ddt: add support for prefetching tables into the ARC

This change adds a new `zpool prefetch -t ddt $pool` command which
causes a pool's DDT to be loaded into the ARC. The primary goal is to
remove the need to "warm" a pool's cache before deduplication stops
slowing write performance. It may also provide a way to reload portions
of a DDT if they have been flushed due to inactivity.

Sponsored-by: iXsystems, Inc.
Sponsored-by: Catalogics, Inc.
Sponsored-by: Klara, Inc.
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Allan Jude <[email protected]>
Signed-off-by: Will Andrews <[email protected]>
Signed-off-by: Fred Weigel <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Signed-off-by: Don Brady <[email protected]>
Co-authored-by: Will Andrews <[email protected]>
Co-authored-by: Don Brady <[email protected]>
Closes #15890 

5 files changed, 58 insertions, 2 deletions

diff --git a/man/Makefile.am b/man/Makefile.am
index 43bb014dd..194bb4721 100644
--- a/man/Makefile.am
+++ b/man/Makefile.am
@@ -83,6 +83,7 @@ dist_man_MANS = \
 	%D%/man8/zpool-list.8 \
 	%D%/man8/zpool-offline.8 \
 	%D%/man8/zpool-online.8 \
+	%D%/man8/zpool-prefetch.8 \
 	%D%/man8/zpool-reguid.8 \
 	%D%/man8/zpool-remove.8 \
 	%D%/man8/zpool-reopen.8 \
diff --git a/man/man7/zpoolprops.7 b/man/man7/zpoolprops.7
index ff21e5300..9aba11ad5 100644
--- a/man/man7/zpoolprops.7
+++ b/man/man7/zpoolprops.7
@@ -73,6 +73,10 @@ The amount of storage used by cloned blocks.
 Percentage of pool space used.
 This property can also be referred to by its shortened column name,
 .Sy cap .
+.It Sy dedupcached
+Total size of the deduplication table currently loaded into the ARC.
+See
+.Xr zpool-prefetch 8 .
 .It Sy dedup_table_size
 Total on-disk size of the deduplication table.
 .It Sy expandsize
diff --git a/man/man8/zpool-prefetch.8 b/man/man8/zpool-prefetch.8
new file mode 100644
index 000000000..57445bd4a
--- /dev/null
+++ b/man/man8/zpool-prefetch.8
@@ -0,0 +1,46 @@
+.\"
+.\" 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 (c) 2023, Klara Inc.
+.\"
+.Dd February 14, 2024
+.Dt ZPOOL-PREFETCH 8
+.Os
+.
+.Sh NAME
+.Nm zpool-prefetch
+.Nd Loads specific types of data for the given pool
+.Sh SYNOPSIS
+.Nm zpool
+.Cm prefetch
+.Fl t Ar type
+.Ar pool
+.Sh DESCRIPTION
+.Bl -tag -width Ds
+.It Xo
+.Nm zpool
+.Cm prefetch
+.Fl t Li ddt
+.Ar pool
+.Xc
+Prefetch data of a specific type for the given pool; specifically the DDT,
+which will improve write I/O performance when the DDT is resident in the ARC.
+.El
diff --git a/man/man8/zpool-status.8 b/man/man8/zpool-status.8
index bbe7a45aa..d570c852d 100644
--- a/man/man8/zpool-status.8
+++ b/man/man8/zpool-status.8
@@ -26,7 +26,7 @@
 .\" Copyright 2017 Nexenta Systems, Inc.
 .\" Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
 .\"
-.Dd March 16, 2022
+.Dd February 14, 2024
 .Dt ZPOOL-STATUS 8
 .Os
 .
@@ -75,6 +75,8 @@ Display a histogram of deduplication statistics, showing the allocated
 and referenced
 .Pq logically referenced in the pool
 block counts and sizes by reference count.
+If repeated, (-DD), also shows statistics on how much of the DDT is resident
+in the ARC.
 .It Fl e
 Only show unhealthy vdevs (not-ONLINE or with errors).
 .It Fl g
diff --git a/man/man8/zpool.8 b/man/man8/zpool.8
index fe44e15ca..2b966b72b 100644
--- a/man/man8/zpool.8
+++ b/man/man8/zpool.8
@@ -26,7 +26,7 @@
 .\" Copyright 2017 Nexenta Systems, Inc.
 .\" Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
 .\"
-.Dd March 16, 2022
+.Dd February 14, 2024
 .Dt ZPOOL 8
 .Os
 .
@@ -168,6 +168,8 @@ specified.
 .
 .Ss Maintenance
 .Bl -tag -width Ds
+.It Xr zpool-prefetch 8
+Prefetches specific types of pool data.
 .It Xr zpool-scrub 8
 Begins a scrub or resumes a paused scrub.
 .It Xr zpool-checkpoint 8
@@ -598,6 +600,7 @@ don't wait.
 .Xr zpool-list 8 ,
 .Xr zpool-offline 8 ,
 .Xr zpool-online 8 ,
+.Xr zpool-prefetch 8 ,
 .Xr zpool-reguid 8 ,
 .Xr zpool-remove 8 ,
 .Xr zpool-reopen 8 ,