Unable to set special_small_blocks to more than 128 k

[Nikratio]

If I understand 624222a correctly, then since ZFS 2.0.0 I should be able to set special_small_blocks to up to 1 MB.

Indeed, on my Debian stable system zfsprops(7) says:

     special_small_blocks=size
       This value represents the threshold block size for including small file blocks into
       the special allocation class.  Blocks smaller than or equal to this value will be as‐
       signed 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 512B up to 1M.  The de‐
       fault size is 0 which means no small file blocks will be allocated in the special
       class.

However, I'm unable to actually set the value:

$ zpool create -f -R /zpools -o ashift=12 -o autotrim=on -o failmode=continue \
  -O acltype=posixacl -O relatime=on -O xattr=sa -O compression=zstd-19 \
  -O checksum=sha256 -O sync=disabled -O special_small_blocks=262144 \
  -O redundant_metadata=most -O recordsize=524288 -O encryption=on \
  -O feature@large_blocks=enabled-O keyformat=passphrase \
  -O keylocation=file:///<path> \
  memory /dev/nbd2 special /dev/nbd1
cannot create 'memory': invalid 'special_small_blocks=262144' property: must be zero or a power of 2 from 512B to 128K

Version:

$ zpool --version
zfs-2.1.5-1~bpo11+1
zfs-kmod-2.1.5-1~bpo11+1
$ uname -a
Linux vostro.rath.org 5.18.0-0.deb11.3-amd64 #1 SMP PREEMPT_DYNAMIC Debian 5.18.14-1~bpo11+1 (2022-07-28) x86_64 GNU/Linux

What am I doing wrong?

[spicyprogramming]

There's a subtle bug here

zfs/lib/libzfs/libzfs_dataset.c

Lines 1242 to 1249 in 6c3c5fc

	int maxbs = SPA_OLD_MAXBLOCKSIZE;
	char buf[64];
	if (zpool_hdl != NULL) {
	char state[64] = "";
	maxbs = zpool_get_prop_int(zpool_hdl,
	ZPOOL_PROP_MAXBLOCKSIZE, NULL);

where we can't read maxblocksize from the pool (since it hasn't been created yet) and use SPA_OLD_MAXBLOCKSIZE (128K) instead as the max value for special_small_blocks.

You should be able to set special_small_blocks=262144 once the pool is created.