From a00433837236507e6ce90c22bf09e6699deef991 Mon Sep 17 00:00:00 2001 From: Matthew Ahrens Date: Thu, 13 Apr 2017 14:35:00 -0700 Subject: [PATCH] OpenZFS 8025 - dbuf_read() creates unnecessary zio_root() for bonus buf Authored by: Matthew Ahrens Reviewed by: Dan Kimmel Reviewed by: Pavel Zakharov Reviewed by: Prashanth Sreenivasa Approved by: Robert Mustacchi Ported-by: Brian Behlendorf dbuf_read() creates a zio_root() to track and wait for all the zio's that may happen as part of this call. However, if the blkptr_t for this buffer is NULL or a hole, we will not create any more zio's, so this zio_root() is unnecessary. This is always the case when calling dbuf_read() on a bonus buffer, because it has no blkptr (it's part of the containing dnode). For workloads that read a lot of bonus buffers (e.g. file creation and removal), creating and destroying these unnecessary zio's can decrease performance by around 3%. The fix is to only create/destroy the zio_root() in dbuf_read() if the blkptr is not NULL and not a hole. Porting Notes: - The error handling for when dbuf_read_impl() fails which was originally added in commit 5f6d0b6f5 has been preserved. OpenZFS-issue: https://www.illumos.org/issues/8025 OpenZFS-commit: https://github.com/openzfs/openzfs/commit/8ec5c7c Closes #6048 --- module/zfs/dbuf.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/module/zfs/dbuf.c b/module/zfs/dbuf.c index 096d183584c3..b0b1bffab9cb 100644 --- a/module/zfs/dbuf.c +++ b/module/zfs/dbuf.c @@ -1170,7 +1170,6 @@ int dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags) { int err = 0; - boolean_t havepzio = (zio != NULL); boolean_t prefetch; dnode_t *dn; @@ -1214,11 +1213,13 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags) DB_DNODE_EXIT(db); } else if (db->db_state == DB_UNCACHED) { spa_t *spa = dn->dn_objset->os_spa; + boolean_t need_wait = B_FALSE; if (zio == NULL && - db->db_blkptr != NULL && !BP_IS_HOLE(db->db_blkptr)) + db->db_blkptr != NULL && !BP_IS_HOLE(db->db_blkptr)) { zio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL); - + need_wait = B_TRUE; + } err = dbuf_read_impl(db, zio, flags); /* dbuf_read_impl has dropped db_mtx for us */ @@ -1230,7 +1231,7 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags) rw_exit(&dn->dn_struct_rwlock); DB_DNODE_EXIT(db); - if (!err && !havepzio && zio != NULL) + if (!err && need_wait) err = zio_wait(zio); } else { /* @@ -1265,7 +1266,6 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags) mutex_exit(&db->db_mtx); } - ASSERT(err || havepzio || db->db_state == DB_CACHED); return (err); }