FreeBSD/Linux Kernel Cross Reference
sys/contrib/openzfs/module/zfs/vdev_cache.c

     /*
      * 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 https://opensource.org/licenses/CDDL-1.0.
     * 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.
     */
    static uint_t zfs_vdev_cache_max = 1 << 14;                     /* 16KB */
    static uint_t zfs_vdev_cache_size = 0;
    static uint_t zfs_vdev_cache_bshift = 16;
    
    #define VCBS (1 << zfs_vdev_cache_bshift)       /* 64KB */
    
    static 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 (TREE_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 = TREE_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;
           uint64_t cache_phase __maybe_unused = P2PHASE(zio->io_offset, VCBS);
   
           ASSERT3U(zio->io_type, ==, ZIO_TYPE_READ);
   
           if (zfs_vdev_cache_size == 0)
                   return (B_FALSE);
   
           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.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 (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;
           }
   }
   
   ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_max, UINT, ZMOD_RW,
           "Inflate reads small than max");
   
   ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_size, UINT, ZMOD_RD,
           "Total size of the per-disk cache");
   
   ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_bshift, UINT, ZMOD_RW,
           "Shift size to inflate reads too");

Cache object: 178833c3eb2c39a2855c5af4a3cffdd8