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 bytes_read;
  43     int64_t cluster_size;
  44     bool compress;
  45     NotifierWithReturn before_write;
  46     QLIST_HEAD(, CowRequest) inflight_reqs;
  47
  48     HBitmap *copy_bitmap;
  49 } BackupBlockJob;
  50
  51 /* See if in-flight requests overlap and wait for them to complete */
  52 static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
  53                                                        int64_t start,
  54                                                        int64_t end)
  55 {
  56     CowRequest *req;
  57     bool retry;
  58
  59     do {
  60         retry = false;
  61         QLIST_FOREACH(req, &job->inflight_reqs, list) {
  62             if (end > req->start_byte && start < req->end_byte) {
  63                 qemu_co_queue_wait(&req->wait_queue, NULL);
  64                 retry = true;
  65                 break;
  66             }
  67         }
  68     } while (retry);
  69 }
  70
  71 /* Keep track of an in-flight request */
  72 static void cow_request_begin(CowRequest *req, BackupBlockJob *job,
  73                               int64_t start, int64_t end)
  74 {
  75     req->start_byte = start;
  76     req->end_byte = end;
  77     qemu_co_queue_init(&req->wait_queue);
  78     QLIST_INSERT_HEAD(&job->inflight_reqs, req, list);
  79 }
  80
  81 /* Forget about a completed request */
  82 static void cow_request_end(CowRequest *req)
  83 {
  84     QLIST_REMOVE(req, list);
  85     qemu_co_queue_restart_all(&req->wait_queue);
  86 }
  87
  88 static int coroutine_fn backup_do_cow(BackupBlockJob *job,
  89                                       int64_t offset, uint64_t bytes,
  90                                       bool *error_is_read,
  91                                       bool is_write_notifier)
  92 {
  93     BlockBackend *blk = job->common.blk;
  94     CowRequest cow_request;
  95     struct iovec iov;
  96     QEMUIOVector bounce_qiov;
  97     void *bounce_buffer = NULL;
  98     int ret = 0;
  99     int64_t start, end; /* bytes */
 100     int n; /* bytes */
 101
 102     qemu_co_rwlock_rdlock(&job->flush_rwlock);
 103
 104     start = QEMU_ALIGN_DOWN(offset, job->cluster_size);
 105     end = QEMU_ALIGN_UP(bytes + offset, job->cluster_size);
 106
 107     trace_backup_do_cow_enter(job, start, offset, bytes);
 108
 109     wait_for_overlapping_requests(job, start, end);
 110     cow_request_begin(&cow_request, job, start, end);
 111
 112     for (; start < end; start += job->cluster_size) {
 113         if (!hbitmap_get(job->copy_bitmap, start / job->cluster_size)) {
 114             trace_backup_do_cow_skip(job, start);
 115             continue; /* already copied */
 116         }
 117         hbitmap_reset(job->copy_bitmap, start / job->cluster_size, 1);
 118
 119         trace_backup_do_cow_process(job, start);
 120
 121         n = MIN(job->cluster_size, job->common.len - start);
 122
 123         if (!bounce_buffer) {
 124             bounce_buffer = blk_blockalign(blk, job->cluster_size);
 125         }
 126         iov.iov_base = bounce_buffer;
 127         iov.iov_len = n;
 128         qemu_iovec_init_external(&bounce_qiov, &iov, 1);
 129
 130         ret = blk_co_preadv(blk, start, bounce_qiov.size, &bounce_qiov,
 131                             is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0);
 132         if (ret < 0) {
 133             trace_backup_do_cow_read_fail(job, start, ret);
 134             if (error_is_read) {
 135                 *error_is_read = true;
 136             }
 137             hbitmap_set(job->copy_bitmap, start / job->cluster_size, 1);
 138             goto out;
 139         }
 140
 141         if (buffer_is_zero(iov.iov_base, iov.iov_len)) {
 142             ret = blk_co_pwrite_zeroes(job->target, start,
 143                                        bounce_qiov.size, BDRV_REQ_MAY_UNMAP);
 144         } else {
 145             ret = blk_co_pwritev(job->target, start,
 146                                  bounce_qiov.size, &bounce_qiov,
 147                                  job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
 148         }
 149         if (ret < 0) {
 150             trace_backup_do_cow_write_fail(job, start, ret);
 151             if (error_is_read) {
 152                 *error_is_read = false;
 153             }
 154             hbitmap_set(job->copy_bitmap, start / job->cluster_size, 1);
 155             goto out;
 156         }
 157
 158         /* Publish progress, guest I/O counts as progress too.  Note that the
 159          * offset field is an opaque progress value, it is not a disk offset.
 160          */
 161         job->bytes_read += n;
 162         job->common.offset += n;
 163     }
 164
 165 out:
 166     if (bounce_buffer) {
 167         qemu_vfree(bounce_buffer);
 168     }
 169
 170     cow_request_end(&cow_request);
 171
 172     trace_backup_do_cow_return(job, offset, bytes, ret);
 173
 174     qemu_co_rwlock_unlock(&job->flush_rwlock);
 175
 176     return ret;
 177 }
 178
 179 static int coroutine_fn backup_before_write_notify(
 180         NotifierWithReturn *notifier,
 181         void *opaque)
 182 {
 183     BackupBlockJob *job = container_of(notifier, BackupBlockJob, before_write);
 184     BdrvTrackedRequest *req = opaque;
 185
 186     assert(req->bs == blk_bs(job->common.blk));
 187     assert(QEMU_IS_ALIGNED(req->offset, BDRV_SECTOR_SIZE));
 188     assert(QEMU_IS_ALIGNED(req->bytes, BDRV_SECTOR_SIZE));
 189
 190     return backup_do_cow(job, req->offset, req->bytes, NULL, true);
 191 }
 192
 193 static void backup_set_speed(BlockJob *job, int64_t speed, Error **errp)
 194 {
 195     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 196
 197     if (speed < 0) {
 198         error_setg(errp, QERR_INVALID_PARAMETER, "speed");
 199         return;
 200     }
 201     ratelimit_set_speed(&s->limit, speed, SLICE_TIME);
 202 }
 203
 204 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 205 {
 206     BdrvDirtyBitmap *bm;
 207     BlockDriverState *bs = blk_bs(job->common.blk);
 208
 209     if (ret < 0 || block_job_is_cancelled(&job->common)) {
 210         /* Merge the successor back into the parent, delete nothing. */
 211         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
 212         assert(bm);
 213     } else {
 214         /* Everything is fine, delete this bitmap and install the backup. */
 215         bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
 216         assert(bm);
 217     }
 218 }
 219
 220 static void backup_commit(BlockJob *job)
 221 {
 222     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 223     if (s->sync_bitmap) {
 224         backup_cleanup_sync_bitmap(s, 0);
 225     }
 226 }
 227
 228 static void backup_abort(BlockJob *job)
 229 {
 230     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 231     if (s->sync_bitmap) {
 232         backup_cleanup_sync_bitmap(s, -1);
 233     }
 234 }
 235
 236 static void backup_clean(BlockJob *job)
 237 {
 238     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 239     assert(s->target);
 240     blk_unref(s->target);
 241     s->target = NULL;
 242 }
 243
 244 static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
 245 {
 246     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 247
 248     blk_set_aio_context(s->target, aio_context);
 249 }
 250
 251 void backup_do_checkpoint(BlockJob *job, Error **errp)
 252 {
 253     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 254     int64_t len;
 255
 256     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);
 257
 258     if (backup_job->sync_mode != MIRROR_SYNC_MODE_NONE) {
 259         error_setg(errp, "The backup job only supports block checkpoint in"
 260                    " sync=none mode");
 261         return;
 262     }
 263
 264     len = DIV_ROUND_UP(backup_job->common.len, backup_job->cluster_size);
 265     hbitmap_set(backup_job->copy_bitmap, 0, len);
 266 }
 267
 268 void backup_wait_for_overlapping_requests(BlockJob *job, int64_t offset,
 269                                           uint64_t bytes)
 270 {
 271     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 272     int64_t start, end;
 273
 274     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);
 275
 276     start = QEMU_ALIGN_DOWN(offset, backup_job->cluster_size);
 277     end = QEMU_ALIGN_UP(offset + bytes, backup_job->cluster_size);
 278     wait_for_overlapping_requests(backup_job, start, end);
 279 }
 280
 281 void backup_cow_request_begin(CowRequest *req, BlockJob *job,
 282                               int64_t offset, uint64_t bytes)
 283 {
 284     BackupBlockJob *backup_job = container_of(job, BackupBlockJob, common);
 285     int64_t start, end;
 286
 287     assert(job->driver->job_type == BLOCK_JOB_TYPE_BACKUP);
 288
 289     start = QEMU_ALIGN_DOWN(offset, backup_job->cluster_size);
 290     end = QEMU_ALIGN_UP(offset + bytes, backup_job->cluster_size);
 291     cow_request_begin(req, backup_job, start, end);
 292 }
 293
 294 void backup_cow_request_end(CowRequest *req)
 295 {
 296     cow_request_end(req);
 297 }
 298
 299 static void backup_drain(BlockJob *job)
 300 {
 301     BackupBlockJob *s = container_of(job, BackupBlockJob, common);
 302
 303     /* Need to keep a reference in case blk_drain triggers execution
 304      * of backup_complete...
 305      */
 306     if (s->target) {
 307         BlockBackend *target = s->target;
 308         blk_ref(target);
 309         blk_drain(target);
 310         blk_unref(target);
 311     }
 312 }
 313
 314 static BlockErrorAction backup_error_action(BackupBlockJob *job,
 315                                             bool read, int error)
 316 {
 317     if (read) {
 318         return block_job_error_action(&job->common, job->on_source_error,
 319                                       true, error);
 320     } else {
 321         return block_job_error_action(&job->common, job->on_target_error,
 322                                       false, error);
 323     }
 324 }
 325
 326 typedef struct {
 327     int ret;
 328 } BackupCompleteData;
 329
 330 static void backup_complete(BlockJob *job, void *opaque)
 331 {
 332     BackupCompleteData *data = opaque;
 333
 334     block_job_completed(job, data->ret);
 335     g_free(data);
 336 }
 337
 338 static bool coroutine_fn yield_and_check(BackupBlockJob *job)
 339 {
 340     if (block_job_is_cancelled(&job->common)) {
 341         return true;
 342     }
 343
 344     /* we need to yield so that bdrv_drain_all() returns.
 345      * (without, VM does not reboot)
 346      */
 347     if (job->common.speed) {
 348         uint64_t delay_ns = ratelimit_calculate_delay(&job->limit,
 349                                                       job->bytes_read);
 350         job->bytes_read = 0;
 351         block_job_sleep_ns(&job->common, delay_ns);
 352     } else {
 353         block_job_sleep_ns(&job->common, 0);
 354     }
 355
 356     if (block_job_is_cancelled(&job->common)) {
 357         return true;
 358     }
 359
 360     return false;
 361 }
 362
 363 static int coroutine_fn backup_run_incremental(BackupBlockJob *job)
 364 {
 365     bool error_is_read;
 366     int ret = 0;
 367     int clusters_per_iter;
 368     uint32_t granularity;
 369     int64_t offset;
 370     int64_t cluster;
 371     int64_t end;
 372     BdrvDirtyBitmapIter *dbi;
 373
 374     granularity = bdrv_dirty_bitmap_granularity(job->sync_bitmap);
 375     clusters_per_iter = MAX((granularity / job->cluster_size), 1);
 376     dbi = bdrv_dirty_iter_new(job->sync_bitmap);
 377
 378     /* Find the next dirty sector(s) */
 379     while ((offset = bdrv_dirty_iter_next(dbi)) >= 0) {
 380         cluster = offset / job->cluster_size;
 381
 382         for (end = cluster + clusters_per_iter; cluster < end; cluster++) {
 383             do {
 384                 if (yield_and_check(job)) {
 385                     goto out;
 386                 }
 387                 ret = backup_do_cow(job, cluster * job->cluster_size,
 388                                     job->cluster_size, &error_is_read,
 389                                     false);
 390                 if ((ret < 0) &&
 391                     backup_error_action(job, error_is_read, -ret) ==
 392                     BLOCK_ERROR_ACTION_REPORT) {
 393                     goto out;
 394                 }
 395             } while (ret < 0);
 396         }
 397
 398         /* If the bitmap granularity is smaller than the backup granularity,
 399          * we need to advance the iterator pointer to the next cluster. */
 400         if (granularity < job->cluster_size) {
 401             bdrv_set_dirty_iter(dbi, cluster * job->cluster_size);
 402         }
 403     }
 404
 405 out:
 406     bdrv_dirty_iter_free(dbi);
 407     return ret;
 408 }
 409
 410 /* init copy_bitmap from sync_bitmap */
 411 static void backup_incremental_init_copy_bitmap(BackupBlockJob *job)
 412 {
 413     BdrvDirtyBitmapIter *dbi;
 414     int64_t offset;
 415     int64_t end = DIV_ROUND_UP(bdrv_dirty_bitmap_size(job->sync_bitmap),
 416                                job->cluster_size);
 417
 418     dbi = bdrv_dirty_iter_new(job->sync_bitmap);
 419     while ((offset = bdrv_dirty_iter_next(dbi)) != -1) {
 420         int64_t cluster = offset / job->cluster_size;
 421         int64_t next_cluster;
 422
 423         offset += bdrv_dirty_bitmap_granularity(job->sync_bitmap);
 424         if (offset >= bdrv_dirty_bitmap_size(job->sync_bitmap)) {
 425             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 426             break;
 427         }
 428
 429         offset = bdrv_dirty_bitmap_next_zero(job->sync_bitmap, offset);
 430         if (offset == -1) {
 431             hbitmap_set(job->copy_bitmap, cluster, end - cluster);
 432             break;
 433         }
 434
 435         next_cluster = DIV_ROUND_UP(offset, job->cluster_size);
 436         hbitmap_set(job->copy_bitmap, cluster, next_cluster - cluster);
 437         if (next_cluster >= end) {
 438             break;
 439         }
 440
 441         bdrv_set_dirty_iter(dbi, next_cluster * job->cluster_size);
 442     }
 443
 444     job->common.offset = job->common.len -
 445                          hbitmap_count(job->copy_bitmap) * job->cluster_size;
 446
 447     bdrv_dirty_iter_free(dbi);
 448 }
 449
 450 static void coroutine_fn backup_run(void *opaque)
 451 {
 452     BackupBlockJob *job = opaque;
 453     BackupCompleteData *data;
 454     BlockDriverState *bs = blk_bs(job->common.blk);
 455     int64_t offset, nb_clusters;
 456     int ret = 0;
 457
 458     QLIST_INIT(&job->inflight_reqs);
 459     qemu_co_rwlock_init(&job->flush_rwlock);
 460
 461     nb_clusters = DIV_ROUND_UP(job->common.len, job->cluster_size);
 462     job->copy_bitmap = hbitmap_alloc(nb_clusters, 0);
 463     if (job->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 464         backup_incremental_init_copy_bitmap(job);
 465     } else {
 466         hbitmap_set(job->copy_bitmap, 0, nb_clusters);
 467     }
 468
 469
 470     job->before_write.notify = backup_before_write_notify;
 471     bdrv_add_before_write_notifier(bs, &job->before_write);
 472
 473     if (job->sync_mode == MIRROR_SYNC_MODE_NONE) {
 474         /* All bits are set in copy_bitmap to allow any cluster to be copied.
 475          * This does not actually require them to be copied. */
 476         while (!block_job_is_cancelled(&job->common)) {
 477             /* Yield until the job is cancelled.  We just let our before_write
 478              * notify callback service CoW requests. */
 479             block_job_yield(&job->common);
 480         }
 481     } else if (job->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 482         ret = backup_run_incremental(job);
 483     } else {
 484         /* Both FULL and TOP SYNC_MODE's require copying.. */
 485         for (offset = 0; offset < job->common.len;
 486              offset += job->cluster_size) {
 487             bool error_is_read;
 488             int alloced = 0;
 489
 490             if (yield_and_check(job)) {
 491                 break;
 492             }
 493
 494             if (job->sync_mode == MIRROR_SYNC_MODE_TOP) {
 495                 int i;
 496                 int64_t n;
 497
 498                 /* Check to see if these blocks are already in the
 499                  * backing file. */
 500
 501                 for (i = 0; i < job->cluster_size;) {
 502                     /* bdrv_is_allocated() only returns true/false based
 503                      * on the first set of sectors it comes across that
 504                      * are are all in the same state.
 505                      * For that reason we must verify each sector in the
 506                      * backup cluster length.  We end up copying more than
 507                      * needed but at some point that is always the case. */
 508                     alloced =
 509                         bdrv_is_allocated(bs, offset + i,
 510                                           job->cluster_size - i, &n);
 511                     i += n;
 512
 513                     if (alloced || n == 0) {
 514                         break;
 515                     }
 516                 }
 517
 518                 /* If the above loop never found any sectors that are in
 519                  * the topmost image, skip this backup. */
 520                 if (alloced == 0) {
 521                     continue;
 522                 }
 523             }
 524             /* FULL sync mode we copy the whole drive. */
 525             if (alloced < 0) {
 526                 ret = alloced;
 527             } else {
 528                 ret = backup_do_cow(job, offset, job->cluster_size,
 529                                     &error_is_read, false);
 530             }
 531             if (ret < 0) {
 532                 /* Depending on error action, fail now or retry cluster */
 533                 BlockErrorAction action =
 534                     backup_error_action(job, error_is_read, -ret);
 535                 if (action == BLOCK_ERROR_ACTION_REPORT) {
 536                     break;
 537                 } else {
 538                     offset -= job->cluster_size;
 539                     continue;
 540                 }
 541             }
 542         }
 543     }
 544
 545     notifier_with_return_remove(&job->before_write);
 546
 547     /* wait until pending backup_do_cow() calls have completed */
 548     qemu_co_rwlock_wrlock(&job->flush_rwlock);
 549     qemu_co_rwlock_unlock(&job->flush_rwlock);
 550     hbitmap_free(job->copy_bitmap);
 551
 552     data = g_malloc(sizeof(*data));
 553     data->ret = ret;
 554     block_job_defer_to_main_loop(&job->common, backup_complete, data);
 555 }
 556
 557 static const BlockJobDriver backup_job_driver = {
 558     .instance_size          = sizeof(BackupBlockJob),
 559     .job_type               = BLOCK_JOB_TYPE_BACKUP,
 560     .start                  = backup_run,
 561     .set_speed              = backup_set_speed,
 562     .commit                 = backup_commit,
 563     .abort                  = backup_abort,
 564     .clean                  = backup_clean,
 565     .attached_aio_context   = backup_attached_aio_context,
 566     .drain                  = backup_drain,
 567 };
 568
 569 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
 570                   BlockDriverState *target, int64_t speed,
 571                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
 572                   bool compress,
 573                   BlockdevOnError on_source_error,
 574                   BlockdevOnError on_target_error,
 575                   int creation_flags,
 576                   BlockCompletionFunc *cb, void *opaque,
 577                   BlockJobTxn *txn, Error **errp)
 578 {
 579     int64_t len;
 580     BlockDriverInfo bdi;
 581     BackupBlockJob *job = NULL;
 582     int ret;
 583
 584     assert(bs);
 585     assert(target);
 586
 587     if (bs == target) {
 588         error_setg(errp, "Source and target cannot be the same");
 589         return NULL;
 590     }
 591
 592     if (!bdrv_is_inserted(bs)) {
 593         error_setg(errp, "Device is not inserted: %s",
 594                    bdrv_get_device_name(bs));
 595         return NULL;
 596     }
 597
 598     if (!bdrv_is_inserted(target)) {
 599         error_setg(errp, "Device is not inserted: %s",
 600                    bdrv_get_device_name(target));
 601         return NULL;
 602     }
 603
 604     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
 605         error_setg(errp, "Compression is not supported for this drive %s",
 606                    bdrv_get_device_name(target));
 607         return NULL;
 608     }
 609
 610     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
 611         return NULL;
 612     }
 613
 614     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
 615         return NULL;
 616     }
 617
 618     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
 619         if (!sync_bitmap) {
 620             error_setg(errp, "must provide a valid bitmap name for "
 621                              "\"incremental\" sync mode");
 622             return NULL;
 623         }
 624
 625         /* Create a new bitmap, and freeze/disable this one. */
 626         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
 627             return NULL;
 628         }
 629     } else if (sync_bitmap) {
 630         error_setg(errp,
 631                    "a sync_bitmap was provided to backup_run, "
 632                    "but received an incompatible sync_mode (%s)",
 633                    MirrorSyncMode_str(sync_mode));
 634         return NULL;
 635     }
 636
 637     len = bdrv_getlength(bs);
 638     if (len < 0) {
 639         error_setg_errno(errp, -len, "unable to get length for '%s'",
 640                          bdrv_get_device_name(bs));
 641         goto error;
 642     }
 643
 644     /* job->common.len is fixed, so we can't allow resize */
 645     job = block_job_create(job_id, &backup_job_driver, bs,
 646                            BLK_PERM_CONSISTENT_READ,
 647                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 648                            BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD,
 649                            speed, creation_flags, cb, opaque, errp);
 650     if (!job) {
 651         goto error;
 652     }
 653
 654     /* The target must match the source in size, so no resize here either */
 655     job->target = blk_new(BLK_PERM_WRITE,
 656                           BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE |
 657                           BLK_PERM_WRITE_UNCHANGED | BLK_PERM_GRAPH_MOD);
 658     ret = blk_insert_bs(job->target, target, errp);
 659     if (ret < 0) {
 660         goto error;
 661     }
 662
 663     job->on_source_error = on_source_error;
 664     job->on_target_error = on_target_error;
 665     job->sync_mode = sync_mode;
 666     job->sync_bitmap = sync_mode == MIRROR_SYNC_MODE_INCREMENTAL ?
 667                        sync_bitmap : NULL;
 668     job->compress = compress;
 669
 670     /* If there is no backing file on the target, we cannot rely on COW if our
 671      * backup cluster size is smaller than the target cluster size. Even for
 672      * targets with a backing file, try to avoid COW if possible. */
 673     ret = bdrv_get_info(target, &bdi);
 674     if (ret == -ENOTSUP && !target->backing) {
 675         /* Cluster size is not defined */
 676         warn_report("The target block device doesn't provide "
 677                     "information about the block size and it doesn't have a "
 678                     "backing file. The default block size of %u bytes is "
 679                     "used. If the actual block size of the target exceeds "
 680                     "this default, the backup may be unusable",
 681                     BACKUP_CLUSTER_SIZE_DEFAULT);
 682         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 683     } else if (ret < 0 && !target->backing) {
 684         error_setg_errno(errp, -ret,
 685             "Couldn't determine the cluster size of the target image, "
 686             "which has no backing file");
 687         error_append_hint(errp,
 688             "Aborting, since this may create an unusable destination image\n");
 689         goto error;
 690     } else if (ret < 0 && target->backing) {
 691         /* Not fatal; just trudge on ahead. */
 692         job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
 693     } else {
 694         job->cluster_size = MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
 695     }
 696
 697     /* Required permissions are already taken with target's blk_new() */
 698     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
 699                        &error_abort);
 700     job->common.len = len;
 701     block_job_txn_add_job(txn, &job->common);
 702
 703     return &job->common;
 704
 705  error:
 706     if (sync_bitmap) {
 707         bdrv_reclaim_dirty_bitmap(bs, sync_bitmap, NULL);
 708     }
 709     if (job) {
 710         backup_clean(&job->common);
 711         block_job_early_fail(&job->common);
 712     }
 713
 714     return NULL;
 715 }