xref: /illumos-gate/usr/src/uts/common/fs/zfs/vdev_cache.c (revision 4d7988d6)
1 /*
2  * CDDL HEADER START
3  *
4  * The contents of this file are subject to the terms of the
5  * Common Development and Distribution License (the "License").
6  * You may not use this file except in compliance with the License.
7  *
8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
9  * or http://www.opensolaris.org/os/licensing.
10  * See the License for the specific language governing permissions
11  * and limitations under the License.
12  *
13  * When distributing Covered Code, include this CDDL HEADER in each
14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
15  * If applicable, add the following below this CDDL HEADER, with the
16  * fields enclosed by brackets "[]" replaced with your own identifying
17  * information: Portions Copyright [yyyy] [name of copyright owner]
18  *
19  * CDDL HEADER END
20  */
21 /*
22  * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
23  * Use is subject to license terms.
24  */
25 /*
26  * Copyright (c) 2013, 2017 by Delphix. All rights reserved.
27  */
28 
29 #include <sys/zfs_context.h>
30 #include <sys/spa.h>
31 #include <sys/vdev_impl.h>
32 #include <sys/zio.h>
33 #include <sys/kstat.h>
34 #include <sys/abd.h>
35 
36 /*
37  * Virtual device read-ahead caching.
38  *
39  * This file implements a simple LRU read-ahead cache.  When the DMU reads
40  * a given block, it will often want other, nearby blocks soon thereafter.
41  * We take advantage of this by reading a larger disk region and caching
42  * the result.  In the best case, this can turn 128 back-to-back 512-byte
43  * reads into a single 64k read followed by 127 cache hits; this reduces
44  * latency dramatically.  In the worst case, it can turn an isolated 512-byte
45  * read into a 64k read, which doesn't affect latency all that much but is
46  * terribly wasteful of bandwidth.  A more intelligent version of the cache
47  * could keep track of access patterns and not do read-ahead unless it sees
48  * at least two temporally close I/Os to the same region.  Currently, only
49  * metadata I/O is inflated.  A futher enhancement could take advantage of
50  * more semantic information about the I/O.  And it could use something
51  * faster than an AVL tree; that was chosen solely for convenience.
52  *
53  * There are five cache operations: allocate, fill, read, write, evict.
54  *
55  * (1) Allocate.  This reserves a cache entry for the specified region.
56  *     We separate the allocate and fill operations so that multiple threads
57  *     don't generate I/O for the same cache miss.
58  *
59  * (2) Fill.  When the I/O for a cache miss completes, the fill routine
60  *     places the data in the previously allocated cache entry.
61  *
62  * (3) Read.  Read data from the cache.
63  *
64  * (4) Write.  Update cache contents after write completion.
65  *
66  * (5) Evict.  When allocating a new entry, we evict the oldest (LRU) entry
67  *     if the total cache size exceeds zfs_vdev_cache_size.
68  */
69 
70 /*
71  * These tunables are for performance analysis.
72  */
73 /*
74  * All i/os smaller than zfs_vdev_cache_max will be turned into
75  * 1<<zfs_vdev_cache_bshift byte reads by the vdev_cache (aka software
76  * track buffer).  At most zfs_vdev_cache_size bytes will be kept in each
77  * vdev's vdev_cache.
78  *
79  * TODO: Note that with the current ZFS code, it turns out that the
80  * vdev cache is not helpful, and in some cases actually harmful.  It
81  * is better if we disable this.  Once some time has passed, we should
82  * actually remove this to simplify the code.  For now we just disable
83  * it by setting the zfs_vdev_cache_size to zero.  Note that Solaris 11
84  * has made these same changes.
85  */
86 int zfs_vdev_cache_max = 1<<14;			/* 16KB */
87 int zfs_vdev_cache_size = 0;
88 int zfs_vdev_cache_bshift = 16;
89 
90 #define	VCBS (1 << zfs_vdev_cache_bshift)	/* 64KB */
91 
92 kstat_t	*vdc_ksp = NULL;
93 
94 typedef struct vdc_stats {
95 	kstat_named_t vdc_stat_delegations;
96 	kstat_named_t vdc_stat_hits;
97 	kstat_named_t vdc_stat_misses;
98 } vdc_stats_t;
99 
100 static vdc_stats_t vdc_stats = {
101 	{ "delegations",	KSTAT_DATA_UINT64 },
102 	{ "hits",		KSTAT_DATA_UINT64 },
103 	{ "misses",		KSTAT_DATA_UINT64 }
104 };
105 
106 #define	VDCSTAT_BUMP(stat)	atomic_inc_64(&vdc_stats.stat.value.ui64);
107 
108 static inline int
vdev_cache_offset_compare(const void * a1,const void * a2)109 vdev_cache_offset_compare(const void *a1, const void *a2)
110 {
111 	const vdev_cache_entry_t *ve1 = (const vdev_cache_entry_t *)a1;
112 	const vdev_cache_entry_t *ve2 = (const vdev_cache_entry_t *)a2;
113 
114 	return (TREE_CMP(ve1->ve_offset, ve2->ve_offset));
115 }
116 
117 static int
vdev_cache_lastused_compare(const void * a1,const void * a2)118 vdev_cache_lastused_compare(const void *a1, const void *a2)
119 {
120 	const vdev_cache_entry_t *ve1 = (const vdev_cache_entry_t *)a1;
121 	const vdev_cache_entry_t *ve2 = (const vdev_cache_entry_t *)a2;
122 
123 	int cmp = TREE_CMP(ve1->ve_lastused, ve2->ve_lastused);
124 	if (likely(cmp))
125 		return (cmp);
126 
127 	/*
128 	 * Among equally old entries, sort by offset to ensure uniqueness.
129 	 */
130 	return (vdev_cache_offset_compare(a1, a2));
131 }
132 
133 /*
134  * Evict the specified entry from the cache.
135  */
136 static void
vdev_cache_evict(vdev_cache_t * vc,vdev_cache_entry_t * ve)137 vdev_cache_evict(vdev_cache_t *vc, vdev_cache_entry_t *ve)
138 {
139 	ASSERT(MUTEX_HELD(&vc->vc_lock));
140 	ASSERT3P(ve->ve_fill_io, ==, NULL);
141 	ASSERT3P(ve->ve_abd, !=, NULL);
142 
143 	avl_remove(&vc->vc_lastused_tree, ve);
144 	avl_remove(&vc->vc_offset_tree, ve);
145 	abd_free(ve->ve_abd);
146 	kmem_free(ve, sizeof (vdev_cache_entry_t));
147 }
148 
149 /*
150  * Allocate an entry in the cache.  At the point we don't have the data,
151  * we're just creating a placeholder so that multiple threads don't all
152  * go off and read the same blocks.
153  */
154 static vdev_cache_entry_t *
vdev_cache_allocate(zio_t * zio)155 vdev_cache_allocate(zio_t *zio)
156 {
157 	vdev_cache_t *vc = &zio->io_vd->vdev_cache;
158 	uint64_t offset = P2ALIGN(zio->io_offset, VCBS);
159 	vdev_cache_entry_t *ve;
160 
161 	ASSERT(MUTEX_HELD(&vc->vc_lock));
162 
163 	if (zfs_vdev_cache_size == 0)
164 		return (NULL);
165 
166 	/*
167 	 * If adding a new entry would exceed the cache size,
168 	 * evict the oldest entry (LRU).
169 	 */
170 	if ((avl_numnodes(&vc->vc_lastused_tree) << zfs_vdev_cache_bshift) >
171 	    zfs_vdev_cache_size) {
172 		ve = avl_first(&vc->vc_lastused_tree);
173 		if (ve->ve_fill_io != NULL)
174 			return (NULL);
175 		ASSERT3U(ve->ve_hits, !=, 0);
176 		vdev_cache_evict(vc, ve);
177 	}
178 
179 	ve = kmem_zalloc(sizeof (vdev_cache_entry_t), KM_SLEEP);
180 	ve->ve_offset = offset;
181 	ve->ve_lastused = ddi_get_lbolt();
182 	ve->ve_abd = abd_alloc_for_io(VCBS, B_TRUE);
183 
184 	avl_add(&vc->vc_offset_tree, ve);
185 	avl_add(&vc->vc_lastused_tree, ve);
186 
187 	return (ve);
188 }
189 
190 static void
vdev_cache_hit(vdev_cache_t * vc,vdev_cache_entry_t * ve,zio_t * zio)191 vdev_cache_hit(vdev_cache_t *vc, vdev_cache_entry_t *ve, zio_t *zio)
192 {
193 	uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);
194 
195 	ASSERT(MUTEX_HELD(&vc->vc_lock));
196 	ASSERT3P(ve->ve_fill_io, ==, NULL);
197 
198 	if (ve->ve_lastused != ddi_get_lbolt()) {
199 		avl_remove(&vc->vc_lastused_tree, ve);
200 		ve->ve_lastused = ddi_get_lbolt();
201 		avl_add(&vc->vc_lastused_tree, ve);
202 	}
203 
204 	ve->ve_hits++;
205 	abd_copy_off(zio->io_abd, ve->ve_abd, 0, cache_phase, zio->io_size);
206 }
207 
208 /*
209  * Fill a previously allocated cache entry with data.
210  */
211 static void
vdev_cache_fill(zio_t * fio)212 vdev_cache_fill(zio_t *fio)
213 {
214 	vdev_t *vd = fio->io_vd;
215 	vdev_cache_t *vc = &vd->vdev_cache;
216 	vdev_cache_entry_t *ve = fio->io_private;
217 	zio_t *pio;
218 
219 	ASSERT3U(fio->io_size, ==, VCBS);
220 
221 	/*
222 	 * Add data to the cache.
223 	 */
224 	mutex_enter(&vc->vc_lock);
225 
226 	ASSERT3P(ve->ve_fill_io, ==, fio);
227 	ASSERT3U(ve->ve_offset, ==, fio->io_offset);
228 	ASSERT3P(ve->ve_abd, ==, fio->io_abd);
229 
230 	ve->ve_fill_io = NULL;
231 
232 	/*
233 	 * Even if this cache line was invalidated by a missed write update,
234 	 * any reads that were queued up before the missed update are still
235 	 * valid, so we can satisfy them from this line before we evict it.
236 	 */
237 	zio_link_t *zl = NULL;
238 	while ((pio = zio_walk_parents(fio, &zl)) != NULL)
239 		vdev_cache_hit(vc, ve, pio);
240 
241 	if (fio->io_error || ve->ve_missed_update)
242 		vdev_cache_evict(vc, ve);
243 
244 	mutex_exit(&vc->vc_lock);
245 }
246 
247 /*
248  * Read data from the cache.  Returns B_TRUE cache hit, B_FALSE on miss.
249  */
250 boolean_t
vdev_cache_read(zio_t * zio)251 vdev_cache_read(zio_t *zio)
252 {
253 	vdev_cache_t *vc = &zio->io_vd->vdev_cache;
254 	vdev_cache_entry_t *ve, ve_search;
255 	uint64_t cache_offset = P2ALIGN(zio->io_offset, VCBS);
256 	uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);
257 	zio_t *fio;
258 
259 	ASSERT3U(zio->io_type, ==, ZIO_TYPE_READ);
260 
261 	if (zio->io_flags & ZIO_FLAG_DONT_CACHE)
262 		return (B_FALSE);
263 
264 	if (zio->io_size > zfs_vdev_cache_max)
265 		return (B_FALSE);
266 
267 	/*
268 	 * If the I/O straddles two or more cache blocks, don't cache it.
269 	 */
270 	if (P2BOUNDARY(zio->io_offset, zio->io_size, VCBS))
271 		return (B_FALSE);
272 
273 	ASSERT3U(cache_phase + zio->io_size, <=, VCBS);
274 
275 	mutex_enter(&vc->vc_lock);
276 
277 	ve_search.ve_offset = cache_offset;
278 	ve = avl_find(&vc->vc_offset_tree, &ve_search, NULL);
279 
280 	if (ve != NULL) {
281 		if (ve->ve_missed_update) {
282 			mutex_exit(&vc->vc_lock);
283 			return (B_FALSE);
284 		}
285 
286 		if ((fio = ve->ve_fill_io) != NULL) {
287 			zio_vdev_io_bypass(zio);
288 			zio_add_child(zio, fio);
289 			mutex_exit(&vc->vc_lock);
290 			VDCSTAT_BUMP(vdc_stat_delegations);
291 			return (B_TRUE);
292 		}
293 
294 		vdev_cache_hit(vc, ve, zio);
295 		zio_vdev_io_bypass(zio);
296 
297 		mutex_exit(&vc->vc_lock);
298 		VDCSTAT_BUMP(vdc_stat_hits);
299 		return (B_TRUE);
300 	}
301 
302 	ve = vdev_cache_allocate(zio);
303 
304 	if (ve == NULL) {
305 		mutex_exit(&vc->vc_lock);
306 		return (B_FALSE);
307 	}
308 
309 	fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,
310 	    ve->ve_abd, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_NOW,
311 	    ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve);
312 
313 	ve->ve_fill_io = fio;
314 	zio_vdev_io_bypass(zio);
315 	zio_add_child(zio, fio);
316 
317 	mutex_exit(&vc->vc_lock);
318 	zio_nowait(fio);
319 	VDCSTAT_BUMP(vdc_stat_misses);
320 
321 	return (B_TRUE);
322 }
323 
324 /*
325  * Update cache contents upon write completion.
326  */
327 void
vdev_cache_write(zio_t * zio)328 vdev_cache_write(zio_t *zio)
329 {
330 	vdev_cache_t *vc = &zio->io_vd->vdev_cache;
331 	vdev_cache_entry_t *ve, ve_search;
332 	uint64_t io_start = zio->io_offset;
333 	uint64_t io_end = io_start + zio->io_size;
334 	uint64_t min_offset = P2ALIGN(io_start, VCBS);
335 	uint64_t max_offset = P2ROUNDUP(io_end, VCBS);
336 	avl_index_t where;
337 
338 	ASSERT3U(zio->io_type, ==, ZIO_TYPE_WRITE);
339 
340 	mutex_enter(&vc->vc_lock);
341 
342 	ve_search.ve_offset = min_offset;
343 	ve = avl_find(&vc->vc_offset_tree, &ve_search, &where);
344 
345 	if (ve == NULL)
346 		ve = avl_nearest(&vc->vc_offset_tree, where, AVL_AFTER);
347 
348 	while (ve != NULL && ve->ve_offset < max_offset) {
349 		uint64_t start = MAX(ve->ve_offset, io_start);
350 		uint64_t end = MIN(ve->ve_offset + VCBS, io_end);
351 
352 		if (ve->ve_fill_io != NULL) {
353 			ve->ve_missed_update = 1;
354 		} else {
355 			abd_copy_off(ve->ve_abd, zio->io_abd,
356 			    start - ve->ve_offset, start - io_start,
357 			    end - start);
358 		}
359 		ve = AVL_NEXT(&vc->vc_offset_tree, ve);
360 	}
361 	mutex_exit(&vc->vc_lock);
362 }
363 
364 void
vdev_cache_purge(vdev_t * vd)365 vdev_cache_purge(vdev_t *vd)
366 {
367 	vdev_cache_t *vc = &vd->vdev_cache;
368 	vdev_cache_entry_t *ve;
369 
370 	mutex_enter(&vc->vc_lock);
371 	while ((ve = avl_first(&vc->vc_offset_tree)) != NULL)
372 		vdev_cache_evict(vc, ve);
373 	mutex_exit(&vc->vc_lock);
374 }
375 
376 void
vdev_cache_init(vdev_t * vd)377 vdev_cache_init(vdev_t *vd)
378 {
379 	vdev_cache_t *vc = &vd->vdev_cache;
380 
381 	mutex_init(&vc->vc_lock, NULL, MUTEX_DEFAULT, NULL);
382 
383 	avl_create(&vc->vc_offset_tree, vdev_cache_offset_compare,
384 	    sizeof (vdev_cache_entry_t),
385 	    offsetof(struct vdev_cache_entry, ve_offset_node));
386 
387 	avl_create(&vc->vc_lastused_tree, vdev_cache_lastused_compare,
388 	    sizeof (vdev_cache_entry_t),
389 	    offsetof(struct vdev_cache_entry, ve_lastused_node));
390 }
391 
392 void
vdev_cache_fini(vdev_t * vd)393 vdev_cache_fini(vdev_t *vd)
394 {
395 	vdev_cache_t *vc = &vd->vdev_cache;
396 
397 	vdev_cache_purge(vd);
398 
399 	avl_destroy(&vc->vc_offset_tree);
400 	avl_destroy(&vc->vc_lastused_tree);
401 
402 	mutex_destroy(&vc->vc_lock);
403 }
404 
405 void
vdev_cache_stat_init(void)406 vdev_cache_stat_init(void)
407 {
408 	vdc_ksp = kstat_create("zfs", 0, "vdev_cache_stats", "misc",
409 	    KSTAT_TYPE_NAMED, sizeof (vdc_stats) / sizeof (kstat_named_t),
410 	    KSTAT_FLAG_VIRTUAL);
411 	if (vdc_ksp != NULL) {
412 		vdc_ksp->ks_data = &vdc_stats;
413 		kstat_install(vdc_ksp);
414 	}
415 }
416 
417 void
vdev_cache_stat_fini(void)418 vdev_cache_stat_fini(void)
419 {
420 	if (vdc_ksp != NULL) {
421 		kstat_delete(vdc_ksp);
422 		vdc_ksp = NULL;
423 	}
424 }
425