Fix ARC stats for embedded blkptrs

Re-factor arc_read() to better account for embedded data blkptrs.
Previously, reading the payload from an embedded blkptr would cause
arcstats such as demand_metadata_misses to be bumped when there was
actually no cache "miss" because the data are already available in
the blkptr.

The following test procedure was used to demonstrate the problem:

   zpool create tank ...
   zfs create -o compression=lz4 tank/fs
   echo blah > /tank/fs/blah
   stat /tank/fs/blah
   grep 'meta.*mis' /proc/spl/kstat/zfs/arcstats

and repeating the last two steps to watch the metadata miss counter
increment.  This can also be demonstrated via the  zfs_arc_miss DTRACE4
probe in arc_read().

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: George Wilson <george.wilson@delphix.com>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #8319

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index 385a2edec34a84536712c3dfa7fd160ec68dfd39..9072b6582bd67beab524f86bf224eaa91980f733 100644 (file)
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -6147,13 +6147,14 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
            (zio_flags & ZIO_FLAG_RAW_ENCRYPT) != 0;
        boolean_t noauth_read = BP_IS_AUTHENTICATED(bp) &&
            (zio_flags & ZIO_FLAG_RAW_ENCRYPT) != 0;
+       boolean_t embedded_bp = !!BP_IS_EMBEDDED(bp);
        int rc = 0;
 
-       ASSERT(!BP_IS_EMBEDDED(bp) ||
+       ASSERT(!embedded_bp ||
            BPE_GET_ETYPE(bp) == BP_EMBEDDED_TYPE_DATA);
 
 top:
-       if (!BP_IS_EMBEDDED(bp)) {
+       if (!embedded_bp) {
                /*
                 * Embedded BP's have no DVA and require no I/O to "read".
                 * Create an anonymous arc buf to back it.
@@ -6253,7 +6254,7 @@ top:
                                    ARC_FLAG_PRESCIENT_PREFETCH);
                        }
 
-                       ASSERT(!BP_IS_EMBEDDED(bp) || !BP_IS_HOLE(bp));
+                       ASSERT(!embedded_bp || !BP_IS_HOLE(bp));
 
                        /* Get a buf with the desired data in it. */
                        rc = arc_buf_alloc_impl(hdr, spa, zb, private,
@@ -6321,14 +6322,17 @@ top:
                }
 
                if (hdr == NULL) {
-                       /* this block is not in the cache */
+                       /*
+                        * This block is not in the cache or it has
+                        * embedded data.
+                        */
                        arc_buf_hdr_t *exists = NULL;
                        arc_buf_contents_t type = BP_GET_BUFC_TYPE(bp);
                        hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize,
                            BP_IS_PROTECTED(bp), BP_GET_COMPRESS(bp), type,
                            encrypted_read);
 
-                       if (!BP_IS_EMBEDDED(bp)) {
+                       if (!embedded_bp) {
                                hdr->b_dva = *BP_IDENTITY(bp);
                                hdr->b_birth = BP_PHYSICAL_BIRTH(bp);
                                exists = buf_hash_insert(hdr, &hash_lock);
@@ -6465,17 +6469,25 @@ top:
                        arc_hdr_clear_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
 
                /*
-                * At this point, we have a level 1 cache miss.  Try again in
-                * L2ARC if possible.
+                * At this point, we have a level 1 cache miss or a blkptr
+                * with embedded data.  Try again in L2ARC if possible.
                 */
                ASSERT3U(HDR_GET_LSIZE(hdr), ==, lsize);
 
-               DTRACE_PROBE4(arc__miss, arc_buf_hdr_t *, hdr, blkptr_t *, bp,
-                   uint64_t, lsize, zbookmark_phys_t *, zb);
-               ARCSTAT_BUMP(arcstat_misses);
-               ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr),
-                   demand, prefetch, !HDR_ISTYPE_METADATA(hdr),
-                   data, metadata, misses);
+               /*
+                * Skip ARC stat bump for block pointers with embedded
+                * data. The data are read from the blkptr itself via
+                * decode_embedded_bp_compressed().
+                */
+               if (!embedded_bp) {
+                       DTRACE_PROBE4(arc__miss, arc_buf_hdr_t *, hdr,
+                           blkptr_t *, bp, uint64_t, lsize,
+                           zbookmark_phys_t *, zb);
+                       ARCSTAT_BUMP(arcstat_misses);
+                       ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr),
+                           demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data,
+                           metadata, misses);
+               }
 
                if (vd != NULL && l2arc_ndev != 0 && !(l2arc_norw && devw)) {
                        /*
@@ -6567,7 +6579,12 @@ top:
                } else {
                        if (vd != NULL)
                                spa_config_exit(spa, SCL_L2ARC, vd);
-                       if (l2arc_ndev != 0) {
+                       /*
+                        * Skip ARC stat bump for block pointers with
+                        * embedded data. The data are read from the blkptr
+                        * itself via decode_embedded_bp_compressed().
+                        */
+                       if (l2arc_ndev != 0 && !embedded_bp) {
                                DTRACE_PROBE1(l2arc__miss,
                                    arc_buf_hdr_t *, hdr);
                                ARCSTAT_BUMP(arcstat_l2_misses);
@@ -6592,7 +6609,7 @@ top:
 
 out:
        /* embedded bps don't actually go to disk */
-       if (!BP_IS_EMBEDDED(bp))
+       if (!embedded_bp)
                spa_read_history_add(spa, zb, *arc_flags);
        return (rc);
 }