Add prefetch property

ZFS prefetch is currently governed by the zfs_prefetch_disable
tunable. However, this is a module-wide settings - if a specific
dataset benefits from prefetch, while others have issue with it,
an optimal solution does not exists.

This commit introduce the "prefetch" tri-state property, which enable
granular control (at dataset/volume level) for prefetching.

This patch does not remove the zfs_prefetch_disable, which remains
a system-wide switch for enable/disable prefetch. However, to avoid
duplication, it would be preferable to deprecate and then remove
the module tunable.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Ameer Hamza <[email protected]>
Signed-off-by: Gionatan Danti <[email protected]>
Co-authored-by: Gionatan Danti <[email protected]>
Closes #15237 
Closes #15436 

1 files changed, 17 insertions, 0 deletions

diff --git a/man/man7/zfsprops.7 b/man/man7/zfsprops.7
index e3674b1f8..59f640437 100644
--- a/man/man7/zfsprops.7
+++ b/man/man7/zfsprops.7
@@ -1613,6 +1613,23 @@ If this property is set to
 then only metadata is cached.
 The default value is
 .Sy all .
+.It Sy prefetch Ns = Ns Sy all Ns | Ns Sy none Ns | Ns Sy metadata
+Controls what speculative prefetch does.
+If this property is set to
+.Sy all ,
+then both user data and metadata are prefetched.
+If this property is set to
+.Sy none ,
+then neither user data nor metadata are prefetched.
+If this property is set to
+.Sy metadata ,
+then only metadata are prefetched.
+The default value is
+.Sy all .
+.Pp
+Please note that the module parameter zfs_disable_prefetch=1 can
+be used to totally disable speculative prefetch, bypassing anything
+this property does.
 .It Sy setuid Ns = Ns Sy on Ns | Ns Sy off
 Controls whether the setuid bit is respected for the file system.
 The default value is