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.
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.
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]
22 * Copyright 2009 Sun Microsystems, Inc. All rights reserved.
23 * Use is subject to license terms.
26 #include <sys/zfs_context.h>
28 #include <sys/vdev_impl.h>
30 #include <sys/kstat.h>
33 * Virtual device read-ahead caching.
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.
49 * There are five cache operations: allocate, fill, read, write, evict.
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.
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.
58 * (3) Read. Read data from the cache.
60 * (4) Write. Update cache contents after write completion.
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.
67 * These tunables are for performance analysis.
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
75 int zfs_vdev_cache_max = 1<<14; /* 16KB */
76 int zfs_vdev_cache_size = 10ULL << 20; /* 10MB */
77 int zfs_vdev_cache_bshift = 16;
79 #define VCBS (1 << zfs_vdev_cache_bshift) /* 64KB */
81 kstat_t *vdc_ksp = NULL;
83 typedef struct vdc_stats {
84 kstat_named_t vdc_stat_delegations;
85 kstat_named_t vdc_stat_hits;
86 kstat_named_t vdc_stat_misses;
89 static vdc_stats_t vdc_stats = {
90 { "delegations", KSTAT_DATA_UINT64 },
91 { "hits", KSTAT_DATA_UINT64 },
92 { "misses", KSTAT_DATA_UINT64 }
95 #define VDCSTAT_BUMP(stat) atomic_add_64(&vdc_stats.stat.value.ui64, 1);
98 vdev_cache_offset_compare(const void *a1, const void *a2)
100 const vdev_cache_entry_t *ve1 = a1;
101 const vdev_cache_entry_t *ve2 = a2;
103 if (ve1->ve_offset < ve2->ve_offset)
105 if (ve1->ve_offset > ve2->ve_offset)
111 vdev_cache_lastused_compare(const void *a1, const void *a2)
113 const vdev_cache_entry_t *ve1 = a1;
114 const vdev_cache_entry_t *ve2 = a2;
116 if (ve1->ve_lastused < ve2->ve_lastused)
118 if (ve1->ve_lastused > ve2->ve_lastused)
122 * Among equally old entries, sort by offset to ensure uniqueness.
124 return (vdev_cache_offset_compare(a1, a2));
128 * Evict the specified entry from the cache.
131 vdev_cache_evict(vdev_cache_t *vc, vdev_cache_entry_t *ve)
133 ASSERT(MUTEX_HELD(&vc->vc_lock));
134 ASSERT(ve->ve_fill_io == NULL);
135 ASSERT(ve->ve_data != NULL);
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));
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.
148 static vdev_cache_entry_t *
149 vdev_cache_allocate(zio_t *zio)
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;
155 ASSERT(MUTEX_HELD(&vc->vc_lock));
157 if (zfs_vdev_cache_size == 0)
161 * If adding a new entry would exceed the cache size,
162 * evict the oldest entry (LRU).
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);
167 if (ve->ve_fill_io != NULL)
169 ASSERT(ve->ve_hits != 0);
170 vdev_cache_evict(vc, ve);
173 ve = kmem_zalloc(sizeof (vdev_cache_entry_t), KM_SLEEP);
174 ve->ve_offset = offset;
175 ve->ve_lastused = ddi_get_lbolt();
176 ve->ve_data = zio_buf_alloc(VCBS);
178 avl_add(&vc->vc_offset_tree, ve);
179 avl_add(&vc->vc_lastused_tree, ve);
185 vdev_cache_hit(vdev_cache_t *vc, vdev_cache_entry_t *ve, zio_t *zio)
187 uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);
189 ASSERT(MUTEX_HELD(&vc->vc_lock));
190 ASSERT(ve->ve_fill_io == NULL);
192 if (ve->ve_lastused != ddi_get_lbolt()) {
193 avl_remove(&vc->vc_lastused_tree, ve);
194 ve->ve_lastused = ddi_get_lbolt();
195 avl_add(&vc->vc_lastused_tree, ve);
199 bcopy(ve->ve_data + cache_phase, zio->io_data, zio->io_size);
203 * Fill a previously allocated cache entry with data.
206 vdev_cache_fill(zio_t *fio)
208 vdev_t *vd = fio->io_vd;
209 vdev_cache_t *vc = &vd->vdev_cache;
210 vdev_cache_entry_t *ve = fio->io_private;
213 ASSERT(fio->io_size == VCBS);
216 * Add data to the cache.
218 mutex_enter(&vc->vc_lock);
220 ASSERT(ve->ve_fill_io == fio);
221 ASSERT(ve->ve_offset == fio->io_offset);
222 ASSERT(ve->ve_data == fio->io_data);
224 ve->ve_fill_io = NULL;
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.
231 while ((pio = zio_walk_parents(fio)) != NULL)
232 vdev_cache_hit(vc, ve, pio);
234 if (fio->io_error || ve->ve_missed_update)
235 vdev_cache_evict(vc, ve);
237 mutex_exit(&vc->vc_lock);
241 * Read data from the cache. Returns 0 on cache hit, errno on a miss.
244 vdev_cache_read(zio_t *zio)
246 vdev_cache_t *vc = &zio->io_vd->vdev_cache;
247 vdev_cache_entry_t *ve, *ve_search;
248 uint64_t cache_offset = P2ALIGN(zio->io_offset, VCBS);
249 ASSERTV(uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);)
252 ASSERT(zio->io_type == ZIO_TYPE_READ);
254 if (zio->io_flags & ZIO_FLAG_DONT_CACHE)
257 if (zio->io_size > zfs_vdev_cache_max)
261 * If the I/O straddles two or more cache blocks, don't cache it.
263 if (P2BOUNDARY(zio->io_offset, zio->io_size, VCBS))
266 ASSERT(cache_phase + zio->io_size <= VCBS);
268 mutex_enter(&vc->vc_lock);
270 ve_search = kmem_alloc(sizeof(vdev_cache_entry_t), KM_SLEEP);
271 ve_search->ve_offset = cache_offset;
272 ve = avl_find(&vc->vc_offset_tree, ve_search, NULL);
273 kmem_free(ve_search, sizeof(vdev_cache_entry_t));
276 if (ve->ve_missed_update) {
277 mutex_exit(&vc->vc_lock);
281 if ((fio = ve->ve_fill_io) != NULL) {
282 zio_vdev_io_bypass(zio);
283 zio_add_child(zio, fio);
284 mutex_exit(&vc->vc_lock);
285 VDCSTAT_BUMP(vdc_stat_delegations);
289 vdev_cache_hit(vc, ve, zio);
290 zio_vdev_io_bypass(zio);
292 mutex_exit(&vc->vc_lock);
293 VDCSTAT_BUMP(vdc_stat_hits);
297 ve = vdev_cache_allocate(zio);
300 mutex_exit(&vc->vc_lock);
304 fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,
305 ve->ve_data, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_CACHE_FILL,
306 ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve);
308 ve->ve_fill_io = fio;
309 zio_vdev_io_bypass(zio);
310 zio_add_child(zio, fio);
312 mutex_exit(&vc->vc_lock);
314 VDCSTAT_BUMP(vdc_stat_misses);
320 * Update cache contents upon write completion.
323 vdev_cache_write(zio_t *zio)
325 vdev_cache_t *vc = &zio->io_vd->vdev_cache;
326 vdev_cache_entry_t *ve, ve_search;
327 uint64_t io_start = zio->io_offset;
328 uint64_t io_end = io_start + zio->io_size;
329 uint64_t min_offset = P2ALIGN(io_start, VCBS);
330 uint64_t max_offset = P2ROUNDUP(io_end, VCBS);
333 ASSERT(zio->io_type == ZIO_TYPE_WRITE);
335 mutex_enter(&vc->vc_lock);
337 ve_search.ve_offset = min_offset;
338 ve = avl_find(&vc->vc_offset_tree, &ve_search, &where);
341 ve = avl_nearest(&vc->vc_offset_tree, where, AVL_AFTER);
343 while (ve != NULL && ve->ve_offset < max_offset) {
344 uint64_t start = MAX(ve->ve_offset, io_start);
345 uint64_t end = MIN(ve->ve_offset + VCBS, io_end);
347 if (ve->ve_fill_io != NULL) {
348 ve->ve_missed_update = 1;
350 bcopy((char *)zio->io_data + start - io_start,
351 ve->ve_data + start - ve->ve_offset, end - start);
353 ve = AVL_NEXT(&vc->vc_offset_tree, ve);
355 mutex_exit(&vc->vc_lock);
359 vdev_cache_purge(vdev_t *vd)
361 vdev_cache_t *vc = &vd->vdev_cache;
362 vdev_cache_entry_t *ve;
364 mutex_enter(&vc->vc_lock);
365 while ((ve = avl_first(&vc->vc_offset_tree)) != NULL)
366 vdev_cache_evict(vc, ve);
367 mutex_exit(&vc->vc_lock);
371 vdev_cache_init(vdev_t *vd)
373 vdev_cache_t *vc = &vd->vdev_cache;
375 mutex_init(&vc->vc_lock, NULL, MUTEX_DEFAULT, NULL);
377 avl_create(&vc->vc_offset_tree, vdev_cache_offset_compare,
378 sizeof (vdev_cache_entry_t),
379 offsetof(struct vdev_cache_entry, ve_offset_node));
381 avl_create(&vc->vc_lastused_tree, vdev_cache_lastused_compare,
382 sizeof (vdev_cache_entry_t),
383 offsetof(struct vdev_cache_entry, ve_lastused_node));
387 vdev_cache_fini(vdev_t *vd)
389 vdev_cache_t *vc = &vd->vdev_cache;
391 vdev_cache_purge(vd);
393 avl_destroy(&vc->vc_offset_tree);
394 avl_destroy(&vc->vc_lastused_tree);
396 mutex_destroy(&vc->vc_lock);
400 vdev_cache_stat_init(void)
402 vdc_ksp = kstat_create("zfs", 0, "vdev_cache_stats", "misc",
403 KSTAT_TYPE_NAMED, sizeof (vdc_stats) / sizeof (kstat_named_t),
405 if (vdc_ksp != NULL) {
406 vdc_ksp->ks_data = &vdc_stats;
407 kstat_install(vdc_ksp);
412 vdev_cache_stat_fini(void)
414 if (vdc_ksp != NULL) {
415 kstat_delete(vdc_ksp);