/*

 * CDDL HEADER START

 *

 * The contents of this file are subject to the terms of the

 * Common Development and Distribution License (the "License").

 * You may not use this file except in compliance with the License.

 *

 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE

 * or http://www.opensolaris.org/os/licensing.

 * See the License for the specific language governing permissions

 * and limitations under the License.

 *

 * When distributing Covered Code, include this CDDL HEADER in each

 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.

 * If applicable, add the following below this CDDL HEADER, with the

 * fields enclosed by brackets "[]" replaced with your own identifying

 * information: Portions Copyright [yyyy] [name of copyright owner]

 *

 * CDDL HEADER END

 */

/*

 * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.

 * Use is subject to license terms.

 */

#include <sys/zfs_context.h>

#include <sys/spa.h>

#include <sys/vdev_impl.h>

#include <sys/zio.h>

#include <sys/kstat.h>

/*

 * Virtual device read-ahead caching.

 *

 * This file implements a simple LRU read-ahead cache.  When the DMU reads

 * a given block, it will often want other, nearby blocks soon thereafter.

 * We take advantage of this by reading a larger disk region and caching

 * the result.  In the best case, this can turn 128 back-to-back 512-byte

 * reads into a single 64k read followed by 127 cache hits; this reduces

 * latency dramatically.  In the worst case, it can turn an isolated 512-byte

 * read into a 64k read, which doesn't affect latency all that much but is

 * terribly wasteful of bandwidth.  A more intelligent version of the cache

 * could keep track of access patterns and not do read-ahead unless it sees

 * at least two temporally close I/Os to the same region.  Currently, only

 * metadata I/O is inflated.  A futher enhancement could take advantage of

 * more semantic information about the I/O.  And it could use something

 * faster than an AVL tree; that was chosen solely for convenience.

 *

 * There are five cache operations: allocate, fill, read, write, evict.

 *

 * (1) Allocate.  This reserves a cache entry for the specified region.

 *     We separate the allocate and fill operations so that multiple threads

 *     don't generate I/O for the same cache miss.

 *

 * (2) Fill.  When the I/O for a cache miss completes, the fill routine

 *     places the data in the previously allocated cache entry.

 *

 * (3) Read.  Read data from the cache.

 *

 * (4) Write.  Update cache contents after write completion.

 *

 * (5) Evict.  When allocating a new entry, we evict the oldest (LRU) entry

 *     if the total cache size exceeds zfs_vdev_cache_size.

 */

/*

 * These tunables are for performance analysis.

 */

/*

 * All i/os smaller than zfs_vdev_cache_max will be turned into

 * 1<<zfs_vdev_cache_bshift byte reads by the vdev_cache (aka software

 * track buffer).  At most zfs_vdev_cache_size bytes will be kept in each

 * vdev's vdev_cache.

 */

int zfs_vdev_cache_max = 1<<14;			/* 16KB */

int zfs_vdev_cache_size = 10ULL << 20;		/* 10MB */

int zfs_vdev_cache_bshift = 16;

#define	VCBS (1 << zfs_vdev_cache_bshift)	/* 64KB */

kstat_t	*vdc_ksp = NULL;

typedef struct vdc_stats {

	kstat_named_t vdc_stat_delegations;

	kstat_named_t vdc_stat_hits;

	kstat_named_t vdc_stat_misses;

} vdc_stats_t;

static vdc_stats_t vdc_stats = {

	{ "delegations",	KSTAT_DATA_UINT64 },

	{ "hits",		KSTAT_DATA_UINT64 },

	{ "misses",		KSTAT_DATA_UINT64 }

};

#define	VDCSTAT_BUMP(stat)	atomic_add_64(&vdc_stats.stat.value.ui64, 1);

static int

vdev_cache_offset_compare(const void *a1, const void *a2)

{

	const vdev_cache_entry_t *ve1 = a1;

	const vdev_cache_entry_t *ve2 = a2;

	if (ve1->ve_offset < ve2->ve_offset)

		return (-1);

	if (ve1->ve_offset > ve2->ve_offset)

		return (1);

	return (0);

}

static int

vdev_cache_lastused_compare(const void *a1, const void *a2)

{

	const vdev_cache_entry_t *ve1 = a1;

	const vdev_cache_entry_t *ve2 = a2;

	if (ve1->ve_lastused < ve2->ve_lastused)

		return (-1);

	if (ve1->ve_lastused > ve2->ve_lastused)

		return (1);

	/*

	 * Among equally old entries, sort by offset to ensure uniqueness.

	 */

	return (vdev_cache_offset_compare(a1, a2));

}

/*

 * Evict the specified entry from the cache.

 */

static void

vdev_cache_evict(vdev_cache_t *vc, vdev_cache_entry_t *ve)

{

	ASSERT(MUTEX_HELD(&vc->vc_lock));

	ASSERT(ve->ve_fill_io == NULL);

	ASSERT(ve->ve_data != NULL);

	avl_remove(&vc->vc_lastused_tree, ve);

	avl_remove(&vc->vc_offset_tree, ve);

	zio_buf_free(ve->ve_data, VCBS);

	kmem_free(ve, sizeof (vdev_cache_entry_t));

}

/*

 * Allocate an entry in the cache.  At the point we don't have the data,

 * we're just creating a placeholder so that multiple threads don't all

 * go off and read the same blocks.

 */

static vdev_cache_entry_t *

vdev_cache_allocate(zio_t *zio)

{

	vdev_cache_t *vc = &zio->io_vd->vdev_cache;

	uint64_t offset = P2ALIGN(zio->io_offset, VCBS);

	vdev_cache_entry_t *ve;

	ASSERT(MUTEX_HELD(&vc->vc_lock));

	if (zfs_vdev_cache_size == 0)

		return (NULL);

	/*

	 * If adding a new entry would exceed the cache size,

	 * evict the oldest entry (LRU).

	 */

	if ((avl_numnodes(&vc->vc_lastused_tree) << zfs_vdev_cache_bshift) >

	    zfs_vdev_cache_size) {

		ve = avl_first(&vc->vc_lastused_tree);

		if (ve->ve_fill_io != NULL)

			return (NULL);

		ASSERT(ve->ve_hits != 0);

		vdev_cache_evict(vc, ve);

	}

	ve = kmem_zalloc(sizeof (vdev_cache_entry_t), KM_SLEEP);

	ve->ve_offset = offset;

	ve->ve_lastused = ddi_get_lbolt();

	ve->ve_data = zio_buf_alloc(VCBS);

	avl_add(&vc->vc_offset_tree, ve);

	avl_add(&vc->vc_lastused_tree, ve);

	return (ve);

}

static void

vdev_cache_hit(vdev_cache_t *vc, vdev_cache_entry_t *ve, zio_t *zio)

{

	uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);

	ASSERT(MUTEX_HELD(&vc->vc_lock));

	ASSERT(ve->ve_fill_io == NULL);

	if (ve->ve_lastused != ddi_get_lbolt()) {

		avl_remove(&vc->vc_lastused_tree, ve);

		ve->ve_lastused = ddi_get_lbolt();

		avl_add(&vc->vc_lastused_tree, ve);

	}

	ve->ve_hits++;

	bcopy(ve->ve_data + cache_phase, zio->io_data, zio->io_size);

}

/*

 * Fill a previously allocated cache entry with data.

 */

static void

vdev_cache_fill(zio_t *fio)

{

	vdev_t *vd = fio->io_vd;

	vdev_cache_t *vc = &vd->vdev_cache;

	vdev_cache_entry_t *ve = fio->io_private;

	zio_t *pio;

	ASSERT(fio->io_size == VCBS);

	/*

	 * Add data to the cache.

	 */

	mutex_enter(&vc->vc_lock);

	ASSERT(ve->ve_fill_io == fio);

	ASSERT(ve->ve_offset == fio->io_offset);

	ASSERT(ve->ve_data == fio->io_data);

	ve->ve_fill_io = NULL;

	/*

	 * Even if this cache line was invalidated by a missed write update,

	 * any reads that were queued up before the missed update are still

	 * valid, so we can satisfy them from this line before we evict it.

	 */

	while ((pio = zio_walk_parents(fio)) != NULL)

		vdev_cache_hit(vc, ve, pio);

	if (fio->io_error || ve->ve_missed_update)

		vdev_cache_evict(vc, ve);

	mutex_exit(&vc->vc_lock);

}

/*

 * Read data from the cache.  Returns 0 on cache hit, errno on a miss.

 */

int

vdev_cache_read(zio_t *zio)

{

	vdev_cache_t *vc = &zio->io_vd->vdev_cache;

	vdev_cache_entry_t *ve, ve_search;

	uint64_t cache_offset = P2ALIGN(zio->io_offset, VCBS);

	uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);

	zio_t *fio;

	ASSERT(zio->io_type == ZIO_TYPE_READ);

	if (zio->io_flags & ZIO_FLAG_DONT_CACHE)

		return (EINVAL);

	if (zio->io_size > zfs_vdev_cache_max)

		return (EOVERFLOW);

	/*

	 * If the I/O straddles two or more cache blocks, don't cache it.

	 */

	if (P2BOUNDARY(zio->io_offset, zio->io_size, VCBS))

		return (EXDEV);

	ASSERT(cache_phase + zio->io_size <= VCBS);

	mutex_enter(&vc->vc_lock);

	ve_search.ve_offset = cache_offset;

	ve = avl_find(&vc->vc_offset_tree, &ve_search, NULL);

	if (ve != NULL) {

		if (ve->ve_missed_update) {

			mutex_exit(&vc->vc_lock);

			return (ESTALE);

		}

		if ((fio = ve->ve_fill_io) != NULL) {

			zio_vdev_io_bypass(zio);

			zio_add_child(zio, fio);

			mutex_exit(&vc->vc_lock);

			VDCSTAT_BUMP(vdc_stat_delegations);

			return (0);

		}

		vdev_cache_hit(vc, ve, zio);

		zio_vdev_io_bypass(zio);

		mutex_exit(&vc->vc_lock);

		VDCSTAT_BUMP(vdc_stat_hits);

		return (0);

	}

	ve = vdev_cache_allocate(zio);

	if (ve == NULL) {

		mutex_exit(&vc->vc_lock);

		return (ENOMEM);

	}

	fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,

	    ve->ve_data, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_CACHE_FILL,

	    ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve);

	ve->ve_fill_io = fio;

	zio_vdev_io_bypass(zio);

	zio_add_child(zio, fio);

	mutex_exit(&vc->vc_lock);

	zio_nowait(fio);

	VDCSTAT_BUMP(vdc_stat_misses);

	return (0);

}

/*

 * Update cache contents upon write completion.

 */

void

vdev_cache_write(zio_t *zio)

{

	vdev_cache_t *vc = &zio->io_vd->vdev_cache;

	vdev_cache_entry_t *ve, ve_search;

	uint64_t io_start = zio->io_offset;

	uint64_t io_end = io_start + zio->io_size;

	uint64_t min_offset = P2ALIGN(io_start, VCBS);

	uint64_t max_offset = P2ROUNDUP(io_end, VCBS);

	avl_index_t where;

	ASSERT(zio->io_type == ZIO_TYPE_WRITE);

	mutex_enter(&vc->vc_lock);

	ve_search.ve_offset = min_offset;

	ve = avl_find(&vc->vc_offset_tree, &ve_search, &where);

	if (ve == NULL)

		ve = avl_nearest(&vc->vc_offset_tree, where, AVL_AFTER);

	while (ve != NULL && ve->ve_offset < max_offset) {

		uint64_t start = MAX(ve->ve_offset, io_start);

		uint64_t end = MIN(ve->ve_offset + VCBS, io_end);

		if (ve->ve_fill_io != NULL) {

			ve->ve_missed_update = 1;

		} else {

			bcopy((char *)zio->io_data + start - io_start,

			    ve->ve_data + start - ve->ve_offset, end - start);

		}

		ve = AVL_NEXT(&vc->vc_offset_tree, ve);

	}

	mutex_exit(&vc->vc_lock);

}

void

vdev_cache_purge(vdev_t *vd)

{

	vdev_cache_t *vc = &vd->vdev_cache;

	vdev_cache_entry_t *ve;

	mutex_enter(&vc->vc_lock);

	while ((ve = avl_first(&vc->vc_offset_tree)) != NULL)

		vdev_cache_evict(vc, ve);

	mutex_exit(&vc->vc_lock);

}

void

vdev_cache_init(vdev_t *vd)

{

	vdev_cache_t *vc = &vd->vdev_cache;

	mutex_init(&vc->vc_lock, NULL, MUTEX_DEFAULT, NULL);

	avl_create(&vc->vc_offset_tree, vdev_cache_offset_compare,

	    sizeof (vdev_cache_entry_t),

	    offsetof(struct vdev_cache_entry, ve_offset_node));

	avl_create(&vc->vc_lastused_tree, vdev_cache_lastused_compare,

	    sizeof (vdev_cache_entry_t),

	    offsetof(struct vdev_cache_entry, ve_lastused_node));

}

void

vdev_cache_fini(vdev_t *vd)

{

	vdev_cache_t *vc = &vd->vdev_cache;

	vdev_cache_purge(vd);

	avl_destroy(&vc->vc_offset_tree);

	avl_destroy(&vc->vc_lastused_tree);

	mutex_destroy(&vc->vc_lock);

}

void

vdev_cache_stat_init(void)

{

	vdc_ksp = kstat_create("zfs", 0, "vdev_cache_stats", "misc",

	    KSTAT_TYPE_NAMED, sizeof (vdc_stats) / sizeof (kstat_named_t),

	    KSTAT_FLAG_VIRTUAL);

	if (vdc_ksp != NULL) {

		vdc_ksp->ks_data = &vdc_stats;

		kstat_install(vdc_ksp);

	}

}

void

vdev_cache_stat_fini(void)

{

	if (vdc_ksp != NULL) {

		kstat_delete(vdc_ksp);

		vdc_ksp = NULL;

	}

}