block/replication.c

   1 /*
   2  * Replication Block filter
   3  *
   4  * Copyright (c) 2016 HUAWEI TECHNOLOGIES CO., LTD.
   5  * Copyright (c) 2016 Intel Corporation
   6  * Copyright (c) 2016 FUJITSU LIMITED
   7  *
   8  * Author:
   9  *   Wen Congyang <wency@cn.fujitsu.com>
  10  *
  11  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  12  * See the COPYING file in the top-level directory.
  13  */
  14
  15 #include "qemu/osdep.h"
  16 #include "qemu-common.h"
  17 #include "block/nbd.h"
  18 #include "block/blockjob.h"
  19 #include "block/block_int.h"
  20 #include "block/block_backup.h"
  21 #include "sysemu/block-backend.h"
  22 #include "qapi/error.h"
  23 #include "replication.h"
  24
  25 typedef enum {
  26     BLOCK_REPLICATION_NONE,             /* block replication is not started */
  27     BLOCK_REPLICATION_RUNNING,          /* block replication is running */
  28     BLOCK_REPLICATION_FAILOVER,         /* failover is running in background */
  29     BLOCK_REPLICATION_FAILOVER_FAILED,  /* failover failed */
  30     BLOCK_REPLICATION_DONE,             /* block replication is done */
  31 } ReplicationStage;
  32
  33 typedef struct BDRVReplicationState {
  34     ReplicationMode mode;
  35     ReplicationStage stage;
  36     BdrvChild *active_disk;
  37     BdrvChild *hidden_disk;
  38     BdrvChild *secondary_disk;
  39     char *top_id;
  40     ReplicationState *rs;
  41     Error *blocker;
  42     int orig_hidden_flags;
  43     int orig_secondary_flags;
  44     int error;
  45 } BDRVReplicationState;
  46
  47 static void replication_start(ReplicationState *rs, ReplicationMode mode,
  48                               Error **errp);
  49 static void replication_do_checkpoint(ReplicationState *rs, Error **errp);
  50 static void replication_get_error(ReplicationState *rs, Error **errp);
  51 static void replication_stop(ReplicationState *rs, bool failover,
  52                              Error **errp);
  53
  54 #define REPLICATION_MODE        "mode"
  55 #define REPLICATION_TOP_ID      "top-id"
  56 static QemuOptsList replication_runtime_opts = {
  57     .name = "replication",
  58     .head = QTAILQ_HEAD_INITIALIZER(replication_runtime_opts.head),
  59     .desc = {
  60         {
  61             .name = REPLICATION_MODE,
  62             .type = QEMU_OPT_STRING,
  63         },
  64         {
  65             .name = REPLICATION_TOP_ID,
  66             .type = QEMU_OPT_STRING,
  67         },
  68         { /* end of list */ }
  69     },
  70 };
  71
  72 static ReplicationOps replication_ops = {
  73     .start = replication_start,
  74     .checkpoint = replication_do_checkpoint,
  75     .get_error = replication_get_error,
  76     .stop = replication_stop,
  77 };
  78
  79 static int replication_open(BlockDriverState *bs, QDict *options,
  80                             int flags, Error **errp)
  81 {
  82     int ret;
  83     BDRVReplicationState *s = bs->opaque;
  84     Error *local_err = NULL;
  85     QemuOpts *opts = NULL;
  86     const char *mode;
  87     const char *top_id;
  88
  89     bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
  90                                false, errp);
  91     if (!bs->file) {
  92         return -EINVAL;
  93     }
  94
  95     ret = -EINVAL;
  96     opts = qemu_opts_create(&replication_runtime_opts, NULL, 0, &error_abort);
  97     qemu_opts_absorb_qdict(opts, options, &local_err);
  98     if (local_err) {
  99         goto fail;
 100     }
 101
 102     mode = qemu_opt_get(opts, REPLICATION_MODE);
 103     if (!mode) {
 104         error_setg(&local_err, "Missing the option mode");
 105         goto fail;
 106     }
 107
 108     if (!strcmp(mode, "primary")) {
 109         s->mode = REPLICATION_MODE_PRIMARY;
 110         top_id = qemu_opt_get(opts, REPLICATION_TOP_ID);
 111         if (top_id) {
 112             error_setg(&local_err, "The primary side does not support option top-id");
 113             goto fail;
 114         }
 115     } else if (!strcmp(mode, "secondary")) {
 116         s->mode = REPLICATION_MODE_SECONDARY;
 117         top_id = qemu_opt_get(opts, REPLICATION_TOP_ID);
 118         s->top_id = g_strdup(top_id);
 119         if (!s->top_id) {
 120             error_setg(&local_err, "Missing the option top-id");
 121             goto fail;
 122         }
 123     } else {
 124         error_setg(&local_err,
 125                    "The option mode's value should be primary or secondary");
 126         goto fail;
 127     }
 128
 129     s->rs = replication_new(bs, &replication_ops);
 130
 131     ret = 0;
 132
 133 fail:
 134     qemu_opts_del(opts);
 135     error_propagate(errp, local_err);
 136
 137     return ret;
 138 }
 139
 140 static void replication_close(BlockDriverState *bs)
 141 {
 142     BDRVReplicationState *s = bs->opaque;
 143
 144     if (s->stage == BLOCK_REPLICATION_RUNNING) {
 145         replication_stop(s->rs, false, NULL);
 146     }
 147     if (s->stage == BLOCK_REPLICATION_FAILOVER) {
 148         block_job_cancel_sync(s->active_disk->bs->job);
 149     }
 150
 151     if (s->mode == REPLICATION_MODE_SECONDARY) {
 152         g_free(s->top_id);
 153     }
 154
 155     replication_remove(s->rs);
 156 }
 157
 158 static void replication_child_perm(BlockDriverState *bs, BdrvChild *c,
 159                                    const BdrvChildRole *role,
 160                                    uint64_t perm, uint64_t shared,
 161                                    uint64_t *nperm, uint64_t *nshared)
 162 {
 163     *nperm = *nshared = BLK_PERM_CONSISTENT_READ \
 164                         | BLK_PERM_WRITE \
 165                         | BLK_PERM_WRITE_UNCHANGED;
 166
 167     return;
 168 }
 169
 170 static int64_t replication_getlength(BlockDriverState *bs)
 171 {
 172     return bdrv_getlength(bs->file->bs);
 173 }
 174
 175 static int replication_get_io_status(BDRVReplicationState *s)
 176 {
 177     switch (s->stage) {
 178     case BLOCK_REPLICATION_NONE:
 179         return -EIO;
 180     case BLOCK_REPLICATION_RUNNING:
 181         return 0;
 182     case BLOCK_REPLICATION_FAILOVER:
 183         return s->mode == REPLICATION_MODE_PRIMARY ? -EIO : 0;
 184     case BLOCK_REPLICATION_FAILOVER_FAILED:
 185         return s->mode == REPLICATION_MODE_PRIMARY ? -EIO : 1;
 186     case BLOCK_REPLICATION_DONE:
 187         /*
 188          * active commit job completes, and active disk and secondary_disk
 189          * is swapped, so we can operate bs->file directly
 190          */
 191         return s->mode == REPLICATION_MODE_PRIMARY ? -EIO : 0;
 192     default:
 193         abort();
 194     }
 195 }
 196
 197 static int replication_return_value(BDRVReplicationState *s, int ret)
 198 {
 199     if (s->mode == REPLICATION_MODE_SECONDARY) {
 200         return ret;
 201     }
 202
 203     if (ret < 0) {
 204         s->error = ret;
 205         ret = 0;
 206     }
 207
 208     return ret;
 209 }
 210
 211 static coroutine_fn int replication_co_readv(BlockDriverState *bs,
 212                                              int64_t sector_num,
 213                                              int remaining_sectors,
 214                                              QEMUIOVector *qiov)
 215 {
 216     BDRVReplicationState *s = bs->opaque;
 217     BdrvChild *child = s->secondary_disk;
 218     BlockJob *job = NULL;
 219     CowRequest req;
 220     int ret;
 221
 222     if (s->mode == REPLICATION_MODE_PRIMARY) {
 223         /* We only use it to forward primary write requests */
 224         return -EIO;
 225     }
 226
 227     ret = replication_get_io_status(s);
 228     if (ret < 0) {
 229         return ret;
 230     }
 231
 232     if (child && child->bs) {
 233         job = child->bs->job;
 234     }
 235
 236     if (job) {
 237         backup_wait_for_overlapping_requests(child->bs->job, sector_num,
 238                                              remaining_sectors);
 239         backup_cow_request_begin(&req, child->bs->job, sector_num,
 240                                  remaining_sectors);
 241         ret = bdrv_co_readv(bs->file, sector_num, remaining_sectors,
 242                             qiov);
 243         backup_cow_request_end(&req);
 244         goto out;
 245     }
 246
 247     ret = bdrv_co_readv(bs->file, sector_num, remaining_sectors, qiov);
 248 out:
 249     return replication_return_value(s, ret);
 250 }
 251
 252 static coroutine_fn int replication_co_writev(BlockDriverState *bs,
 253                                               int64_t sector_num,
 254                                               int remaining_sectors,
 255                                               QEMUIOVector *qiov)
 256 {
 257     BDRVReplicationState *s = bs->opaque;
 258     QEMUIOVector hd_qiov;
 259     uint64_t bytes_done = 0;
 260     BdrvChild *top = bs->file;
 261     BdrvChild *base = s->secondary_disk;
 262     BdrvChild *target;
 263     int ret, n;
 264
 265     ret = replication_get_io_status(s);
 266     if (ret < 0) {
 267         goto out;
 268     }
 269
 270     if (ret == 0) {
 271         ret = bdrv_co_writev(top, sector_num,
 272                              remaining_sectors, qiov);
 273         return replication_return_value(s, ret);
 274     }
 275
 276     /*
 277      * Failover failed, only write to active disk if the sectors
 278      * have already been allocated in active disk/hidden disk.
 279      */
 280     qemu_iovec_init(&hd_qiov, qiov->niov);
 281     while (remaining_sectors > 0) {
 282         ret = bdrv_is_allocated_above(top->bs, base->bs, sector_num,
 283                                       remaining_sectors, &n);
 284         if (ret < 0) {
 285             goto out1;
 286         }
 287
 288         qemu_iovec_reset(&hd_qiov);
 289         qemu_iovec_concat(&hd_qiov, qiov, bytes_done, n * BDRV_SECTOR_SIZE);
 290
 291         target = ret ? top : base;
 292         ret = bdrv_co_writev(target, sector_num, n, &hd_qiov);
 293         if (ret < 0) {
 294             goto out1;
 295         }
 296
 297         remaining_sectors -= n;
 298         sector_num += n;
 299         bytes_done += n * BDRV_SECTOR_SIZE;
 300     }
 301
 302 out1:
 303     qemu_iovec_destroy(&hd_qiov);
 304 out:
 305     return ret;
 306 }
 307
 308 static bool replication_recurse_is_first_non_filter(BlockDriverState *bs,
 309                                                     BlockDriverState *candidate)
 310 {
 311     return bdrv_recurse_is_first_non_filter(bs->file->bs, candidate);
 312 }
 313
 314 static void secondary_do_checkpoint(BDRVReplicationState *s, Error **errp)
 315 {
 316     Error *local_err = NULL;
 317     int ret;
 318
 319     if (!s->secondary_disk->bs->job) {
 320         error_setg(errp, "Backup job was cancelled unexpectedly");
 321         return;
 322     }
 323
 324     backup_do_checkpoint(s->secondary_disk->bs->job, &local_err);
 325     if (local_err) {
 326         error_propagate(errp, local_err);
 327         return;
 328     }
 329
 330     ret = s->active_disk->bs->drv->bdrv_make_empty(s->active_disk->bs);
 331     if (ret < 0) {
 332         error_setg(errp, "Cannot make active disk empty");
 333         return;
 334     }
 335
 336     ret = s->hidden_disk->bs->drv->bdrv_make_empty(s->hidden_disk->bs);
 337     if (ret < 0) {
 338         error_setg(errp, "Cannot make hidden disk empty");
 339         return;
 340     }
 341 }
 342
 343 static void reopen_backing_file(BlockDriverState *bs, bool writable,
 344                                 Error **errp)
 345 {
 346     BDRVReplicationState *s = bs->opaque;
 347     BlockReopenQueue *reopen_queue = NULL;
 348     int orig_hidden_flags, orig_secondary_flags;
 349     int new_hidden_flags, new_secondary_flags;
 350     Error *local_err = NULL;
 351
 352     if (writable) {
 353         orig_hidden_flags = s->orig_hidden_flags =
 354                                 bdrv_get_flags(s->hidden_disk->bs);
 355         new_hidden_flags = (orig_hidden_flags | BDRV_O_RDWR) &
 356                                                     ~BDRV_O_INACTIVE;
 357         orig_secondary_flags = s->orig_secondary_flags =
 358                                 bdrv_get_flags(s->secondary_disk->bs);
 359         new_secondary_flags = (orig_secondary_flags | BDRV_O_RDWR) &
 360                                                      ~BDRV_O_INACTIVE;
 361     } else {
 362         orig_hidden_flags = (s->orig_hidden_flags | BDRV_O_RDWR) &
 363                                                     ~BDRV_O_INACTIVE;
 364         new_hidden_flags = s->orig_hidden_flags;
 365         orig_secondary_flags = (s->orig_secondary_flags | BDRV_O_RDWR) &
 366                                                     ~BDRV_O_INACTIVE;
 367         new_secondary_flags = s->orig_secondary_flags;
 368     }
 369
 370     if (orig_hidden_flags != new_hidden_flags) {
 371         reopen_queue = bdrv_reopen_queue(reopen_queue, s->hidden_disk->bs, NULL,
 372                                          new_hidden_flags);
 373     }
 374
 375     if (!(orig_secondary_flags & BDRV_O_RDWR)) {
 376         reopen_queue = bdrv_reopen_queue(reopen_queue, s->secondary_disk->bs,
 377                                          NULL, new_secondary_flags);
 378     }
 379
 380     if (reopen_queue) {
 381         bdrv_reopen_multiple(bdrv_get_aio_context(bs),
 382                              reopen_queue, &local_err);
 383         error_propagate(errp, local_err);
 384     }
 385 }
 386
 387 static void backup_job_cleanup(BlockDriverState *bs)
 388 {
 389     BDRVReplicationState *s = bs->opaque;
 390     BlockDriverState *top_bs;
 391
 392     top_bs = bdrv_lookup_bs(s->top_id, s->top_id, NULL);
 393     if (!top_bs) {
 394         return;
 395     }
 396     bdrv_op_unblock_all(top_bs, s->blocker);
 397     error_free(s->blocker);
 398     reopen_backing_file(bs, false, NULL);
 399 }
 400
 401 static void backup_job_completed(void *opaque, int ret)
 402 {
 403     BlockDriverState *bs = opaque;
 404     BDRVReplicationState *s = bs->opaque;
 405
 406     if (s->stage != BLOCK_REPLICATION_FAILOVER) {
 407         /* The backup job is cancelled unexpectedly */
 408         s->error = -EIO;
 409     }
 410
 411     backup_job_cleanup(bs);
 412 }
 413
 414 static bool check_top_bs(BlockDriverState *top_bs, BlockDriverState *bs)
 415 {
 416     BdrvChild *child;
 417
 418     /* The bs itself is the top_bs */
 419     if (top_bs == bs) {
 420         return true;
 421     }
 422
 423     /* Iterate over top_bs's children */
 424     QLIST_FOREACH(child, &top_bs->children, next) {
 425         if (child->bs == bs || check_top_bs(child->bs, bs)) {
 426             return true;
 427         }
 428     }
 429
 430     return false;
 431 }
 432
 433 static void replication_start(ReplicationState *rs, ReplicationMode mode,
 434                               Error **errp)
 435 {
 436     BlockDriverState *bs = rs->opaque;
 437     BDRVReplicationState *s;
 438     BlockDriverState *top_bs;
 439     int64_t active_length, hidden_length, disk_length;
 440     AioContext *aio_context;
 441     Error *local_err = NULL;
 442     BlockJob *job;
 443
 444     aio_context = bdrv_get_aio_context(bs);
 445     aio_context_acquire(aio_context);
 446     s = bs->opaque;
 447
 448     if (s->stage != BLOCK_REPLICATION_NONE) {
 449         error_setg(errp, "Block replication is running or done");
 450         aio_context_release(aio_context);
 451         return;
 452     }
 453
 454     if (s->mode != mode) {
 455         error_setg(errp, "The parameter mode's value is invalid, needs %d,"
 456                    " but got %d", s->mode, mode);
 457         aio_context_release(aio_context);
 458         return;
 459     }
 460
 461     switch (s->mode) {
 462     case REPLICATION_MODE_PRIMARY:
 463         break;
 464     case REPLICATION_MODE_SECONDARY:
 465         s->active_disk = bs->file;
 466         if (!s->active_disk || !s->active_disk->bs ||
 467                                     !s->active_disk->bs->backing) {
 468             error_setg(errp, "Active disk doesn't have backing file");
 469             aio_context_release(aio_context);
 470             return;
 471         }
 472
 473         s->hidden_disk = s->active_disk->bs->backing;
 474         if (!s->hidden_disk->bs || !s->hidden_disk->bs->backing) {
 475             error_setg(errp, "Hidden disk doesn't have backing file");
 476             aio_context_release(aio_context);
 477             return;
 478         }
 479
 480         s->secondary_disk = s->hidden_disk->bs->backing;
 481         if (!s->secondary_disk->bs || !bdrv_has_blk(s->secondary_disk->bs)) {
 482             error_setg(errp, "The secondary disk doesn't have block backend");
 483             aio_context_release(aio_context);
 484             return;
 485         }
 486
 487         /* verify the length */
 488         active_length = bdrv_getlength(s->active_disk->bs);
 489         hidden_length = bdrv_getlength(s->hidden_disk->bs);
 490         disk_length = bdrv_getlength(s->secondary_disk->bs);
 491         if (active_length < 0 || hidden_length < 0 || disk_length < 0 ||
 492             active_length != hidden_length || hidden_length != disk_length) {
 493             error_setg(errp, "Active disk, hidden disk, secondary disk's length"
 494                        " are not the same");
 495             aio_context_release(aio_context);
 496             return;
 497         }
 498
 499         if (!s->active_disk->bs->drv->bdrv_make_empty ||
 500             !s->hidden_disk->bs->drv->bdrv_make_empty) {
 501             error_setg(errp,
 502                        "Active disk or hidden disk doesn't support make_empty");
 503             aio_context_release(aio_context);
 504             return;
 505         }
 506
 507         /* reopen the backing file in r/w mode */
 508         reopen_backing_file(bs, true, &local_err);
 509         if (local_err) {
 510             error_propagate(errp, local_err);
 511             aio_context_release(aio_context);
 512             return;
 513         }
 514
 515         /* start backup job now */
 516         error_setg(&s->blocker,
 517                    "Block device is in use by internal backup job");
 518
 519         top_bs = bdrv_lookup_bs(s->top_id, s->top_id, NULL);
 520         if (!top_bs || !bdrv_is_root_node(top_bs) ||
 521             !check_top_bs(top_bs, bs)) {
 522             error_setg(errp, "No top_bs or it is invalid");
 523             reopen_backing_file(bs, false, NULL);
 524             aio_context_release(aio_context);
 525             return;
 526         }
 527         bdrv_op_block_all(top_bs, s->blocker);
 528         bdrv_op_unblock(top_bs, BLOCK_OP_TYPE_DATAPLANE, s->blocker);
 529
 530         job = backup_job_create(NULL, s->secondary_disk->bs, s->hidden_disk->bs,
 531                                 0, MIRROR_SYNC_MODE_NONE, NULL, false,
 532                                 BLOCKDEV_ON_ERROR_REPORT,
 533                                 BLOCKDEV_ON_ERROR_REPORT, BLOCK_JOB_INTERNAL,
 534                                 backup_job_completed, bs, NULL, &local_err);
 535         if (local_err) {
 536             error_propagate(errp, local_err);
 537             backup_job_cleanup(bs);
 538             aio_context_release(aio_context);
 539             return;
 540         }
 541         block_job_start(job);
 542         break;
 543     default:
 544         aio_context_release(aio_context);
 545         abort();
 546     }
 547
 548     s->stage = BLOCK_REPLICATION_RUNNING;
 549
 550     if (s->mode == REPLICATION_MODE_SECONDARY) {
 551         secondary_do_checkpoint(s, errp);
 552     }
 553
 554     s->error = 0;
 555     aio_context_release(aio_context);
 556 }
 557
 558 static void replication_do_checkpoint(ReplicationState *rs, Error **errp)
 559 {
 560     BlockDriverState *bs = rs->opaque;
 561     BDRVReplicationState *s;
 562     AioContext *aio_context;
 563
 564     aio_context = bdrv_get_aio_context(bs);
 565     aio_context_acquire(aio_context);
 566     s = bs->opaque;
 567
 568     if (s->mode == REPLICATION_MODE_SECONDARY) {
 569         secondary_do_checkpoint(s, errp);
 570     }
 571     aio_context_release(aio_context);
 572 }
 573
 574 static void replication_get_error(ReplicationState *rs, Error **errp)
 575 {
 576     BlockDriverState *bs = rs->opaque;
 577     BDRVReplicationState *s;
 578     AioContext *aio_context;
 579
 580     aio_context = bdrv_get_aio_context(bs);
 581     aio_context_acquire(aio_context);
 582     s = bs->opaque;
 583
 584     if (s->stage != BLOCK_REPLICATION_RUNNING) {
 585         error_setg(errp, "Block replication is not running");
 586         aio_context_release(aio_context);
 587         return;
 588     }
 589
 590     if (s->error) {
 591         error_setg(errp, "I/O error occurred");
 592         aio_context_release(aio_context);
 593         return;
 594     }
 595     aio_context_release(aio_context);
 596 }
 597
 598 static void replication_done(void *opaque, int ret)
 599 {
 600     BlockDriverState *bs = opaque;
 601     BDRVReplicationState *s = bs->opaque;
 602
 603     if (ret == 0) {
 604         s->stage = BLOCK_REPLICATION_DONE;
 605
 606         /* refresh top bs's filename */
 607         bdrv_refresh_filename(bs);
 608         s->active_disk = NULL;
 609         s->secondary_disk = NULL;
 610         s->hidden_disk = NULL;
 611         s->error = 0;
 612     } else {
 613         s->stage = BLOCK_REPLICATION_FAILOVER_FAILED;
 614         s->error = -EIO;
 615     }
 616 }
 617
 618 static void replication_stop(ReplicationState *rs, bool failover, Error **errp)
 619 {
 620     BlockDriverState *bs = rs->opaque;
 621     BDRVReplicationState *s;
 622     AioContext *aio_context;
 623
 624     aio_context = bdrv_get_aio_context(bs);
 625     aio_context_acquire(aio_context);
 626     s = bs->opaque;
 627
 628     if (s->stage != BLOCK_REPLICATION_RUNNING) {
 629         error_setg(errp, "Block replication is not running");
 630         aio_context_release(aio_context);
 631         return;
 632     }
 633
 634     switch (s->mode) {
 635     case REPLICATION_MODE_PRIMARY:
 636         s->stage = BLOCK_REPLICATION_DONE;
 637         s->error = 0;
 638         break;
 639     case REPLICATION_MODE_SECONDARY:
 640         /*
 641          * This BDS will be closed, and the job should be completed
 642          * before the BDS is closed, because we will access hidden
 643          * disk, secondary disk in backup_job_completed().
 644          */
 645         if (s->secondary_disk->bs->job) {
 646             block_job_cancel_sync(s->secondary_disk->bs->job);
 647         }
 648
 649         if (!failover) {
 650             secondary_do_checkpoint(s, errp);
 651             s->stage = BLOCK_REPLICATION_DONE;
 652             aio_context_release(aio_context);
 653             return;
 654         }
 655
 656         s->stage = BLOCK_REPLICATION_FAILOVER;
 657         commit_active_start(NULL, s->active_disk->bs, s->secondary_disk->bs,
 658                             BLOCK_JOB_INTERNAL, 0, BLOCKDEV_ON_ERROR_REPORT,
 659                             NULL, replication_done, bs, true, errp);
 660         break;
 661     default:
 662         aio_context_release(aio_context);
 663         abort();
 664     }
 665     aio_context_release(aio_context);
 666 }
 667
 668 BlockDriver bdrv_replication = {
 669     .format_name                = "replication",
 670     .protocol_name              = "replication",
 671     .instance_size              = sizeof(BDRVReplicationState),
 672
 673     .bdrv_open                  = replication_open,
 674     .bdrv_close                 = replication_close,
 675     .bdrv_child_perm            = replication_child_perm,
 676
 677     .bdrv_getlength             = replication_getlength,
 678     .bdrv_co_readv              = replication_co_readv,
 679     .bdrv_co_writev             = replication_co_writev,
 680
 681     .is_filter                  = true,
 682     .bdrv_recurse_is_first_non_filter = replication_recurse_is_first_non_filter,
 683
 684     .has_variable_length        = true,
 685 };
 686
 687 static void bdrv_replication_init(void)
 688 {
 689     bdrv_register(&bdrv_replication);
 690 }
 691
 692 block_init(bdrv_replication_init);