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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 2008 Sun Microsystems, Inc. All rights reserved.
23 * Use is subject to license terms.
24 */
25
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26#include <sys/zfs_context.h>
27#include <sys/spa.h>
28#include <sys/vdev_impl.h>
29#include <sys/zio.h>
30#include <sys/kstat.h>
31
32/*
33 * Virtual device read-ahead caching.
34 *
35 * This file implements a simple LRU read-ahead cache. When the DMU reads
36 * a given block, it will often want other, nearby blocks soon thereafter.
37 * We take advantage of this by reading a larger disk region and caching
38 * the result. In the best case, this can turn 128 back-to-back 512-byte
39 * reads into a single 64k read followed by 127 cache hits; this reduces
40 * latency dramatically. In the worst case, it can turn an isolated 512-byte
41 * read into a 64k read, which doesn't affect latency all that much but is
42 * terribly wasteful of bandwidth. A more intelligent version of the cache
43 * could keep track of access patterns and not do read-ahead unless it sees
44 * at least two temporally close I/Os to the same region. Currently, only
45 * metadata I/O is inflated. A futher enhancement could take advantage of
46 * more semantic information about the I/O. And it could use something
47 * faster than an AVL tree; that was chosen solely for convenience.
48 *
49 * There are five cache operations: allocate, fill, read, write, evict.
50 *
51 * (1) Allocate. This reserves a cache entry for the specified region.
52 * We separate the allocate and fill operations so that multiple threads
53 * don't generate I/O for the same cache miss.
54 *
55 * (2) Fill. When the I/O for a cache miss completes, the fill routine
56 * places the data in the previously allocated cache entry.
57 *
58 * (3) Read. Read data from the cache.
59 *
60 * (4) Write. Update cache contents after write completion.
61 *
62 * (5) Evict. When allocating a new entry, we evict the oldest (LRU) entry
63 * if the total cache size exceeds zfs_vdev_cache_size.
64 */
65
66/*
67 * These tunables are for performance analysis.
68 */
69/*
70 * All i/os smaller than zfs_vdev_cache_max will be turned into
71 * 1<<zfs_vdev_cache_bshift byte reads by the vdev_cache (aka software
72 * track buffer). At most zfs_vdev_cache_size bytes will be kept in each
73 * vdev's vdev_cache.
74 */
75int zfs_vdev_cache_max = 1<<14; /* 16KB */
76int zfs_vdev_cache_size = 10ULL << 20; /* 10MB */
77int zfs_vdev_cache_bshift = 16;
78
79#define VCBS (1 << zfs_vdev_cache_bshift) /* 64KB */
80
81kstat_t *vdc_ksp = NULL;
82
83typedef struct vdc_stats {
84 kstat_named_t vdc_stat_delegations;
85 kstat_named_t vdc_stat_hits;
86 kstat_named_t vdc_stat_misses;
87} vdc_stats_t;
88
89static vdc_stats_t vdc_stats = {
90 { "delegations", KSTAT_DATA_UINT64 },
91 { "hits", KSTAT_DATA_UINT64 },
92 { "misses", KSTAT_DATA_UINT64 }
93};
94
95#define VDCSTAT_BUMP(stat) atomic_add_64(&vdc_stats.stat.value.ui64, 1);
96
97static int
98vdev_cache_offset_compare(const void *a1, const void *a2)
99{
100 const vdev_cache_entry_t *ve1 = a1;
101 const vdev_cache_entry_t *ve2 = a2;
102
103 if (ve1->ve_offset < ve2->ve_offset)
104 return (-1);
105 if (ve1->ve_offset > ve2->ve_offset)
106 return (1);
107 return (0);
108}
109
110static int
111vdev_cache_lastused_compare(const void *a1, const void *a2)
112{
113 const vdev_cache_entry_t *ve1 = a1;
114 const vdev_cache_entry_t *ve2 = a2;
115
116 if (ve1->ve_lastused < ve2->ve_lastused)
117 return (-1);
118 if (ve1->ve_lastused > ve2->ve_lastused)
119 return (1);
120
121 /*
122 * Among equally old entries, sort by offset to ensure uniqueness.
123 */
124 return (vdev_cache_offset_compare(a1, a2));
125}
126
127/*
128 * Evict the specified entry from the cache.
129 */
130static void
131vdev_cache_evict(vdev_cache_t *vc, vdev_cache_entry_t *ve)
132{
133 ASSERT(MUTEX_HELD(&vc->vc_lock));
134 ASSERT(ve->ve_fill_io == NULL);
135 ASSERT(ve->ve_data != NULL);
136
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137 avl_remove(&vc->vc_lastused_tree, ve);
138 avl_remove(&vc->vc_offset_tree, ve);
139 zio_buf_free(ve->ve_data, VCBS);
140 kmem_free(ve, sizeof (vdev_cache_entry_t));
141}
142
143/*
144 * Allocate an entry in the cache. At the point we don't have the data,
145 * we're just creating a placeholder so that multiple threads don't all
146 * go off and read the same blocks.
147 */
148static vdev_cache_entry_t *
149vdev_cache_allocate(zio_t *zio)
150{
151 vdev_cache_t *vc = &zio->io_vd->vdev_cache;
152 uint64_t offset = P2ALIGN(zio->io_offset, VCBS);
153 vdev_cache_entry_t *ve;
154
155 ASSERT(MUTEX_HELD(&vc->vc_lock));
156
157 if (zfs_vdev_cache_size == 0)
158 return (NULL);
159
160 /*
161 * If adding a new entry would exceed the cache size,
162 * evict the oldest entry (LRU).
163 */
164 if ((avl_numnodes(&vc->vc_lastused_tree) << zfs_vdev_cache_bshift) >
165 zfs_vdev_cache_size) {
166 ve = avl_first(&vc->vc_lastused_tree);
b128c09f 167 if (ve->ve_fill_io != NULL)
34dc7c2f 168 return (NULL);
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169 ASSERT(ve->ve_hits != 0);
170 vdev_cache_evict(vc, ve);
171 }
172
173 ve = kmem_zalloc(sizeof (vdev_cache_entry_t), KM_SLEEP);
174 ve->ve_offset = offset;
175 ve->ve_lastused = lbolt;
176 ve->ve_data = zio_buf_alloc(VCBS);
177
178 avl_add(&vc->vc_offset_tree, ve);
179 avl_add(&vc->vc_lastused_tree, ve);
180
181 return (ve);
182}
183
184static void
185vdev_cache_hit(vdev_cache_t *vc, vdev_cache_entry_t *ve, zio_t *zio)
186{
187 uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);
188
189 ASSERT(MUTEX_HELD(&vc->vc_lock));
190 ASSERT(ve->ve_fill_io == NULL);
191
192 if (ve->ve_lastused != lbolt) {
193 avl_remove(&vc->vc_lastused_tree, ve);
194 ve->ve_lastused = lbolt;
195 avl_add(&vc->vc_lastused_tree, ve);
196 }
197
198 ve->ve_hits++;
199 bcopy(ve->ve_data + cache_phase, zio->io_data, zio->io_size);
200}
201
202/*
203 * Fill a previously allocated cache entry with data.
204 */
205static void
206vdev_cache_fill(zio_t *zio)
207{
208 vdev_t *vd = zio->io_vd;
209 vdev_cache_t *vc = &vd->vdev_cache;
210 vdev_cache_entry_t *ve = zio->io_private;
211 zio_t *dio;
212
213 ASSERT(zio->io_size == VCBS);
214
215 /*
216 * Add data to the cache.
217 */
218 mutex_enter(&vc->vc_lock);
219
220 ASSERT(ve->ve_fill_io == zio);
221 ASSERT(ve->ve_offset == zio->io_offset);
222 ASSERT(ve->ve_data == zio->io_data);
223
224 ve->ve_fill_io = NULL;
225
226 /*
227 * Even if this cache line was invalidated by a missed write update,
228 * any reads that were queued up before the missed update are still
229 * valid, so we can satisfy them from this line before we evict it.
230 */
231 for (dio = zio->io_delegate_list; dio; dio = dio->io_delegate_next)
232 vdev_cache_hit(vc, ve, dio);
233
234 if (zio->io_error || ve->ve_missed_update)
235 vdev_cache_evict(vc, ve);
236
237 mutex_exit(&vc->vc_lock);
238
239 while ((dio = zio->io_delegate_list) != NULL) {
240 zio->io_delegate_list = dio->io_delegate_next;
241 dio->io_delegate_next = NULL;
242 dio->io_error = zio->io_error;
243 zio_execute(dio);
244 }
245}
246
247/*
248 * Read data from the cache. Returns 0 on cache hit, errno on a miss.
249 */
250int
251vdev_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 ASSERT(zio->io_type == ZIO_TYPE_READ);
260
261 if (zio->io_flags & ZIO_FLAG_DONT_CACHE)
262 return (EINVAL);
263
264 if (zio->io_size > zfs_vdev_cache_max)
265 return (EOVERFLOW);
266
267 /*
268 * If the I/O straddles two or more cache blocks, don't cache it.
269 */
b128c09f 270 if (P2BOUNDARY(zio->io_offset, zio->io_size, VCBS))
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271 return (EXDEV);
272
273 ASSERT(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 (ESTALE);
284 }
285
286 if ((fio = ve->ve_fill_io) != NULL) {
287 zio->io_delegate_next = fio->io_delegate_list;
288 fio->io_delegate_list = zio;
289 zio_vdev_io_bypass(zio);
290 mutex_exit(&vc->vc_lock);
291 VDCSTAT_BUMP(vdc_stat_delegations);
292 return (0);
293 }
294
295 vdev_cache_hit(vc, ve, zio);
296 zio_vdev_io_bypass(zio);
297
298 mutex_exit(&vc->vc_lock);
299 zio_execute(zio);
300 VDCSTAT_BUMP(vdc_stat_hits);
301 return (0);
302 }
303
304 ve = vdev_cache_allocate(zio);
305
306 if (ve == NULL) {
307 mutex_exit(&vc->vc_lock);
308 return (ENOMEM);
309 }
310
b128c09f 311 fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,
34dc7c2f 312 ve->ve_data, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_CACHE_FILL,
b128c09f 313 ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve);
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314
315 ve->ve_fill_io = fio;
316 fio->io_delegate_list = zio;
317 zio_vdev_io_bypass(zio);
318
319 mutex_exit(&vc->vc_lock);
320 zio_nowait(fio);
321 VDCSTAT_BUMP(vdc_stat_misses);
322
323 return (0);
324}
325
326/*
327 * Update cache contents upon write completion.
328 */
329void
330vdev_cache_write(zio_t *zio)
331{
332 vdev_cache_t *vc = &zio->io_vd->vdev_cache;
333 vdev_cache_entry_t *ve, ve_search;
334 uint64_t io_start = zio->io_offset;
335 uint64_t io_end = io_start + zio->io_size;
336 uint64_t min_offset = P2ALIGN(io_start, VCBS);
337 uint64_t max_offset = P2ROUNDUP(io_end, VCBS);
338 avl_index_t where;
339
340 ASSERT(zio->io_type == ZIO_TYPE_WRITE);
341
342 mutex_enter(&vc->vc_lock);
343
344 ve_search.ve_offset = min_offset;
345 ve = avl_find(&vc->vc_offset_tree, &ve_search, &where);
346
347 if (ve == NULL)
348 ve = avl_nearest(&vc->vc_offset_tree, where, AVL_AFTER);
349
350 while (ve != NULL && ve->ve_offset < max_offset) {
351 uint64_t start = MAX(ve->ve_offset, io_start);
352 uint64_t end = MIN(ve->ve_offset + VCBS, io_end);
353
354 if (ve->ve_fill_io != NULL) {
355 ve->ve_missed_update = 1;
356 } else {
357 bcopy((char *)zio->io_data + start - io_start,
358 ve->ve_data + start - ve->ve_offset, end - start);
359 }
360 ve = AVL_NEXT(&vc->vc_offset_tree, ve);
361 }
362 mutex_exit(&vc->vc_lock);
363}
364
365void
366vdev_cache_purge(vdev_t *vd)
367{
368 vdev_cache_t *vc = &vd->vdev_cache;
369 vdev_cache_entry_t *ve;
370
371 mutex_enter(&vc->vc_lock);
372 while ((ve = avl_first(&vc->vc_offset_tree)) != NULL)
373 vdev_cache_evict(vc, ve);
374 mutex_exit(&vc->vc_lock);
375}
376
377void
378vdev_cache_init(vdev_t *vd)
379{
380 vdev_cache_t *vc = &vd->vdev_cache;
381
382 mutex_init(&vc->vc_lock, NULL, MUTEX_DEFAULT, NULL);
383
384 avl_create(&vc->vc_offset_tree, vdev_cache_offset_compare,
385 sizeof (vdev_cache_entry_t),
386 offsetof(struct vdev_cache_entry, ve_offset_node));
387
388 avl_create(&vc->vc_lastused_tree, vdev_cache_lastused_compare,
389 sizeof (vdev_cache_entry_t),
390 offsetof(struct vdev_cache_entry, ve_lastused_node));
391}
392
393void
394vdev_cache_fini(vdev_t *vd)
395{
396 vdev_cache_t *vc = &vd->vdev_cache;
397
398 vdev_cache_purge(vd);
399
400 avl_destroy(&vc->vc_offset_tree);
401 avl_destroy(&vc->vc_lastused_tree);
402
403 mutex_destroy(&vc->vc_lock);
404}
405
406void
407vdev_cache_stat_init(void)
408{
409 vdc_ksp = kstat_create("zfs", 0, "vdev_cache_stats", "misc",
410 KSTAT_TYPE_NAMED, sizeof (vdc_stats) / sizeof (kstat_named_t),
411 KSTAT_FLAG_VIRTUAL);
412 if (vdc_ksp != NULL) {
413 vdc_ksp->ks_data = &vdc_stats;
414 kstat_install(vdc_ksp);
415 }
416}
417
418void
419vdev_cache_stat_fini(void)
420{
421 if (vdc_ksp != NULL) {
422 kstat_delete(vdc_ksp);
423 vdc_ksp = NULL;
424 }
425}