block/fvd-prefetch.c

   1 /*
   2  * Copyright (c) 2010-2011 IBM
   3  *
   4  * Authors:
   5  *         Chunqiang Tang <ctang@us.ibm.com>
   6  *
   7  * This work is licensed under the terms of the GNU GPL, version 2.
   8  * See the COPYING file in the top-level directory.
   9  */
  10
  11 /*=============================================================================
  12  *  A short description: this FVD module implements the function of
  13  *  prefetching data from the base image and storing it in the FVD image.
  14  *============================================================================*/
  15
  16 static void resume_prefetch (BlockDriverState * bs, int64_t current_time);
  17 static void do_next_prefetch_read (BlockDriverState * bs, int64_t current_time);
  18
  19 void fvd_init_prefetch (void *opaque)
  20 {
  21     BlockDriverState * bs = opaque;
  22     BDRVFvdState *s = bs->opaque;
  23     FvdAIOCB *acb;
  24     int i;
  25
  26     QDEBUG ("Start prefetching\n");
  27
  28     if (bdrv_find_format ("blksim") == NULL) {
  29         /* In simulation mode, the random seed should not be initialized here.*/
  30         srandom (time (NULL) + getpid () + getpid () * 987654 + random ());
  31     }
  32
  33     s->prefetch_acb =
  34         my_qemu_malloc (sizeof (FvdAIOCB *) * s->num_prefetch_slots);
  35
  36     for (i = 0; i < s->num_prefetch_slots; i++) {
  37         acb = s->prefetch_acb[i] =
  38             my_qemu_aio_get (&fvd_aio_pool, bs, null_prefetch_cb, NULL);
  39
  40         if (!acb) {
  41             s->prefetch_error = TRUE;
  42             int j;
  43             for (j = 0; j < i; j++) {
  44                 my_qemu_aio_unref (s->prefetch_acb[j]);
  45                 s->prefetch_acb[j] = NULL;
  46             }
  47
  48             my_qemu_free (s->prefetch_acb);
  49             s->prefetch_acb = NULL;
  50             fprintf (stderr,
  51                      "qemu_aio_get() failed and cannot start prefetching.\n");
  52             return;
  53         }
  54
  55         acb->type = OP_COPY;
  56     }
  57
  58     s->prefetch_state = PREFETCH_STATE_RUNNING;
  59
  60     for (i = 0; i < s->num_prefetch_slots; i++) {
  61         acb = s->prefetch_acb[i];
  62         acb->copy.buffered_sector_begin = acb->copy.buffered_sector_end = 0;
  63         QLIST_INIT (&acb->copy_lock.dependent_writes);
  64         acb->copy_lock.next.le_prev = NULL;
  65         acb->copy.hd_acb = NULL;
  66         acb->sector_num = 0;
  67         acb->nb_sectors = 0;
  68         acb->copy.iov.iov_len = s->sectors_per_prefetch * 512;
  69         acb->copy.buf = acb->copy.iov.iov_base =
  70             my_qemu_blockalign (bs->backing_hd, acb->copy.iov.iov_len);
  71         qemu_iovec_init_external (&acb->copy.qiov, &acb->copy.iov, 1);
  72     }
  73
  74     if (s->prefetch_timer) {
  75         timer_free(s->prefetch_timer);
  76         s->prefetch_timer =
  77             timer_new_ns(QEMU_CLOCK_REALTIME, (QEMUTimerCB *) resume_prefetch, bs);
  78     }
  79
  80     s->pause_prefetch_requested = FALSE;
  81     s->unclaimed_prefetch_region_start = 0;
  82     s->prefetch_read_throughput = -1;        /* Indicate not initialized. */
  83     s->prefetch_write_throughput = -1;        /* Indicate not initialized. */
  84     s->prefetch_read_time = 0;
  85     s->prefetch_write_time = 0;
  86     s->prefetch_data_read = 0;
  87     s->prefetch_data_written = 0;
  88     s->next_prefetch_read_slot = 0;
  89     s->num_filled_prefetch_slots = 0;
  90     s->prefetch_read_active = FALSE;
  91
  92     do_next_prefetch_read (bs, qemu_clock_get_ns(QEMU_CLOCK_REALTIME));
  93 }
  94
  95 static void pause_prefetch (BDRVFvdState * s)
  96 {
  97     int64_t ms = 1 + (int64_t) ((random () / ((double) RAND_MAX))
  98                                 * s->prefetch_throttle_time);
  99     QDEBUG ("Pause prefetch for %" PRId64 " milliseconds\n", ms);
 100     /* When the timer expires, it goes to resume_prefetch(). */
 101     timer_mod(s->prefetch_timer, qemu_clock_get_ns(QEMU_CLOCK_REALTIME) + ms);
 102 }
 103
 104 static void terminate_prefetch (BlockDriverState * bs, int final_state)
 105 {
 106     BDRVFvdState *s = bs->opaque;
 107     int i;
 108
 109     ASSERT (!s->prefetch_read_active && s->num_filled_prefetch_slots == 0);
 110
 111     for (i = 0; i < s->num_prefetch_slots; i++) {
 112         if (s->prefetch_acb) {
 113             my_qemu_vfree (s->prefetch_acb[i]->copy.buf);
 114             my_qemu_aio_unref (s->prefetch_acb[i]);
 115             s->prefetch_acb[i] = NULL;
 116         }
 117     }
 118     my_qemu_free (s->prefetch_acb);
 119     s->prefetch_acb = NULL;
 120
 121     if (s->prefetch_timer) {
 122         timer_del(s->prefetch_timer);
 123         timer_free(s->prefetch_timer);
 124         s->prefetch_timer = NULL;
 125     }
 126
 127     if (final_state == PREFETCH_STATE_FINISHED) {
 128         if (s->prefetch_error) {
 129             s->prefetch_state = PREFETCH_STATE_DISABLED;
 130         } else {
 131             s->prefetch_state = PREFETCH_STATE_FINISHED;
 132         }
 133     } else {
 134         s->prefetch_state = final_state;
 135     }
 136
 137     if (s->prefetch_state == PREFETCH_STATE_FINISHED) {
 138         QDEBUG ("FVD prefetching finished successfully.\n");
 139
 140         if (s->stale_bitmap) {
 141             memset (s->stale_bitmap, 0xFF, s->bitmap_size);
 142             if (s->fresh_bitmap && s->fresh_bitmap != s->stale_bitmap) {
 143                 memset (s->fresh_bitmap, 0xFF, s->bitmap_size);
 144             }
 145         }
 146
 147         /* Flush the table since its entries may be dirty due to 'soft-write'
 148          * by prefetching or copy-on-read. */
 149         flush_metadata_to_disk (bs);
 150
 151         /* Update the on-disk header. */
 152         FvdHeader header;
 153         read_fvd_header (s, &header);
 154         header.all_data_in_fvd_img = TRUE;
 155         update_fvd_header (s, &header);
 156         s->copy_on_read = FALSE;
 157     } else if (s->prefetch_state == PREFETCH_STATE_DISABLED) {
 158         QDEBUG ("FVD disk prefetching disabled.\n");
 159     }
 160 }
 161
 162 static void do_next_prefetch_read (BlockDriverState * bs, int64_t current_time)
 163 {
 164     FvdAIOCB *acb;
 165     BDRVFvdState *s = bs->opaque;
 166     int64_t begin, end;
 167
 168     ASSERT (!s->prefetch_read_active
 169             && s->num_filled_prefetch_slots < s->num_prefetch_slots
 170             && !s->pause_prefetch_requested);
 171
 172     /* Find the next region to prefetch. */
 173     begin = s->unclaimed_prefetch_region_start;
 174     while (1) {
 175         if (begin >= s->nb_sectors_in_base_img) {
 176             s->unclaimed_prefetch_region_start = s->nb_sectors_in_base_img;
 177             if (s->num_filled_prefetch_slots == 0) {
 178                 terminate_prefetch (bs, PREFETCH_STATE_FINISHED);
 179             }
 180             return;
 181         }
 182         end = begin + s->sectors_per_prefetch;
 183         if (end > s->nb_sectors_in_base_img) {
 184             end = s->nb_sectors_in_base_img;
 185         }
 186         if (find_region_in_base_img (s, &begin, &end)) {
 187             break;
 188         }
 189         begin = end;
 190     }
 191
 192     ASSERT (begin % s->block_size == 0
 193             && (end % s->block_size == 0 || end == s->nb_sectors_in_base_img));
 194
 195     acb = s->prefetch_acb[s->next_prefetch_read_slot];
 196     acb->copy.buffered_sector_begin = acb->sector_num = begin;
 197     acb->copy.buffered_sector_end = s->unclaimed_prefetch_region_start = end;
 198     acb->nb_sectors = end - begin;
 199     acb->copy.qiov.size = acb->copy.iov.iov_len = acb->nb_sectors * 512;
 200     acb->copy.iov.iov_base = acb->copy.buf;
 201     acb->copy.last_prefetch_op_start_time = current_time;
 202     acb->copy.hd_acb = bdrv_aio_readv (bs->backing_hd, acb->sector_num,
 203                                        &acb->copy.qiov, acb->nb_sectors,
 204                                        finish_prefetch_read, acb);
 205
 206
 207     if (acb->copy.hd_acb == NULL) {
 208         QDEBUG ("PREFETCH: error when starting read for sector_num=%" PRId64
 209                 " nb_sectors=%d\n", acb->sector_num, acb->nb_sectors);
 210         s->prefetch_error = TRUE;
 211         s->prefetch_state = PREFETCH_STATE_DISABLED;
 212         if (s->num_filled_prefetch_slots == 0) {
 213             terminate_prefetch (bs, PREFETCH_STATE_DISABLED);
 214         }
 215     } else {
 216         s->prefetch_read_active = TRUE;
 217         QDEBUG ("PREFETCH: start read for sector_num=%" PRId64
 218                 " nb_sectors=%d total_prefetched_bytes=%" PRId64 "\n",
 219                 acb->sector_num, acb->nb_sectors, s->total_prefetch_data);
 220 #ifdef FVD_DEBUG
 221     s->total_prefetch_data += acb->copy.iov.iov_len;
 222 #endif
 223     }
 224 }
 225
 226 static void finish_prefetch_write (void *opaque, int ret)
 227 {
 228     FvdAIOCB *acb = (FvdAIOCB *) opaque;
 229     BlockDriverState *bs = acb->common.bs;
 230     BDRVFvdState *s = bs->opaque;
 231     int64_t begin, end;
 232     const int64_t current_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
 233
 234     ASSERT (acb->nb_sectors > 0 && s->num_filled_prefetch_slots > 0);
 235
 236     QLIST_REMOVE (acb, copy_lock.next);
 237     restart_dependent_writes (acb);
 238     acb->copy.hd_acb = NULL;
 239     QLIST_INIT (&acb->copy_lock.dependent_writes);
 240
 241     if (ret != 0) {
 242         QDEBUG ("PREFETCH: finished write with error for sector_num=%" PRId64
 243                 " nb_sectors=%d\n", acb->sector_num, acb->nb_sectors);
 244         s->num_filled_prefetch_slots = 0;
 245         s->prefetch_error = TRUE;
 246         s->prefetch_state = PREFETCH_STATE_DISABLED;
 247         if (!s->prefetch_read_active) {
 248             terminate_prefetch (bs, PREFETCH_STATE_DISABLED);
 249         }
 250         return;
 251     }
 252
 253     /* No need to update the on-disk bitmap or the stale bitmap. See Section
 254      * 3.3.4 of the FVD-cow paper. */
 255     update_fresh_bitmap (acb->sector_num, acb->nb_sectors, s);
 256
 257     const int64_t write_time =
 258         current_time - acb->copy.last_prefetch_op_start_time;
 259     s->prefetch_write_time += write_time;
 260     s->prefetch_data_written += acb->nb_sectors * 512;
 261
 262     QDEBUG ("PREFETCH: write_finished  sector_num=%" PRId64
 263             " nb_sectors=%d  write_time=%d (ms)\n", acb->sector_num,
 264             acb->nb_sectors, (int) write_time);
 265
 266     /* Calculate throughput and determine if it needs to pause prefetching due
 267      * to low throughput. */
 268     if (s->prefetch_timer && s->prefetch_throttle_time > 0
 269         && !s->pause_prefetch_requested
 270         && s->prefetch_write_time > s->prefetch_write_throughput_measure_time) {
 271         const double this_round_throughput =
 272             s->prefetch_data_written / (double) s->prefetch_write_time;
 273         if (s->prefetch_write_throughput < 0) {
 274             /* Previously not initialized. */
 275             s->prefetch_write_throughput = this_round_throughput;
 276         } else {
 277             s->prefetch_write_throughput =
 278                 s->prefetch_perf_calc_alpha * s->prefetch_write_throughput +
 279                 (1 - s->prefetch_perf_calc_alpha) * this_round_throughput;
 280         }
 281         if (s->prefetch_write_throughput < s->prefetch_min_write_throughput) {
 282             QDEBUG ("PREFETCH: slow_write  this_write=%d (ms)  "
 283                     "this_write_throughput=%.3lf (MB/s)   "
 284                     "avg_write_throughput=%.3lf (MB/s)\n",
 285                     (int) write_time,
 286                     this_round_throughput / 1048576 * 1000,
 287                     s->prefetch_write_throughput / 1048576 * 1000);
 288
 289             /* Make a randomized decision to pause prefetching. This avoids
 290              * pausing all contending FVD drivers. See Section 3.4.2 of the
 291              * FVD-cow paper. */
 292             if (random () > (RAND_MAX / 2)) {
 293                 QDEBUG ("PREFETCH: pause requested.\n");
 294                 s->pause_prefetch_requested = TRUE;
 295             } else {
 296                 QDEBUG ("PREFETCH: continue due to 50%% probability, despite "
 297                         "slow write.\n");
 298                 s->prefetch_write_throughput = -1; /*Indicate not initialized.*/
 299             }
 300         } else {
 301             QDEBUG ("PREFETCH: this_write_throughput=%.3lf (MB/s)   "
 302                     "avg_write_throughput=%.3lf (MB/s)\n",
 303                     this_round_throughput / 1048576 * 1000,
 304                     s->prefetch_write_throughput / 1048576 * 1000);
 305         }
 306
 307         /* Preparing for measuring the next round of throughput. */
 308         s->prefetch_data_written = 0;
 309         s->prefetch_write_time = 0;
 310     }
 311
 312     /* Find in this prefetch slot the next section of prefetched but
 313      * not-yet-written data. */
 314     begin = acb->sector_num + acb->nb_sectors;
 315     if (begin < acb->copy.buffered_sector_end) {
 316         end = acb->copy.buffered_sector_end;
 317         if (find_region_in_base_img (s, &begin, &end)) {
 318             acb->sector_num = begin;
 319             acb->nb_sectors = end - begin;
 320             acb->copy.iov.iov_base = acb->copy.buf +
 321                             (begin - acb->copy.buffered_sector_begin) * 512;
 322             acb->copy.qiov.size = acb->copy.iov.iov_len = acb->nb_sectors * 512;
 323             QDEBUG ("PREFETCH: write_data  sector_num=%" PRId64
 324                     " nb_sectors=%d\n", acb->sector_num, acb->nb_sectors);
 325             acb->copy.hd_acb = store_data (TRUE, acb, bs, acb->sector_num,
 326                                            &acb->copy.qiov, acb->nb_sectors,
 327                                            finish_prefetch_write, acb);
 328             if (acb->copy.hd_acb == NULL) {
 329                 QDEBUG ("PREFETCH: error in starting bdrv_aio_writev().\n");
 330                 s->num_filled_prefetch_slots = 0;
 331                 s->prefetch_error = TRUE;
 332                 s->prefetch_state = PREFETCH_STATE_DISABLED;
 333                 if (!s->prefetch_read_active) {
 334                     terminate_prefetch (bs, PREFETCH_STATE_DISABLED);
 335                 }
 336             } else {
 337                 acb->copy_lock.begin = begin;
 338                 acb->copy_lock.end = end;
 339                 QLIST_INSERT_HEAD (&s->copy_locks, acb, copy_lock.next);
 340             }
 341
 342             return;
 343         }
 344     }
 345
 346     s->num_filled_prefetch_slots--;
 347
 348     if (s->prefetch_state == PREFETCH_STATE_DISABLED) {
 349         if (s->num_filled_prefetch_slots == 0 && !s->prefetch_read_active) {
 350             terminate_prefetch (bs, PREFETCH_STATE_DISABLED);
 351         }
 352         return;
 353     }
 354
 355     if (begin >= s->nb_sectors_in_base_img) {
 356         /* Prefetching finished. */
 357         ASSERT (s->num_filled_prefetch_slots == 0 && !s->prefetch_read_active);
 358         terminate_prefetch (bs, PREFETCH_STATE_FINISHED);
 359         return;
 360     }
 361
 362     if (s->pause_prefetch_requested) {
 363         if (s->num_filled_prefetch_slots == 0) {
 364             if (!s->prefetch_read_active) {
 365                 pause_prefetch (s);
 366             } else {
 367                 QDEBUG ("PREFETCH: wait for the read operation to finish in "
 368                         "order to pause prefetch.\n");
 369             }
 370             return;
 371         }
 372     }
 373
 374     /* Write out data in the next prefetched slot. */
 375     while (s->num_filled_prefetch_slots > 0) {
 376         int k = s->next_prefetch_read_slot - s->num_filled_prefetch_slots;
 377         if (k < 0) {
 378             k += s->num_prefetch_slots;
 379         }
 380         acb = s->prefetch_acb[k];
 381
 382         int64_t begin = acb->copy.buffered_sector_begin;
 383         int64_t end = acb->copy.buffered_sector_end;
 384         if (find_region_in_base_img (s, &begin, &end)) {
 385             acb->copy.last_prefetch_op_start_time = current_time;
 386             acb->sector_num = begin;
 387             acb->nb_sectors = end - begin;
 388             acb->copy.iov.iov_base =
 389                 acb->copy.buf + (begin - acb->copy.buffered_sector_begin) * 512;
 390             acb->copy.qiov.size = acb->copy.iov.iov_len = acb->nb_sectors * 512;
 391             QDEBUG ("PREFETCH: writes data: sector_num=%" PRId64
 392                     " nb_sectors=%d\n", acb->sector_num, acb->nb_sectors);
 393             acb->copy.hd_acb = store_data (TRUE, acb, bs, acb->sector_num,
 394                                            &acb->copy.qiov, acb->nb_sectors,
 395                                            finish_prefetch_write, acb);
 396
 397             if (acb->copy.hd_acb == NULL) {
 398                 QDEBUG ("PREFETCH: error cannot get a control block to write "
 399                         "a prefetched block.\n");
 400                 s->prefetch_error = TRUE;
 401                 s->prefetch_state = PREFETCH_STATE_DISABLED;
 402                 s->num_filled_prefetch_slots = 0;
 403                 if (!s->prefetch_read_active) {
 404                     terminate_prefetch (bs, PREFETCH_STATE_DISABLED);
 405                 }
 406                 return;
 407             }
 408
 409             acb->copy_lock.begin = begin;
 410             acb->copy_lock.end = end;
 411             QLIST_INSERT_HEAD (&s->copy_locks, acb, copy_lock.next);
 412             break;
 413         } else {
 414             QDEBUG ("PREFETCH: discard prefetched data as they have been "
 415                     "covered: sector_num=%" PRId64 " nb_sectors=%d\n",
 416                     acb->sector_num, acb->nb_sectors);
 417             s->num_filled_prefetch_slots--;
 418         }
 419     }
 420
 421     /* If the reader was stopped due to lack of slots, start the reader. */
 422     if (!s->prefetch_read_active && !s->pause_prefetch_requested) {
 423         do_next_prefetch_read (bs, current_time);
 424     }
 425 }
 426
 427 static void finish_prefetch_read (void *opaque, int ret)
 428 {
 429     FvdAIOCB *acb = (FvdAIOCB *) opaque;
 430     BlockDriverState *bs = acb->common.bs;
 431     BDRVFvdState *s = bs->opaque;
 432
 433     ASSERT (s->prefetch_read_active && s->num_filled_prefetch_slots >= 0
 434             && s->num_filled_prefetch_slots < s->num_prefetch_slots);
 435
 436     s->prefetch_read_active = FALSE;
 437     acb->copy.hd_acb = NULL;
 438
 439     if (s->prefetch_state == PREFETCH_STATE_DISABLED) {
 440         if (s->num_filled_prefetch_slots == 0) {
 441             terminate_prefetch (bs, PREFETCH_STATE_DISABLED);
 442         }
 443         return;
 444     }
 445
 446     if (ret != 0) {
 447         QDEBUG ("PREFETCH: read_error  sector_num=%" PRId64 " nb_sectors=%d.\n",
 448                 acb->sector_num, acb->nb_sectors);
 449         s->prefetch_error = TRUE;
 450         s->prefetch_state = PREFETCH_STATE_DISABLED;
 451         if (s->num_filled_prefetch_slots == 0) {
 452             terminate_prefetch (bs, PREFETCH_STATE_DISABLED);
 453         }
 454         return;
 455     }
 456
 457     const int64_t current_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
 458     const int64_t read_time = current_time -
 459                         acb->copy.last_prefetch_op_start_time;
 460     s->prefetch_read_time += read_time;
 461     s->prefetch_data_read += acb->nb_sectors * 512;
 462
 463     QDEBUG ("PREFETCH: read_finished  sector_num=%" PRId64
 464             " nb_sectors=%d  read_time=%d (ms)\n", acb->sector_num,
 465             acb->nb_sectors, (int) read_time);
 466
 467     /* Calculate throughput and determine if it needs to pause prefetching due
 468      * to low throughput. */
 469     if (s->prefetch_timer && s->prefetch_throttle_time > 0
 470         && !s->pause_prefetch_requested
 471         && s->prefetch_read_time > s->prefetch_read_throughput_measure_time) {
 472         const double this_round_throughput =
 473             s->prefetch_data_read / (double) s->prefetch_read_time;
 474         if (s->prefetch_read_throughput < 0) {
 475             /* Previously not initialized. */
 476             s->prefetch_read_throughput = this_round_throughput;
 477         } else {
 478             s->prefetch_read_throughput = s->prefetch_perf_calc_alpha *
 479                 s->prefetch_read_throughput +
 480                 (1 - s->prefetch_perf_calc_alpha) * this_round_throughput;
 481         }
 482         if (s->prefetch_read_throughput < s->prefetch_min_read_throughput) {
 483             QDEBUG ("PREFETCH: slow_read read_time=%d (ms)   "
 484                     "this_read_throughput=%.3lf (MB/s) "
 485                     "avg_read_throughput=%.3lf (MB/s)\n",
 486                     (int) read_time, this_round_throughput / 1048576 * 1000,
 487                     s->prefetch_read_throughput / 1048576 * 1000);
 488
 489             /* Make a randomized decision to pause prefetching. This avoids
 490              * pausing all contending FVD drivers. See Section 3.4.2 of the
 491              * FVD-cow paper. */
 492             if (random () > (RAND_MAX / 2)) {
 493                 QDEBUG ("PREFETCH: pause requested.\n");
 494                 s->pause_prefetch_requested = TRUE;
 495             } else {
 496                 QDEBUG ("PREFETCH: continue due to 50%% probability, "
 497                         "despite slow read.\n");
 498                 s->prefetch_read_throughput = -1; /*Indicate not initialized.*/
 499             }
 500         } else {
 501             QDEBUG ("PREFETCH: this_read_throughput=%.3lf (MB/s)    "
 502                     "avg_read_throughput=%.3lf (MB/s)\n",
 503                     this_round_throughput / 1048576 * 1000,
 504                     s->prefetch_read_throughput / 1048576 * 1000);
 505         }
 506
 507         /* Preparing for measuring the next round of throughput. */
 508         s->prefetch_data_read = 0;
 509         s->prefetch_read_time = 0;
 510     }
 511
 512     if (s->num_filled_prefetch_slots > 0) {
 513         /* There is one ongoing write for prefetched data. This slot will be
 514          * written out later. */
 515         s->num_filled_prefetch_slots++;
 516         s->next_prefetch_read_slot++;
 517         if (s->next_prefetch_read_slot >= s->num_prefetch_slots) {
 518             s->next_prefetch_read_slot = 0;
 519         }
 520     } else {
 521         /* The writer is not active. Start the writer. */
 522         int64_t begin = acb->copy.buffered_sector_begin;
 523         int64_t end = acb->copy.buffered_sector_end;
 524         if (find_region_in_base_img (s, &begin, &end)) {
 525             acb->copy.last_prefetch_op_start_time = current_time;
 526             acb->sector_num = begin;
 527             acb->nb_sectors = end - begin;
 528             acb->copy.iov.iov_base =
 529                 acb->copy.buf + (begin - acb->copy.buffered_sector_begin) * 512;
 530             acb->copy.qiov.size = acb->copy.iov.iov_len = acb->nb_sectors * 512;
 531             QDEBUG ("PREFETCH: writes_data sector_num=%" PRId64
 532                     " nb_sectors=%d\n", acb->sector_num, acb->nb_sectors);
 533             acb->copy.hd_acb = store_data (TRUE, acb, bs, acb->sector_num,
 534                                            &acb->copy.qiov, acb->nb_sectors,
 535                                            finish_prefetch_write, acb);
 536
 537             if (acb->copy.hd_acb == NULL) {
 538                 QDEBUG ("PREFETCH: error cannot get control block to write a "
 539                         "prefetched block.\n");
 540                 s->prefetch_error = TRUE;
 541                 s->prefetch_state = PREFETCH_STATE_DISABLED;
 542                 if (s->num_filled_prefetch_slots == 0) {
 543                     terminate_prefetch (bs, PREFETCH_STATE_DISABLED);
 544                 }
 545                 return;
 546             }
 547
 548             acb->copy_lock.begin = begin;
 549             acb->copy_lock.end = end;
 550             QLIST_INSERT_HEAD (&s->copy_locks, acb, copy_lock.next);
 551             s->num_filled_prefetch_slots++;
 552             s->next_prefetch_read_slot++;
 553             if (s->next_prefetch_read_slot >= s->num_prefetch_slots) {
 554                 s->next_prefetch_read_slot = 0;
 555             }
 556         } else {
 557             /* The current prefetch slot will be reused to prefetch the next
 558              * bunch of data. */
 559             QDEBUG ("PREFETCH: discard prefetched data as they have been "
 560                     "covered: sector_num=%" PRId64 " nb_sectors=%d\n",
 561                     acb->sector_num, acb->nb_sectors);
 562         }
 563     }
 564
 565     if (s->num_filled_prefetch_slots >= s->num_prefetch_slots) {
 566         QDEBUG ("PREFETCH: halt read because no slot is available.\n");
 567     } else {
 568         if (s->pause_prefetch_requested) {
 569             if (s->num_filled_prefetch_slots == 0) {
 570                 pause_prefetch (s);
 571             }
 572         } else {
 573             do_next_prefetch_read (bs, current_time);
 574         }
 575     }
 576 }
 577
 578 static void resume_prefetch (BlockDriverState * bs, int64_t current_time)
 579 {
 580     BDRVFvdState *s = bs->opaque;
 581
 582     if (s->prefetch_state != PREFETCH_STATE_RUNNING) {
 583         return;
 584     }
 585
 586     ASSERT (s->num_filled_prefetch_slots == 0 && !s->prefetch_read_active);
 587     QDEBUG ("PREFETCH: resume.\n");
 588
 589     s->pause_prefetch_requested = FALSE;
 590     s->prefetch_read_throughput = -1;        /* Indicate not initialized. */
 591     s->prefetch_write_throughput = -1;        /* Indicate not initialized. */
 592     s->prefetch_read_time = 0;
 593     s->prefetch_write_time = 0;
 594     s->prefetch_data_read = 0;
 595     s->prefetch_data_written = 0;
 596
 597     do_next_prefetch_read (bs, qemu_clock_get_ns(QEMU_CLOCK_REALTIME));
 598 }