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OpenZFS 7143 - dbuf_read() creates unnecessary zio_root() for bonus buf

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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.

Changes sponsored by Intel Corp.

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by:  Alex Zhuravlev <alexey.zhuravlev@intel.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue openzfs/openzfs#137
Closes #4803 
Closes #5382
This commit is contained in:
Brian Behlendorf 2016-12-01 16:50:11 -07:00 committed by GitHub
parent ba712624d6
commit b0319c1faa
1 changed files with 4 additions and 3 deletions
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7
module/zfs/dbuf.c
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@ -21,7 +21,7 @@
/*
* Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
* Copyright 2011 Nexenta Systems, Inc. All rights reserved.
* Copyright (c) 2012, 2016 by Delphix. All rights reserved.
* Copyright (c) 2012, 2015 by Delphix. All rights reserved.
* Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
* Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
*/
@ -1207,7 +1207,8 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
} else if (db->db_state == DB_UNCACHED) {
spa_t *spa = dn->dn_objset->os_spa;
if (zio == NULL)
if (zio == NULL &&
db->db_blkptr != NULL && !BP_IS_HOLE(db->db_blkptr))
zio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
err = dbuf_read_impl(db, zio, flags);
@ -1221,7 +1222,7 @@ 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)
if (!err && !havepzio && zio != NULL)
err = zio_wait(zio);
} else {
/*