aboutsummaryrefslogtreecommitdiff
path: root/sys/kern/vfs_cluster.c
blob: 40fa3be52f938a1524764b401dc152c6197e54ed (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
/*-
 * Copyright (c) 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	@(#)vfs_cluster.c	8.7 (Berkeley) 2/13/94
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/proc.h>
#include <sys/buf.h>
#include <sys/vnode.h>
#include <sys/mount.h>
#include <sys/trace.h>
#include <sys/malloc.h>
#include <sys/resourcevar.h>

#ifdef DEBUG
#include <vm/vm.h>
#include <sys/sysctl.h>
int doreallocblks = 1;
struct ctldebug debug13 = { "doreallocblks", &doreallocblks };
#else
/* XXX for cluster_write */
#define doreallocblks 1
#endif

/*
 * Local declarations
 */
struct buf *cluster_newbuf __P((struct vnode *, struct buf *, long, daddr_t,
	    daddr_t, long, int));
struct buf *cluster_rbuild __P((struct vnode *, u_quad_t, struct buf *,
	    daddr_t, daddr_t, long, int, long));
void	    cluster_wbuild __P((struct vnode *, struct buf *, long,
	    daddr_t, int, daddr_t));
struct cluster_save *cluster_collectbufs __P((struct vnode *, struct buf *));

#ifdef DIAGNOSTIC
/*
 * Set to 1 if reads of block zero should cause readahead to be done.
 * Set to 0 treats a read of block zero as a non-sequential read.
 *
 * Setting to one assumes that most reads of block zero of files are due to
 * sequential passes over the files (e.g. cat, sum) where additional blocks
 * will soon be needed.  Setting to zero assumes that the majority are
 * surgical strikes to get particular info (e.g. size, file) where readahead
 * blocks will not be used and, in fact, push out other potentially useful
 * blocks from the cache.  The former seems intuitive, but some quick tests
 * showed that the latter performed better from a system-wide point of view.
 */
int	doclusterraz = 0;
#define ISSEQREAD(vp, blk) \
	(((blk) != 0 || doclusterraz) && \
	 ((blk) == (vp)->v_lastr + 1 || (blk) == (vp)->v_lastr))
#else
#define ISSEQREAD(vp, blk) \
	((blk) != 0 && ((blk) == (vp)->v_lastr + 1 || (blk) == (vp)->v_lastr))
#endif

/*
 * This replaces bread.  If this is a bread at the beginning of a file and
 * lastr is 0, we assume this is the first read and we'll read up to two
 * blocks if they are sequential.  After that, we'll do regular read ahead
 * in clustered chunks.
 *
 * There are 4 or 5 cases depending on how you count:
 *	Desired block is in the cache:
 *	    1 Not sequential access (0 I/Os).
 *	    2 Access is sequential, do read-ahead (1 ASYNC).
 *	Desired block is not in cache:
 *	    3 Not sequential access (1 SYNC).
 *	    4 Sequential access, next block is contiguous (1 SYNC).
 *	    5 Sequential access, next block is not contiguous (1 SYNC, 1 ASYNC)
 *
 * There are potentially two buffers that require I/O.
 * 	bp is the block requested.
 *	rbp is the read-ahead block.
 *	If either is NULL, then you don't have to do the I/O.
 */
int
cluster_read(vp, filesize, lblkno, size, cred, bpp)
	struct vnode *vp;
	u_quad_t filesize;
	daddr_t lblkno;
	long size;
	struct ucred *cred;
	struct buf **bpp;
{
	struct buf *bp, *rbp;
	daddr_t blkno, ioblkno;
	long flags;
	int error, num_ra, alreadyincore;

#ifdef DIAGNOSTIC
	if (size == 0)
		panic("cluster_read: size = 0");
#endif

	error = 0;
	flags = B_READ;
	*bpp = bp = getblk(vp, lblkno, size, 0, 0);
	if (bp->b_flags & B_CACHE) {
		/*
		 * Desired block is in cache; do any readahead ASYNC.
		 * Case 1, 2.
		 */
		trace(TR_BREADHIT, pack(vp, size), lblkno);
		flags |= B_ASYNC;
		ioblkno = lblkno + (vp->v_ralen ? vp->v_ralen : 1);
		alreadyincore = (int)incore(vp, ioblkno);
		bp = NULL;
	} else {
		/* Block wasn't in cache, case 3, 4, 5. */
		trace(TR_BREADMISS, pack(vp, size), lblkno);
		bp->b_flags |= B_READ;
		ioblkno = lblkno;
		alreadyincore = 0;
		curproc->p_stats->p_ru.ru_inblock++;		/* XXX */
	}
	/*
	 * XXX
	 * Replace 1 with a window size based on some permutation of
	 * maxcontig and rot_delay.  This will let you figure out how
	 * many blocks you should read-ahead (case 2, 4, 5).
	 *
	 * If the access isn't sequential, reset the window to 1.
	 * Note that a read to the same block is considered sequential.
	 * This catches the case where the file is being read sequentially,
	 * but at smaller than the filesystem block size.
	 */
	rbp = NULL;
	if (!ISSEQREAD(vp, lblkno)) {
		vp->v_ralen = 0;
		vp->v_maxra = lblkno;
	} else if ((ioblkno + 1) * size <= filesize && !alreadyincore &&
	    !(error = VOP_BMAP(vp, ioblkno, NULL, &blkno, &num_ra)) &&
	    blkno != -1) {
		/*
		 * Reading sequentially, and the next block is not in the
		 * cache.  We are going to try reading ahead.
		 */
		if (num_ra) {
			/*
			 * If our desired readahead block had been read
			 * in a previous readahead but is no longer in
			 * core, then we may be reading ahead too far
			 * or are not using our readahead very rapidly.
			 * In this case we scale back the window.
			 */
			if (!alreadyincore && ioblkno <= vp->v_maxra)
				vp->v_ralen = max(vp->v_ralen >> 1, 1);
			/*
			 * There are more sequential blocks than our current
			 * window allows, scale up.  Ideally we want to get
			 * in sync with the filesystem maxcontig value.
			 */
			else if (num_ra > vp->v_ralen && lblkno != vp->v_lastr)
				vp->v_ralen = vp->v_ralen ?
					min(num_ra, vp->v_ralen << 1) : 1;

			if (num_ra > vp->v_ralen)
				num_ra = vp->v_ralen;
		}

		if (num_ra)				/* case 2, 4 */
			rbp = cluster_rbuild(vp, filesize,
			    bp, ioblkno, blkno, size, num_ra, flags);
		else if (ioblkno == lblkno) {
			bp->b_blkno = blkno;
			/* Case 5: check how many blocks to read ahead */
			++ioblkno;
			if ((ioblkno + 1) * size > filesize ||
			    incore(vp, ioblkno) || (error = VOP_BMAP(vp,
			     ioblkno, NULL, &blkno, &num_ra)) || blkno == -1)
				goto skip_readahead;
			/*
			 * Adjust readahead as above
			 */
			if (num_ra) {
				if (!alreadyincore && ioblkno <= vp->v_maxra)
					vp->v_ralen = max(vp->v_ralen >> 1, 1);
				else if (num_ra > vp->v_ralen &&
					 lblkno != vp->v_lastr)
					vp->v_ralen = vp->v_ralen ?
						min(num_ra,vp->v_ralen<<1) : 1;
				if (num_ra > vp->v_ralen)
					num_ra = vp->v_ralen;
			}
			flags |= B_ASYNC;
			if (num_ra)
				rbp = cluster_rbuild(vp, filesize,
				    NULL, ioblkno, blkno, size, num_ra, flags);
			else {
				rbp = getblk(vp, ioblkno, size, 0, 0);
				rbp->b_flags |= flags;
				rbp->b_blkno = blkno;
			}
		} else {
			/* case 2; read ahead single block */
			rbp = getblk(vp, ioblkno, size, 0, 0);
			rbp->b_flags |= flags;
			rbp->b_blkno = blkno;
		}

		if (rbp == bp)			/* case 4 */
			rbp = NULL;
		else if (rbp) {			/* case 2, 5 */
			trace(TR_BREADMISSRA,
			    pack(vp, (num_ra + 1) * size), ioblkno);
			curproc->p_stats->p_ru.ru_inblock++;	/* XXX */
		}
	}

	/* XXX Kirk, do we need to make sure the bp has creds? */
skip_readahead:
	if (bp)
		if (bp->b_flags & (B_DONE | B_DELWRI))
			panic("cluster_read: DONE bp");
		else 
			error = VOP_STRATEGY(bp);

	if (rbp)
		if (error || rbp->b_flags & (B_DONE | B_DELWRI)) {
			rbp->b_flags &= ~(B_ASYNC | B_READ);
			brelse(rbp);
		} else
			(void) VOP_STRATEGY(rbp);

	/*
	 * Recalculate our maximum readahead
	 */
	if (rbp == NULL)
		rbp = bp;
	if (rbp)
		vp->v_maxra = rbp->b_lblkno + (rbp->b_bufsize / size) - 1;

	if (bp)
		return(biowait(bp));
	return(error);
}

/*
 * If blocks are contiguous on disk, use this to provide clustered
 * read ahead.  We will read as many blocks as possible sequentially
 * and then parcel them up into logical blocks in the buffer hash table.
 */
struct buf *
cluster_rbuild(vp, filesize, bp, lbn, blkno, size, run, flags)
	struct vnode *vp;
	u_quad_t filesize;
	struct buf *bp;
	daddr_t lbn;
	daddr_t blkno;
	long size;
	int run;
	long flags;
{
	struct cluster_save *b_save;
	struct buf *tbp;
	daddr_t bn;
	int i, inc;

#ifdef DIAGNOSTIC
	if (size != vp->v_mount->mnt_stat.f_iosize)
		panic("cluster_rbuild: size %d != filesize %d\n",
			size, vp->v_mount->mnt_stat.f_iosize);
#endif
	if (size * (lbn + run + 1) > filesize)
		--run;
	if (run == 0) {
		if (!bp) {
			bp = getblk(vp, lbn, size, 0, 0);
			bp->b_blkno = blkno;
			bp->b_flags |= flags;
		}
		return(bp);
	}

	bp = cluster_newbuf(vp, bp, flags, blkno, lbn, size, run + 1);
	if (bp->b_flags & (B_DONE | B_DELWRI))
		return (bp);

	b_save = malloc(sizeof(struct buf *) * run + sizeof(struct cluster_save),
	    M_SEGMENT, M_WAITOK);
	b_save->bs_bufsize = b_save->bs_bcount = size;
	b_save->bs_nchildren = 0;
	b_save->bs_children = (struct buf **)(b_save + 1);
	b_save->bs_saveaddr = bp->b_saveaddr;
	bp->b_saveaddr = (caddr_t) b_save;

	inc = btodb(size);
	for (bn = blkno + inc, i = 1; i <= run; ++i, bn += inc) {
		if (incore(vp, lbn + i)) {
			if (i == 1) {
				bp->b_saveaddr = b_save->bs_saveaddr;
				bp->b_flags &= ~B_CALL;
				bp->b_iodone = NULL;
				allocbuf(bp, size);
				free(b_save, M_SEGMENT);
			} else
				allocbuf(bp, size * i);
			break;
		}
		tbp = getblk(vp, lbn + i, 0, 0, 0);
		/*
		 * getblk may return some memory in the buffer if there were
		 * no empty buffers to shed it to.  If there is currently
		 * memory in the buffer, we move it down size bytes to make
		 * room for the valid pages that cluster_callback will insert.
		 * We do this now so we don't have to do it at interrupt time
		 * in the callback routine.
		 */
		if (tbp->b_bufsize != 0) {
			caddr_t bdata = (char *)tbp->b_data;

			if (tbp->b_bufsize + size > MAXBSIZE)
				panic("cluster_rbuild: too much memory");
			if (tbp->b_bufsize > size) {
				/*
				 * XXX if the source and destination regions
				 * overlap we have to copy backward to avoid
				 * clobbering any valid pages (i.e. pagemove
				 * implementations typically can't handle
				 * overlap).
				 */
				bdata += tbp->b_bufsize;
				while (bdata > (char *)tbp->b_data) {
					bdata -= CLBYTES;
					pagemove(bdata, bdata + size, CLBYTES);
				}
			} else 
				pagemove(bdata, bdata + size, tbp->b_bufsize);
		}
		tbp->b_blkno = bn;
		tbp->b_flags |= flags | B_READ | B_ASYNC;
		++b_save->bs_nchildren;
		b_save->bs_children[i - 1] = tbp;
	}
	return(bp);
}

/*
 * Either get a new buffer or grow the existing one.
 */
struct buf *
cluster_newbuf(vp, bp, flags, blkno, lblkno, size, run)
	struct vnode *vp;
	struct buf *bp;
	long flags;
	daddr_t blkno;
	daddr_t lblkno;
	long size;
	int run;
{
	if (!bp) {
		bp = getblk(vp, lblkno, size, 0, 0);
		if (bp->b_flags & (B_DONE | B_DELWRI)) {
			bp->b_blkno = blkno;
			return(bp);
		}
	}
	allocbuf(bp, run * size);
	bp->b_blkno = blkno;
	bp->b_iodone = cluster_callback;
	bp->b_flags |= flags | B_CALL;
	return(bp);
}

/*
 * Cleanup after a clustered read or write.
 * This is complicated by the fact that any of the buffers might have
 * extra memory (if there were no empty buffer headers at allocbuf time)
 * that we will need to shift around.
 */
void
cluster_callback(bp)
	struct buf *bp;
{
	struct cluster_save *b_save;
	struct buf **bpp, *tbp;
	long bsize;
	caddr_t cp;
	int error = 0;

	/*
	 * Must propogate errors to all the components.
	 */
	if (bp->b_flags & B_ERROR)
		error = bp->b_error;

	b_save = (struct cluster_save *)(bp->b_saveaddr);
	bp->b_saveaddr = b_save->bs_saveaddr;

	bsize = b_save->bs_bufsize;
	cp = (char *)bp->b_data + bsize;
	/*
	 * Move memory from the large cluster buffer into the component
	 * buffers and mark IO as done on these.
	 */
	for (bpp = b_save->bs_children; b_save->bs_nchildren--; ++bpp) {
		tbp = *bpp;
		pagemove(cp, tbp->b_data, bsize);
		tbp->b_bufsize += bsize;
		tbp->b_bcount = bsize;
		if (error) {
			tbp->b_flags |= B_ERROR;
			tbp->b_error = error;
		}
		biodone(tbp);
		bp->b_bufsize -= bsize;
		cp += bsize;
	}
	/*
	 * If there was excess memory in the cluster buffer,
	 * slide it up adjacent to the remaining valid data.
	 */
	if (bp->b_bufsize != bsize) {
		if (bp->b_bufsize < bsize)
			panic("cluster_callback: too little memory");
		pagemove(cp, (char *)bp->b_data + bsize, bp->b_bufsize - bsize);
	}
	bp->b_bcount = bsize;
	bp->b_iodone = NULL;
	free(b_save, M_SEGMENT);
	if (bp->b_flags & B_ASYNC)
		brelse(bp);
	else {
		bp->b_flags &= ~B_WANTED;
		wakeup((caddr_t)bp);
	}
}

/*
 * Do clustered write for FFS.
 *
 * Three cases:
 *	1. Write is not sequential (write asynchronously)
 *	Write is sequential:
 *	2.	beginning of cluster - begin cluster
 *	3.	middle of a cluster - add to cluster
 *	4.	end of a cluster - asynchronously write cluster
 */
void
cluster_write(bp, filesize)
        struct buf *bp;
	u_quad_t filesize;
{
        struct vnode *vp;
        daddr_t lbn;
        int maxclen, cursize;

        vp = bp->b_vp;
        lbn = bp->b_lblkno;

	/* Initialize vnode to beginning of file. */
	if (lbn == 0)
		vp->v_lasta = vp->v_clen = vp->v_cstart = vp->v_lastw = 0;

        if (vp->v_clen == 0 || lbn != vp->v_lastw + 1 ||
	    (bp->b_blkno != vp->v_lasta + btodb(bp->b_bcount))) {
		maxclen = MAXBSIZE / vp->v_mount->mnt_stat.f_iosize - 1;
		if (vp->v_clen != 0) {
			/*
			 * Next block is not sequential.
			 *
			 * If we are not writing at end of file, the process
			 * seeked to another point in the file since its
			 * last write, or we have reached our maximum
			 * cluster size, then push the previous cluster.
			 * Otherwise try reallocating to make it sequential.
			 */
			cursize = vp->v_lastw - vp->v_cstart + 1;
			if (!doreallocblks ||
			    (lbn + 1) * bp->b_bcount != filesize ||
			    lbn != vp->v_lastw + 1 || vp->v_clen <= cursize) {
				cluster_wbuild(vp, NULL, bp->b_bcount,
				    vp->v_cstart, cursize, lbn);
			} else {
				struct buf **bpp, **endbp;
				struct cluster_save *buflist;

				buflist = cluster_collectbufs(vp, bp);
				endbp = &buflist->bs_children
				    [buflist->bs_nchildren - 1];
				if (VOP_REALLOCBLKS(vp, buflist)) {
					/*
					 * Failed, push the previous cluster.
					 */
					for (bpp = buflist->bs_children;
					     bpp < endbp; bpp++)
						brelse(*bpp);
					free(buflist, M_SEGMENT);
					cluster_wbuild(vp, NULL, bp->b_bcount,
					    vp->v_cstart, cursize, lbn);
				} else {
					/*
					 * Succeeded, keep building cluster.
					 */
					for (bpp = buflist->bs_children;
					     bpp <= endbp; bpp++)
						bdwrite(*bpp);
					free(buflist, M_SEGMENT);
					vp->v_lastw = lbn;
					vp->v_lasta = bp->b_blkno;
					return;
				}
			}
		}
		/*
		 * Consider beginning a cluster.
		 * If at end of file, make cluster as large as possible,
		 * otherwise find size of existing cluster.
		 */
		if ((lbn + 1) * bp->b_bcount != filesize &&
		    (VOP_BMAP(vp, lbn, NULL, &bp->b_blkno, &maxclen) ||
		     bp->b_blkno == -1)) {
			bawrite(bp);
			vp->v_clen = 0;
			vp->v_lasta = bp->b_blkno;
			vp->v_cstart = lbn + 1;
			vp->v_lastw = lbn;
			return;
		}
                vp->v_clen = maxclen;
                if (maxclen == 0) {		/* I/O not contiguous */
			vp->v_cstart = lbn + 1;
                        bawrite(bp);
                } else {			/* Wait for rest of cluster */
			vp->v_cstart = lbn;
                        bdwrite(bp);
		}
	} else if (lbn == vp->v_cstart + vp->v_clen) {
		/*
		 * At end of cluster, write it out.
		 */
		cluster_wbuild(vp, bp, bp->b_bcount, vp->v_cstart,
		    vp->v_clen + 1, lbn);
		vp->v_clen = 0;
		vp->v_cstart = lbn + 1;
	} else
		/*
		 * In the middle of a cluster, so just delay the
		 * I/O for now.
		 */
		bdwrite(bp);
	vp->v_lastw = lbn;
	vp->v_lasta = bp->b_blkno;
}


/*
 * This is an awful lot like cluster_rbuild...wish they could be combined.
 * The last lbn argument is the current block on which I/O is being
 * performed.  Check to see that it doesn't fall in the middle of
 * the current block (if last_bp == NULL).
 */
void
cluster_wbuild(vp, last_bp, size, start_lbn, len, lbn)
	struct vnode *vp;
	struct buf *last_bp;
	long size;
	daddr_t start_lbn;
	int len;
	daddr_t	lbn;
{
	struct cluster_save *b_save;
	struct buf *bp, *tbp;
	caddr_t	cp;
	int i, s;

#ifdef DIAGNOSTIC
	if (size != vp->v_mount->mnt_stat.f_iosize)
		panic("cluster_wbuild: size %d != filesize %d\n",
			size, vp->v_mount->mnt_stat.f_iosize);
#endif
redo:
	while ((!incore(vp, start_lbn) || start_lbn == lbn) && len) {
		++start_lbn;
		--len;
	}

	/* Get more memory for current buffer */
	if (len <= 1) {
		if (last_bp) {
			bawrite(last_bp);
		} else if (len) {
			bp = getblk(vp, start_lbn, size, 0, 0);
			bawrite(bp);
		}
		return;
	}

	bp = getblk(vp, start_lbn, size, 0, 0);
	if (!(bp->b_flags & B_DELWRI)) {
		++start_lbn;
		--len;
		brelse(bp);
		goto redo;
	}

	/*
	 * Extra memory in the buffer, punt on this buffer.
	 * XXX we could handle this in most cases, but we would have to
	 * push the extra memory down to after our max possible cluster
	 * size and then potentially pull it back up if the cluster was
	 * terminated prematurely--too much hassle.
	 */
	if (bp->b_bcount != bp->b_bufsize) {
		++start_lbn;
		--len;
		bawrite(bp);
		goto redo;
	}

	--len;
	b_save = malloc(sizeof(struct buf *) * len + sizeof(struct cluster_save),
	    M_SEGMENT, M_WAITOK);
	b_save->bs_bcount = bp->b_bcount;
	b_save->bs_bufsize = bp->b_bufsize;
	b_save->bs_nchildren = 0;
	b_save->bs_children = (struct buf **)(b_save + 1);
	b_save->bs_saveaddr = bp->b_saveaddr;
	bp->b_saveaddr = (caddr_t) b_save;

	bp->b_flags |= B_CALL;
	bp->b_iodone = cluster_callback;
	cp = (char *)bp->b_data + size;
	for (++start_lbn, i = 0; i < len; ++i, ++start_lbn) {
		/*
		 * Block is not in core or the non-sequential block
		 * ending our cluster was part of the cluster (in which
		 * case we don't want to write it twice).
		 */
		if (!incore(vp, start_lbn) ||
		    last_bp == NULL && start_lbn == lbn)
			break;

		/*
		 * Get the desired block buffer (unless it is the final
		 * sequential block whose buffer was passed in explictly
		 * as last_bp).
		 */
		if (last_bp == NULL || start_lbn != lbn) {
			tbp = getblk(vp, start_lbn, size, 0, 0);
			if (!(tbp->b_flags & B_DELWRI)) {
				brelse(tbp);
				break;
			}
		} else
			tbp = last_bp;

		++b_save->bs_nchildren;

		/* Move memory from children to parent */
		if (tbp->b_blkno != (bp->b_blkno + btodb(bp->b_bufsize))) {
			printf("Clustered Block: %d addr %x bufsize: %d\n",
			    bp->b_lblkno, bp->b_blkno, bp->b_bufsize);
			printf("Child Block: %d addr: %x\n", tbp->b_lblkno,
			    tbp->b_blkno);
			panic("Clustered write to wrong blocks");
		}

		pagemove(tbp->b_data, cp, size);
		bp->b_bcount += size;
		bp->b_bufsize += size;

		tbp->b_bufsize -= size;
		tbp->b_flags &= ~(B_READ | B_DONE | B_ERROR | B_DELWRI);
		tbp->b_flags |= (B_ASYNC | B_AGE);
		s = splbio();
		reassignbuf(tbp, tbp->b_vp);		/* put on clean list */
		++tbp->b_vp->v_numoutput;
		splx(s);
		b_save->bs_children[i] = tbp;

		cp += size;
	}

	if (i == 0) {
		/* None to cluster */
		bp->b_saveaddr = b_save->bs_saveaddr;
		bp->b_flags &= ~B_CALL;
		bp->b_iodone = NULL;
		free(b_save, M_SEGMENT);
	}
	bawrite(bp);
	if (i < len) {
		len -= i + 1;
		start_lbn += 1;
		goto redo;
	}
}

/*
 * Collect together all the buffers in a cluster.
 * Plus add one additional buffer.
 */
struct cluster_save *
cluster_collectbufs(vp, last_bp)
	struct vnode *vp;
	struct buf *last_bp;
{
	struct cluster_save *buflist;
	daddr_t	lbn;
	int i, len;

	len = vp->v_lastw - vp->v_cstart + 1;
	buflist = malloc(sizeof(struct buf *) * (len + 1) + sizeof(*buflist),
	    M_SEGMENT, M_WAITOK);
	buflist->bs_nchildren = 0;
	buflist->bs_children = (struct buf **)(buflist + 1);
	for (lbn = vp->v_cstart, i = 0; i < len; lbn++, i++)
		    (void)bread(vp, lbn, last_bp->b_bcount, NOCRED,
			&buflist->bs_children[i]);
	buflist->bs_children[i] = last_bp;
	buflist->bs_nchildren = i + 1;
	return (buflist);
}