zfs-builds-mm/zfs-0.8.5/module/zfs/vdev_cache.c

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2020-10-22 13:48:46 +02:00
/*
* 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.
*/
/*
* Copyright (c) 2013, 2016 by Delphix. All rights reserved.
*/
#include <sys/zfs_context.h>
#include <sys/spa.h>
#include <sys/vdev_impl.h>
#include <sys/zio.h>
#include <sys/kstat.h>
#include <sys/abd.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 further 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.
*
* TODO: Note that with the current ZFS code, it turns out that the
* vdev cache is not helpful, and in some cases actually harmful. It
* is better if we disable this. Once some time has passed, we should
* actually remove this to simplify the code. For now we just disable
* it by setting the zfs_vdev_cache_size to zero. Note that Solaris 11
* has made these same changes.
*/
int zfs_vdev_cache_max = 1<<14; /* 16KB */
int zfs_vdev_cache_size = 0;
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_inc_64(&vdc_stats.stat.value.ui64);
static inline int
vdev_cache_offset_compare(const void *a1, const void *a2)
{
const vdev_cache_entry_t *ve1 = (const vdev_cache_entry_t *)a1;
const vdev_cache_entry_t *ve2 = (const vdev_cache_entry_t *)a2;
return (AVL_CMP(ve1->ve_offset, ve2->ve_offset));
}
static int
vdev_cache_lastused_compare(const void *a1, const void *a2)
{
const vdev_cache_entry_t *ve1 = (const vdev_cache_entry_t *)a1;
const vdev_cache_entry_t *ve2 = (const vdev_cache_entry_t *)a2;
int cmp = AVL_CMP(ve1->ve_lastused, ve2->ve_lastused);
if (likely(cmp))
return (cmp);
/*
* 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));
ASSERT3P(ve->ve_fill_io, ==, NULL);
ASSERT3P(ve->ve_abd, !=, NULL);
avl_remove(&vc->vc_lastused_tree, ve);
avl_remove(&vc->vc_offset_tree, ve);
abd_free(ve->ve_abd);
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);
ASSERT3U(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_abd = abd_alloc_for_io(VCBS, B_TRUE);
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));
ASSERT3P(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++;
abd_copy_off(zio->io_abd, ve->ve_abd, 0, cache_phase, 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;
ASSERT3U(fio->io_size, ==, VCBS);
/*
* Add data to the cache.
*/
mutex_enter(&vc->vc_lock);
ASSERT3P(ve->ve_fill_io, ==, fio);
ASSERT3U(ve->ve_offset, ==, fio->io_offset);
ASSERT3P(ve->ve_abd, ==, fio->io_abd);
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.
*/
zio_link_t *zl = NULL;
while ((pio = zio_walk_parents(fio, &zl)) != 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 B_TRUE cache hit, B_FALSE on miss.
*/
boolean_t
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);
zio_t *fio;
ASSERTV(uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS));
ASSERT3U(zio->io_type, ==, ZIO_TYPE_READ);
if (zio->io_flags & ZIO_FLAG_DONT_CACHE)
return (B_FALSE);
if (zio->io_size > zfs_vdev_cache_max)
return (B_FALSE);
/*
* If the I/O straddles two or more cache blocks, don't cache it.
*/
if (P2BOUNDARY(zio->io_offset, zio->io_size, VCBS))
return (B_FALSE);
ASSERT3U(cache_phase + zio->io_size, <=, VCBS);
mutex_enter(&vc->vc_lock);
ve_search = kmem_alloc(sizeof (vdev_cache_entry_t), KM_SLEEP);
ve_search->ve_offset = cache_offset;
ve = avl_find(&vc->vc_offset_tree, ve_search, NULL);
kmem_free(ve_search, sizeof (vdev_cache_entry_t));
if (ve != NULL) {
if (ve->ve_missed_update) {
mutex_exit(&vc->vc_lock);
return (B_FALSE);
}
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 (B_TRUE);
}
vdev_cache_hit(vc, ve, zio);
zio_vdev_io_bypass(zio);
mutex_exit(&vc->vc_lock);
VDCSTAT_BUMP(vdc_stat_hits);
return (B_TRUE);
}
ve = vdev_cache_allocate(zio);
if (ve == NULL) {
mutex_exit(&vc->vc_lock);
return (B_FALSE);
}
fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,
ve->ve_abd, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_NOW,
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 (B_TRUE);
}
/*
* 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;
ASSERT3U(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 {
abd_copy_off(ve->ve_abd, zio->io_abd,
start - ve->ve_offset, start - io_start,
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;
}
}
#if defined(_KERNEL)
module_param(zfs_vdev_cache_max, int, 0644);
MODULE_PARM_DESC(zfs_vdev_cache_max, "Inflate reads small than max");
module_param(zfs_vdev_cache_size, int, 0444);
MODULE_PARM_DESC(zfs_vdev_cache_size, "Total size of the per-disk cache");
module_param(zfs_vdev_cache_bshift, int, 0644);
MODULE_PARM_DESC(zfs_vdev_cache_bshift, "Shift size to inflate reads too");
#endif