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, 2015 by Delphix. All rights reserved.
29 #include <sys/zfs_context.h>
31 #include <sys/vdev_impl.h>
33 #include <sys/kstat.h>
36 * Virtual device read-ahead caching.
38 * This file implements a simple LRU read-ahead cache. When the DMU reads
39 * a given block, it will often want other, nearby blocks soon thereafter.
40 * We take advantage of this by reading a larger disk region and caching
41 * the result. In the best case, this can turn 128 back-to-back 512-byte
42 * reads into a single 64k read followed by 127 cache hits; this reduces
43 * latency dramatically. In the worst case, it can turn an isolated 512-byte
44 * read into a 64k read, which doesn't affect latency all that much but is
45 * terribly wasteful of bandwidth. A more intelligent version of the cache
46 * could keep track of access patterns and not do read-ahead unless it sees
47 * at least two temporally close I/Os to the same region. Currently, only
48 * metadata I/O is inflated. A futher enhancement could take advantage of
49 * more semantic information about the I/O. And it could use something
50 * faster than an AVL tree; that was chosen solely for convenience.
52 * There are five cache operations: allocate, fill, read, write, evict.
54 * (1) Allocate. This reserves a cache entry for the specified region.
55 * We separate the allocate and fill operations so that multiple threads
56 * don't generate I/O for the same cache miss.
58 * (2) Fill. When the I/O for a cache miss completes, the fill routine
59 * places the data in the previously allocated cache entry.
61 * (3) Read. Read data from the cache.
63 * (4) Write. Update cache contents after write completion.
65 * (5) Evict. When allocating a new entry, we evict the oldest (LRU) entry
66 * if the total cache size exceeds zfs_vdev_cache_size.
70 * These tunables are for performance analysis.
73 * All i/os smaller than zfs_vdev_cache_max will be turned into
74 * 1<<zfs_vdev_cache_bshift byte reads by the vdev_cache (aka software
75 * track buffer). At most zfs_vdev_cache_size bytes will be kept in each
78 * TODO: Note that with the current ZFS code, it turns out that the
79 * vdev cache is not helpful, and in some cases actually harmful. It
80 * is better if we disable this. Once some time has passed, we should
81 * actually remove this to simplify the code. For now we just disable
82 * it by setting the zfs_vdev_cache_size to zero. Note that Solaris 11
83 * has made these same changes.
85 int zfs_vdev_cache_max
= 1<<14; /* 16KB */
86 int zfs_vdev_cache_size
= 0;
87 int zfs_vdev_cache_bshift
= 16;
89 #define VCBS (1 << zfs_vdev_cache_bshift) /* 64KB */
91 kstat_t
*vdc_ksp
= NULL
;
93 typedef struct vdc_stats
{
94 kstat_named_t vdc_stat_delegations
;
95 kstat_named_t vdc_stat_hits
;
96 kstat_named_t vdc_stat_misses
;
99 static vdc_stats_t vdc_stats
= {
100 { "delegations", KSTAT_DATA_UINT64
},
101 { "hits", KSTAT_DATA_UINT64
},
102 { "misses", KSTAT_DATA_UINT64
}
105 #define VDCSTAT_BUMP(stat) atomic_inc_64(&vdc_stats.stat.value.ui64);
108 vdev_cache_offset_compare(const void *a1
, const void *a2
)
110 const vdev_cache_entry_t
*ve1
= (const vdev_cache_entry_t
*)a1
;
111 const vdev_cache_entry_t
*ve2
= (const vdev_cache_entry_t
*)a2
;
113 return (AVL_CMP(ve1
->ve_offset
, ve2
->ve_offset
));
117 vdev_cache_lastused_compare(const void *a1
, const void *a2
)
119 const vdev_cache_entry_t
*ve1
= (const vdev_cache_entry_t
*)a1
;
120 const vdev_cache_entry_t
*ve2
= (const vdev_cache_entry_t
*)a2
;
122 int cmp
= AVL_CMP(ve1
->ve_lastused
, ve2
->ve_lastused
);
127 * Among equally old entries, sort by offset to ensure uniqueness.
129 return (vdev_cache_offset_compare(a1
, a2
));
133 * Evict the specified entry from the cache.
136 vdev_cache_evict(vdev_cache_t
*vc
, vdev_cache_entry_t
*ve
)
138 ASSERT(MUTEX_HELD(&vc
->vc_lock
));
139 ASSERT(ve
->ve_fill_io
== NULL
);
140 ASSERT(ve
->ve_data
!= NULL
);
142 avl_remove(&vc
->vc_lastused_tree
, ve
);
143 avl_remove(&vc
->vc_offset_tree
, ve
);
144 zio_buf_free(ve
->ve_data
, VCBS
);
145 kmem_free(ve
, sizeof (vdev_cache_entry_t
));
149 * Allocate an entry in the cache. At the point we don't have the data,
150 * we're just creating a placeholder so that multiple threads don't all
151 * go off and read the same blocks.
153 static vdev_cache_entry_t
*
154 vdev_cache_allocate(zio_t
*zio
)
156 vdev_cache_t
*vc
= &zio
->io_vd
->vdev_cache
;
157 uint64_t offset
= P2ALIGN(zio
->io_offset
, VCBS
);
158 vdev_cache_entry_t
*ve
;
160 ASSERT(MUTEX_HELD(&vc
->vc_lock
));
162 if (zfs_vdev_cache_size
== 0)
166 * If adding a new entry would exceed the cache size,
167 * evict the oldest entry (LRU).
169 if ((avl_numnodes(&vc
->vc_lastused_tree
) << zfs_vdev_cache_bshift
) >
170 zfs_vdev_cache_size
) {
171 ve
= avl_first(&vc
->vc_lastused_tree
);
172 if (ve
->ve_fill_io
!= NULL
)
174 ASSERT(ve
->ve_hits
!= 0);
175 vdev_cache_evict(vc
, ve
);
178 ve
= kmem_zalloc(sizeof (vdev_cache_entry_t
), KM_SLEEP
);
179 ve
->ve_offset
= offset
;
180 ve
->ve_lastused
= ddi_get_lbolt();
181 ve
->ve_data
= zio_buf_alloc(VCBS
);
183 avl_add(&vc
->vc_offset_tree
, ve
);
184 avl_add(&vc
->vc_lastused_tree
, ve
);
190 vdev_cache_hit(vdev_cache_t
*vc
, vdev_cache_entry_t
*ve
, zio_t
*zio
)
192 uint64_t cache_phase
= P2PHASE(zio
->io_offset
, VCBS
);
194 ASSERT(MUTEX_HELD(&vc
->vc_lock
));
195 ASSERT(ve
->ve_fill_io
== NULL
);
197 if (ve
->ve_lastused
!= ddi_get_lbolt()) {
198 avl_remove(&vc
->vc_lastused_tree
, ve
);
199 ve
->ve_lastused
= ddi_get_lbolt();
200 avl_add(&vc
->vc_lastused_tree
, ve
);
204 bcopy(ve
->ve_data
+ cache_phase
, zio
->io_data
, zio
->io_size
);
208 * Fill a previously allocated cache entry with data.
211 vdev_cache_fill(zio_t
*fio
)
213 vdev_t
*vd
= fio
->io_vd
;
214 vdev_cache_t
*vc
= &vd
->vdev_cache
;
215 vdev_cache_entry_t
*ve
= fio
->io_private
;
219 ASSERT(fio
->io_size
== VCBS
);
222 * Add data to the cache.
224 mutex_enter(&vc
->vc_lock
);
226 ASSERT(ve
->ve_fill_io
== fio
);
227 ASSERT(ve
->ve_offset
== fio
->io_offset
);
228 ASSERT(ve
->ve_data
== fio
->io_data
);
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.
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 ASSERT(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 ASSERT(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_data
, 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 ASSERT(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 bcopy((char *)zio
->io_data
+ start
- io_start
,
358 ve
->ve_data
+ start
- ve
->ve_offset
, end
- start
);
360 ve
= AVL_NEXT(&vc
->vc_offset_tree
, ve
);
362 mutex_exit(&vc
->vc_lock
);
366 vdev_cache_purge(vdev_t
*vd
)
368 vdev_cache_t
*vc
= &vd
->vdev_cache
;
369 vdev_cache_entry_t
*ve
;
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
);
378 vdev_cache_init(vdev_t
*vd
)
380 vdev_cache_t
*vc
= &vd
->vdev_cache
;
382 mutex_init(&vc
->vc_lock
, NULL
, MUTEX_DEFAULT
, NULL
);
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
));
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
));
394 vdev_cache_fini(vdev_t
*vd
)
396 vdev_cache_t
*vc
= &vd
->vdev_cache
;
398 vdev_cache_purge(vd
);
400 avl_destroy(&vc
->vc_offset_tree
);
401 avl_destroy(&vc
->vc_lastused_tree
);
403 mutex_destroy(&vc
->vc_lock
);
407 vdev_cache_stat_init(void)
409 vdc_ksp
= kstat_create("zfs", 0, "vdev_cache_stats", "misc",
410 KSTAT_TYPE_NAMED
, sizeof (vdc_stats
) / sizeof (kstat_named_t
),
412 if (vdc_ksp
!= NULL
) {
413 vdc_ksp
->ks_data
= &vdc_stats
;
414 kstat_install(vdc_ksp
);
419 vdev_cache_stat_fini(void)
421 if (vdc_ksp
!= NULL
) {
422 kstat_delete(vdc_ksp
);
427 #if defined(_KERNEL) && defined(HAVE_SPL)
428 module_param(zfs_vdev_cache_max
, int, 0644);
429 MODULE_PARM_DESC(zfs_vdev_cache_max
, "Inflate reads small than max");
431 module_param(zfs_vdev_cache_size
, int, 0444);
432 MODULE_PARM_DESC(zfs_vdev_cache_size
, "Total size of the per-disk cache");
434 module_param(zfs_vdev_cache_bshift
, int, 0644);
435 MODULE_PARM_DESC(zfs_vdev_cache_bshift
, "Shift size to inflate reads too");