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Wire O_DIRECT also to Uncached I/O (#17218)

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Before Direct I/O was implemented, I've implemented lighter version
I called Uncached I/O.  It uses normal DMU/ARC data path with some
optimizations, but evicts data from caches as soon as possible and
reasonable.  Originally I wired it only to a primarycache property,
but now completing the integration all the way up to the VFS.

While Direct I/O has the lowest possible memory bandwidth usage,
it also has a significant number of limitations.  It require I/Os
to be page aligned, does not allow speculative prefetch, etc.  The
Uncached I/O does not have those limitations, but instead require
additional memory copy, though still one less than regular cached
I/O.  As such it should fill the gap in between.  Considering this
I've disabled annoying EINVAL errors on misaligned requests, adding
a tunable for those who wants to test their applications.

To pass the information between the layers I had to change a number
of APIs.  But as side effect upper layers can now control not only
the caching, but also speculative prefetch.  I haven't wired it to
VFS yet, since it require looking on some OS specifics.  But while
there I've implemented speculative prefetch of indirect blocks for
Direct I/O, controllable via all the same mechanisms.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Fixes #17027
Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
This commit is contained in:
Alexander Motin
2025-05-13 17:26:55 -04:00
committed by GitHub
parent e2ba0f7643
commit 734eba251d
35 changed files with 397 additions and 294 deletions
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man/man4/zfs.4
+6 -1
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@@ -304,7 +304,7 @@ Default dnode block size as a power of 2.
.It Sy zfs_default_ibs Ns = Ns Sy 17 Po 128 KiB Pc Pq int
Default dnode indirect block size as a power of 2.
.
.It Sy zfs_dio_enabled Ns = Ns Sy 0 Ns | Ns 1 Pq int
.It Sy zfs_dio_enabled Ns = Ns Sy 1 Ns | Ns 0 Pq int
Enable Direct I/O.
If this setting is 0, then all I/O requests will be directed through the ARC
acting as though the dataset property
@@ -312,6 +312,11 @@ acting as though the dataset property
was set to
.Sy disabled .
.
.It Sy zfs_dio_strict Ns = Ns Sy 0 Ns | Ns 1 Pq int
Strictly enforce alignment for Direct I/O requests, returning
.Sy EINVAL
if not page-aligned instead of silently falling back to uncached I/O.
.
.It Sy zfs_history_output_max Ns = Ns Sy 1048576 Ns B Po 1 MiB Pc Pq u64
When attempting to log an output nvlist of an ioctl in the on-disk history,
the output will not be stored if it is larger than this size (in bytes).