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 * Copyright (c) 2013, 2016 by Delphix. All rights reserved.
29 #include <sys/zfs_context.h>
31 #include <sys/vdev_impl.h>
33 #include <sys/kstat.h>
37 * Virtual device read-ahead caching.
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 further 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.
53 * There are five cache operations: allocate, fill, read, write, evict.
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.
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.
62 * (3) Read. Read data from the cache.
64 * (4) Write. Update cache contents after write completion.
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.
71 * These tunables are for performance analysis.
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
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.
86 int zfs_vdev_cache_max = 1<<14; /* 16KB */
87 int zfs_vdev_cache_size = 0;
88 int zfs_vdev_cache_bshift = 16;
90 #define VCBS (1 << zfs_vdev_cache_bshift) /* 64KB */
92 kstat_t *vdc_ksp = NULL;
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;
100 static vdc_stats_t vdc_stats = {
101 { "delegations", KSTAT_DATA_UINT64 },
102 { "hits", KSTAT_DATA_UINT64 },
103 { "misses", KSTAT_DATA_UINT64 }
106 #define VDCSTAT_BUMP(stat) atomic_inc_64(&vdc_stats.stat.value.ui64);
109 vdev_cache_offset_compare(const void *a1, const void *a2)
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;
114 return (TREE_CMP(ve1->ve_offset, ve2->ve_offset));
118 vdev_cache_lastused_compare(const void *a1, const void *a2)
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;
123 int cmp = TREE_CMP(ve1->ve_lastused, ve2->ve_lastused);
128 * Among equally old entries, sort by offset to ensure uniqueness.
130 return (vdev_cache_offset_compare(a1, a2));
134 * Evict the specified entry from the cache.
137 vdev_cache_evict(vdev_cache_t *vc, vdev_cache_entry_t *ve)
139 ASSERT(MUTEX_HELD(&vc->vc_lock));
140 ASSERT3P(ve->ve_fill_io, ==, NULL);
141 ASSERT3P(ve->ve_abd, !=, NULL);
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));
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.
154 static vdev_cache_entry_t *
155 vdev_cache_allocate(zio_t *zio)
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;
161 ASSERT(MUTEX_HELD(&vc->vc_lock));
163 if (zfs_vdev_cache_size == 0)
167 * If adding a new entry would exceed the cache size,
168 * evict the oldest entry (LRU).
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)
175 ASSERT3U(ve->ve_hits, !=, 0);
176 vdev_cache_evict(vc, ve);
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);
184 avl_add(&vc->vc_offset_tree, ve);
185 avl_add(&vc->vc_lastused_tree, ve);
191 vdev_cache_hit(vdev_cache_t *vc, vdev_cache_entry_t *ve, zio_t *zio)
193 uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);
195 ASSERT(MUTEX_HELD(&vc->vc_lock));
196 ASSERT3P(ve->ve_fill_io, ==, NULL);
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);
205 abd_copy_off(zio->io_abd, ve->ve_abd, 0, cache_phase, zio->io_size);
209 * Fill a previously allocated cache entry with data.
212 vdev_cache_fill(zio_t *fio)
214 vdev_t *vd = fio->io_vd;
215 vdev_cache_t *vc = &vd->vdev_cache;
216 vdev_cache_entry_t *ve = fio->io_private;
219 ASSERT3U(fio->io_size, ==, VCBS);
222 * Add data to the cache.
224 mutex_enter(&vc->vc_lock);
226 ASSERT3P(ve->ve_fill_io, ==, fio);
227 ASSERT3U(ve->ve_offset, ==, fio->io_offset);
228 ASSERT3P(ve->ve_abd, ==, fio->io_abd);
230 ve->ve_fill_io = NULL;
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.
237 zio_link_t *zl = NULL;
238 while ((pio = zio_walk_parents(fio, &zl)) != NULL)
239 vdev_cache_hit(vc, ve, pio);
241 if (fio->io_error || ve->ve_missed_update)
242 vdev_cache_evict(vc, ve);
244 mutex_exit(&vc->vc_lock);
248 * Read data from the cache. Returns B_TRUE cache hit, B_FALSE on miss.
251 vdev_cache_read(zio_t *zio)
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);
257 ASSERTV(uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS));
259 ASSERT3U(zio->io_type, ==, ZIO_TYPE_READ);
261 if (zio->io_flags & ZIO_FLAG_DONT_CACHE)
264 if (zio->io_size > zfs_vdev_cache_max)
268 * If the I/O straddles two or more cache blocks, don't cache it.
270 if (P2BOUNDARY(zio->io_offset, zio->io_size, VCBS))
273 ASSERT3U(cache_phase + zio->io_size, <=, VCBS);
275 mutex_enter(&vc->vc_lock);
277 ve_search = kmem_alloc(sizeof (vdev_cache_entry_t), KM_SLEEP);
278 ve_search->ve_offset = cache_offset;
279 ve = avl_find(&vc->vc_offset_tree, ve_search, NULL);
280 kmem_free(ve_search, sizeof (vdev_cache_entry_t));
283 if (ve->ve_missed_update) {
284 mutex_exit(&vc->vc_lock);
288 if ((fio = ve->ve_fill_io) != NULL) {
289 zio_vdev_io_bypass(zio);
290 zio_add_child(zio, fio);
291 mutex_exit(&vc->vc_lock);
292 VDCSTAT_BUMP(vdc_stat_delegations);
296 vdev_cache_hit(vc, ve, zio);
297 zio_vdev_io_bypass(zio);
299 mutex_exit(&vc->vc_lock);
300 VDCSTAT_BUMP(vdc_stat_hits);
304 ve = vdev_cache_allocate(zio);
307 mutex_exit(&vc->vc_lock);
311 fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,
312 ve->ve_abd, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_NOW,
313 ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve);
315 ve->ve_fill_io = fio;
316 zio_vdev_io_bypass(zio);
317 zio_add_child(zio, fio);
319 mutex_exit(&vc->vc_lock);
321 VDCSTAT_BUMP(vdc_stat_misses);
327 * Update cache contents upon write completion.
330 vdev_cache_write(zio_t *zio)
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);
340 ASSERT3U(zio->io_type, ==, ZIO_TYPE_WRITE);
342 mutex_enter(&vc->vc_lock);
344 ve_search.ve_offset = min_offset;
345 ve = avl_find(&vc->vc_offset_tree, &ve_search, &where);
348 ve = avl_nearest(&vc->vc_offset_tree, where, AVL_AFTER);
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);
354 if (ve->ve_fill_io != NULL) {
355 ve->ve_missed_update = 1;
357 abd_copy_off(ve->ve_abd, zio->io_abd,
358 start - ve->ve_offset, start - io_start,
361 ve = AVL_NEXT(&vc->vc_offset_tree, ve);
363 mutex_exit(&vc->vc_lock);
367 vdev_cache_purge(vdev_t *vd)
369 vdev_cache_t *vc = &vd->vdev_cache;
370 vdev_cache_entry_t *ve;
372 mutex_enter(&vc->vc_lock);
373 while ((ve = avl_first(&vc->vc_offset_tree)) != NULL)
374 vdev_cache_evict(vc, ve);
375 mutex_exit(&vc->vc_lock);
379 vdev_cache_init(vdev_t *vd)
381 vdev_cache_t *vc = &vd->vdev_cache;
383 mutex_init(&vc->vc_lock, NULL, MUTEX_DEFAULT, NULL);
385 avl_create(&vc->vc_offset_tree, vdev_cache_offset_compare,
386 sizeof (vdev_cache_entry_t),
387 offsetof(struct vdev_cache_entry, ve_offset_node));
389 avl_create(&vc->vc_lastused_tree, vdev_cache_lastused_compare,
390 sizeof (vdev_cache_entry_t),
391 offsetof(struct vdev_cache_entry, ve_lastused_node));
395 vdev_cache_fini(vdev_t *vd)
397 vdev_cache_t *vc = &vd->vdev_cache;
399 vdev_cache_purge(vd);
401 avl_destroy(&vc->vc_offset_tree);
402 avl_destroy(&vc->vc_lastused_tree);
404 mutex_destroy(&vc->vc_lock);
408 vdev_cache_stat_init(void)
410 vdc_ksp = kstat_create("zfs", 0, "vdev_cache_stats", "misc",
411 KSTAT_TYPE_NAMED, sizeof (vdc_stats) / sizeof (kstat_named_t),
413 if (vdc_ksp != NULL) {
414 vdc_ksp->ks_data = &vdc_stats;
415 kstat_install(vdc_ksp);
420 vdev_cache_stat_fini(void)
422 if (vdc_ksp != NULL) {
423 kstat_delete(vdc_ksp);
429 ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_max, INT, ZMOD_RW,
430 "Inflate reads small than max");
432 ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_size, INT, ZMOD_RD,
433 "Total size of the per-disk cache");
435 ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_bshift, INT, ZMOD_RW,
436 "Shift size to inflate reads too");