commit(BlockJob *job)
 235 {
 236     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 237     if (s->sync_bitmap) {
 238         backup_cleanup_sync_bitmap(s, 0);
 239     }
 240 }
 241
 242 static void backup_abort(BlockJob *job)
 243 {
 244     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 245     if (s->sync_bitmap) {
 246         backup_cleanup_sync_bitmap(s, -1);
 247     }
 248 }
 249
 250 static void backup_clean(BlockJob *job)
 251 {
 252     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 253     assert(s->target);
 254     blk_unref(s->target);
 255     s->target = NULL;
 256 }
 257
 258 static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
 259 {
 260     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 261
 262     blk_set_aio_context(s->target, aio_context);
 263 }
 264
 265 void backup_do_checkpoint(BlockJob *job, Error **errp)
 266 {
 267     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 268     int64_t len;
 269
 270     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);
 271
 272     if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
 273         error_setg(errp, "The backup job only supports block checkpoint in"
 274                    " sync=none mode");
 275         return;
 276     }
 277
 278     len = DIV_ROUND_UP(backup_job->common.len, backup_job->cluster_size);
 279     bitmap_zero(backup_job->done_bitmap, len);
 280 }
 281
 282 void backup_wait_for_overlapping_requests(BlockJob *job, int64_t sector_num,
 283                                           int nb_sectors)
 284 {
 285     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 286     int64_t sectors_per_cluster = cluster_size_sectors(backup_job);
 287     int64_t start, end;
 288
 289     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);
 290
 291     start = sector_num / sectors_per_cluster;
 292     end = DIV_ROUND_UP(sector_num + nb_sectors, sectors_per_cluster);
 293     wait_for_overlapping_requests(backup_job, start, end);
 294 }
 295
 296 void backup_cow_request_begin(CowRequest *req, BlockJob *job,
 297                               int64_t sector_num,
 298                               int nb_sectors)
 299 {
 300     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 301     int64_t sectors_per_cluster = cluster_size_sectors(backup_job);
 302     int64_t start, end;
 303
 304     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);
 305
 306     start = sector_num / sectors_per_cluster;
 307     end = DIV_ROUND_UP(sector_num + nb_sectors, sectors_per_cluster);
 308     cow_request_begin(req, backup_job, start, end);
 309 }
 310
 311 void backup_cow_request_end(CowRequest *req)
 312 {
 313     cow_request_end(req);
 314 }
 315
 316 static void backup_drain(BlockJob *job)
 317 {
 318     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 319
 320     /* Need to keep a reference in case blk_drain triggers execution
 321      * of backup_complete...
 322      */
 323     if (s->target) {
 324         BlockBackend *target = s->target;
 325         blk_ref(target);
 326         blk_drain(target);
 327         blk_unref(target);
 328     }
 329 }
 330
 331 static BlockErrorAction backup_error_action(BackupBlockJob *job,
 332                                             bool read, int error)
 333 {
 334     if (read) {
 335         return block_job_error_action(&job->common, job->on_source_error,
 336                                       true, error);
 337     } else {
 338         return block_job_error_action(&job->common, job->on_target_error,
 339                                       false, error);
 340     }
 341 }
 342
 343 typedef struct {
 344     int ret;
 345 } BackupCompleteData;
 346
 347 static void backup_complete(BlockJob *job, void *opaque)
 348 {
 349     BackupCompleteData *data = opaque;
 350
 351     block_job_completed(job, data->ret);
 352     g_free(data);
 353 }
 354
 355 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 356 {
 357     if (block_job_is_cancelled(&job->common)) {
 358         return true;
 359     }
 360
 361     /* we need to yield so that bdrv_drain_all() returns.
 362      * (without, VM does not reboot)
 363      */
 364     if (job->common.speed) {
 365         uint64_t delay_ns = ratelimit_calculate_delay(&job->limit,
 366                                                       job->sectors_read *
 367                                                       BDRV_SECTOR_SIZE);
 368         job->sectors_read = 0;
 369         block_job_sleep_ns(&job->common, QEMU_CLOCK_REALTIME, delay_ns);
 370     } else {
 371         block_job_sleep_ns(&job->common, QEMU_CLOCK_REALTIME, 0);
 372     }
 373
 374     if (block_job_is_cancelled(&job->common)) {
 375         return true;
 376     }
 377
 378     return false;
 379 }
 380
 381 static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
 382 {
 383     bool error_is_read;
 384     int ret = 0;
 385     int clusters_per_iter;
 386     uint32_t granularity;
 387     int64_t sector;
 388     int64_t cluster;
 389     int64_t end;
 390     int64_t last_cluster = -1;
 391     int64_t sectors_per_cluster = cluster_size_sectors(job);
 392     BdrvDirtyBitmapIter *dbi;
 393
 394     granularity = bdrv_dirty_bitmap_granularity(job->sync_bitmap);
 395     clusters_per_iter = MAX((granularity / job->cluster_size), 1);
 396     dbi = bdrv_dirty_iter_new(job->sync_bitmap, 0);
 397
 398     /* Find the next dirty sector(s) */
 399     while ((sector = bdrv_dirty_iter_next(dbi)) != -1) {
 400         cluster = sector / sectors_per_cluster;
 401
 402         /* Fake progress updates for any clusters we skipped */
 403         if (cluster != last_cluster + 1) {
 404             job->common.offset += ((cluster - last_cluster - 1) *
 405                                    job->cluster_size);
 406         }
 407
 408         for (end = cluster + clusters_per_iter; cluster < end; cluster++) {
 409             do {
 410                 if (yield_and_check(job)) {
 411                     goto out;
 412                 }
 413                 ret = backup_do_cow(job, cluster * sectors_per_cluster,
 414                                     sectors_per_cluster, &error_is_read,
 415                                     false);
 416                 if ((ret < 0) &&
 417                     backup_error_action(job, error_is_read, -ret) ==
 418                     BLOCK_ERROR_ACTION_REPORT) {
 419                     goto out;
 420                 }
 421             } while (ret < 0);
 422         }
 423
 424         /* If the bitmap granularity is smaller than the backup granularity,
 425          * we need to advance the iterator pointer to the next cluster. */
 426         if (granularity < job->cluster_size) {
 427             bdrv_set_dirty_iter(dbi, cluster * sectors_per_cluster);
 428         }
 429
 430         last_cluster = cluster - 1;
 431     }
 432
 433     /* Play some final catchup with the progress meter */
 434     end = DIV_ROUND_UP(job->common.len, job->cluster_size);
 435     if (last_cluster + 1 < end) {
 436         job->common.offset += ((end - last_cluster - 1) * job->cluster_size);
 437     }
 438
 439 out:
 440     bdrv_dirty_iter_free(dbi);
 441     return ret;
 442 }
 443
 444 static void coroutine_fn backup_run(void *opaque)
 445 {
 446     BackupBlockJob *job = opaque;
 447     BackupCompleteData *data;
 448     BlockDriverState *bs = blk_bs(job->common.blk);
 449     int64_t start, end;
 450     int64_t sectors_per_cluster = cluster_size_sectors(job);
 451     int ret = 0;
 452
 453     QLIST_INIT(&job->inflight_reqs);
 454     qemu_co_rwlock_init(&job->flush_rwlock);
 455
 456     start = 0;
 457     end = DIV_ROUND_UP(job->common.len, job->cluster_size);
 458
 459     job->done_bitmap = bitmap_new(end);
 460
 461     job->before_write.notify = backup_before_write_notify;
 462     bdrv_add_before_write_notifier(bs, &job->before_write);
 463
 464     if (job->sync_mode == MIRROR_SYNC_MODE_NONE) {
 465         while (!block_job_is_cancelled(&job->common)) {
 466             /* Yield until the job is cancelled.  We just let our before_write
 467              * notify callback service CoW requests. */
 468             block_job_yield(&job->common);
 469         }
 470     } else if (job->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 471         ret = backup_run_incremental(job);
 472     } else {
 473         /* Both FULL and TOP SYNC_MODE's require copying.. */
 474         for (; start < end; start++) {
 475             bool error_is_read;
 476             int alloced = 0;
 477
 478             if (yield_and_check(job)) {
 479                 break;
 480             }
 481
 482             if (job->sync_mode == MIRROR_SYNC_MODE_TOP) {
 483                 int i, n;
 484
 485                 /* Check to see if these blocks are already in the
 486                  * backing file. */
 487
 488                 for (i = 0; i < sectors_per_cluster;) {
 489                     /* bdrv_is_allocated() only returns true/false based
 490                      * on the first set of sectors it comes across that
 491                      * are are all in the same state.
 492                      * For that reason we must verify each sector in the
 493                      * backup cluster length.  We end up copying more than
 494                      * needed but at some point that is always the case. */
 495                     alloced =
 496                         bdrv_is_allocated(bs,
 497                                 start * sectors_per_cluster + i,
 498                                 sectors_per_cluster - i, &n);
 499                     i += n;
 500
 501                     if (alloced || n == 0) {
 502                         break;
 503                     }
 504                 }
 505
 506                 /* If the above loop never found any sectors that are in
 507                  * the topmost image, skip this backup. */
 508                 if (alloced == 0) {
 509                     continue;
 510                 }
 511             }
 512             /* FULL sync mode we copy the whole drive. */
 513             if (alloced < 0) {
 514                 ret = alloced;
 515             } else {
 516                 ret = backup_do_cow(job, start * sectors_per_cluster,
 517                                     sectors_per_cluster, &error_is_read,
 518                                     false);
 519             }
 520             if (ret < 0) {
 521                 /* Depending on error action, fail now or retry cluster */
 522                 BlockErrorAction action =
 523                     backup_error_action(job, error_is_read, -ret);
 524                 if (action == BLOCK_ERROR_ACTION_REPORT) {
 525                     break;
 526                 } else {
 527                     start--;
 528                     continue;
 529                 }
 530             }
 531         }
 532     }
 533
 534     notifier_with_return_remove(&job->before_write);
 535
 536     /* wait until pending backup_do_cow() calls have completed */
 537     qemu_co_rwlock_wrlock(&job->flush_rwlock);
 538     qemu_co_rwlock_unlock(&job->flush_rwlock);
 539     g_free(job->done_bitmap);
 540
 541     data = g_malloc(sizeof(*data));
 542     data->ret = ret;
 543     block_job_defer_to_main_loop(&job->common, backup_complete, data);
 544 }
 545
 546 static const BlockJobDriver backup_job_driver = {
 547     .instance_size          = sizeof(BackupBlockJob),
 548     .job_type               = BLOCK_JOB_TYPE_BACKUP,
 549     .start                  = backup_run,
 550     .set_speed              = backup_set_speed,
 551     .commit                 = backup_commit,
 552     .abort                  = backup_abort,
 553     .clean                  = backup_clean,
 554     .attached_aio_context   = backup_attached_aio_context,
 555     .drain                  = backup_drain,
 556 };
 557
 558 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
 559                   BlockDriverState *target, int64_t speed,
 560                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
 561                   bool compress,
 562                   BlockdevOnError on_source_error,
 563                   BlockdevOnError on_target_error,
 564                   int creation_flags,
 565                   BlockCompletionFunc *cb, void *opaque,
 566                   BlockJobTxn *txn, Error **errp)
 567 {
 568     int64_t len;
 569     BlockDriverInfo bdi;
 570     BackupBlockJob *job = NULL;
 571     int ret;
 572
 573     assert(bs);
 574     assert(target);
 575
 576     if (bs == target) {
 577         error_setg(errp, "Source and target cannot be the same");
 578         return NULL;
 579     }
 580
 581     if (!bdrv_is_inserted(bs)) {
 582         error_setg(errp, "Device is not inserted: %s",
 583                    bdrv_get_device_name(bs));
 584         return NULL;
 585     }
 586
 587     if (!bdrv_is_inserted(target)) {
 588         error_setg(errp, "Device is not inserted: %s",
 589                    bdrv_get_device_name(target));
 590         return NULL;
 591     }
 592
 593     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
 594         error_setg(errp, "Compression is not supported for this drive %s",
 595                    bdrv_get_device_name(target));
 596         return NULL;
 597     }
 598
 599     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
 600         return NULL;
 601     }
 602
 603     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
 604         return NULL;
 605     }
 606
 607     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 608         if (!sync_bitmap) {
 609             error_setg(errp, "must provide a valid bitmap name for "
 610                              "\"incremental\" sync mode");
 611             return NULL;
 612         }
 613
 614         /* Create a new bitmap, and freeze/disable this one. */
 615         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
 616             return NULL;
 617         }
 618     } else if (sync_bitmap) {
 619         error_setg(errp,
 620                    "a sync_bitmap was provided to backup_run, "
 621                    "but received an incompatible sync_mode (%s)",
 622                    MirrorSyncMode_lookup[sync_mode]);
 623         return NULL;
 624     }
 625
 626     len = bdrv_getlength(bs);
 627     if (len < 0) {
 628         error_setg_errno(errp, -len, "unable to get length for '%s'",
 629                          bdrv_get_device_name(bs));
 630         goto error;
 631     }
 632
 633     /* job->common.len is fixed, so we can't allow resize */
 634     job = block_job_create(job_id, &backup_job_driver, bs,
 635                            BLK_PERM_CONSISTENT_READ,
 636                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 637                            BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD,
 638                            speed, creation_flags, cb, opaque, errp);
 639     if (!job) {
 640         goto error;
 641     }
 642
 643     /* The target must match the source in size, so no resize here either */
 644     job->target = blk_new(BLK_PERM_WRITE,
 645                           BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 646                           BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD);
 647     ret = blk_insert_bs(job->target, target, errp);
 648     if (ret < 0) {
 649         goto error;
 650     }
 651
 652     job->on_source_error = on_source_error;
 653     job->on_target_error = on_target_error;
 654     job->sync_mode = sync_mode;
 655     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
 656                        sync_bitmap : NULL;
 657     job->compress = compress;
 658
 659     /* If there is no backing file on the target, we cannot rely on COW if our
 660      * backup cluster size is smaller than the target cluster size. Even for
 661      * targets with a backing file, try to avoid COW if possible. */
 662     ret = bdrv_get_info(target, &bdi);
 663     if (ret == -ENOTSUP && !target->backing) {
 664         /* Cluster size is not defined */
 665         error_report("WARNING: The target block device doesn't provide "
 666                      "information about the block size and it doesn't have a "
 667                      "backing file. The default block size of %u bytes is "
 668                      "used. If the actual block size of the target exceeds "
 669                      "this default, the backup may be unusable",
 670                      BACKUP_CLUSTER_SIZE_DEFAULT);
 671         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 672     } else if (ret < 0 && !target->backing) {
 673         error_setg_errno(errp, -ret,
 674             "Couldn't determine the cluster size of the target image, "
 675             "which has no backing file");
 676         error_append_hint(errp,
 677             "Aborting, since this may create an unusable destination image\n");
 678         goto error;
 679     } else if (ret < 0 && target->backing) {
 680         /* Not fatal; just trudge on ahead. */
 681         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 682     } else {
 683         job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
 684     }
 685
 686     /* Required permissions are already taken with target's blk_new() */
 687     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
 688                        &error_abort);
 689     job->common.len = len;
 690     block_job_txn_add_job(txn, &job->common);
 691
 692     return &job->common;
 693
 694  error:
 695     if (sync_bitmap) {
 696         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
 697     }
 698     if (job) {
 699         backup_clean(&job->common);
 700         block_job_early_fail(&job->common);
 701     }
 702
 703     return NULL;
 704 }