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Relax special_small_blocks restrictions

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special_small_blocks is applied to blocks after compression, so it
makes no sense to demand its values to be power of 2.  At most
they could be multiple of 512, but that would still buy us nothing,
so lets allow them be any within SPA_MAXBLOCKSIZE.

Also special_small_blocks does not really need to depend on the
set recordsize, enabled pool features or presence of special vdev.
At worst in any of those cases it will just do nothing, so we
should not complicate users lives by artificial limitations.

While there, polish comments for recordsize and volblocksize.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #17497
This commit is contained in:
Alexander Motin
2025-07-02 14:11:37 -04:00
committed by GitHub
parent 17ee0fd4fa
commit 4e92aee233
13 changed files with 31 additions and 193 deletions
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man/man7/zfsprops.7
+16 -13
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@@ -541,10 +541,16 @@ The
.Sy blocksize
cannot be changed once the volume has been written, so it should be set at
volume creation time.
The default
.Sy blocksize
for volumes is 16 KiB.
Any power of 2 from 512 bytes to 128 KiB is valid.
The size specified must be a power of two greater than or equal to
.Ar 512 B
and less than or equal to
.Ar 128 KiB .
If the
.Sy large_blocks
feature is enabled on the pool, the size may be up to
.Ar 16 MiB .
The default size is
.Ar 16 KiB .
.Pp
This property can also be referred to by its shortened column name,
.Sy volblock .
@@ -1282,10 +1288,12 @@ This feature must be enabled to be used
.It Sy special_small_blocks Ns = Ns Ar size
This value represents the threshold block size for including small file
or zvol blocks into the special allocation class.
Blocks smaller than or equal to this
value will be assigned to the special allocation class while greater blocks
will be assigned to the regular class.
Valid values are zero or a power of two from 512 up to 1048576 (1 MiB).
Blocks smaller than or equal to this value after compression and encryption
will be assigned to the special allocation class, while greater blocks will
be assigned to the regular class.
Valid values are from 0 to maximum block size (
.Ar 16 MiB
).
The default size is 0 which means no small file or zvol blocks
will be allocated in the special class.
.Pp
@@ -1569,11 +1577,6 @@ See
.Xr zpool-features 7
for details on ZFS feature flags.
.Pp
However, blocks larger than
.Ar 1 MiB
can have an impact on i/o latency (e.g. tying up a spinning disk for
~300ms), and also potentially on the memory allocator.
.Pp
Note that maximum size is still limited by default to
.Ar 1 MiB
on x86_32, see