Not working tunable, i can't set value for zfs_arc_max

[Vichislitel]

I can't set value for zfs_arc_max . trying use kstat and windows registry.
How i can change it ?
without reboot not working, and after reboot not working.
If i change value in regestry ***\zfs_arc\zfs_arc_max
after reboot PC, value reset to zero.

System information

Kernel Version | windows 11
Architecture | x64
OpenZFS Version | zfswin2.2.6rc10

[lundman]

The kstat way no longer has any glue connecting it, as we have moved to Registry.

So upstream does do weird things with zfs_arc_max and zfs_arc_min, as they update arc_c_max and arc_c_min respectively, then return to 0 value.
This doesn't quite work for Windows and Registry, as they are 0 for next boot.

Let's think about the right answer for us.

[lundman]

Hmm no, not sure that is it, I set zfs_arc_max to 0x123456789abcdeff in Registry, but the code:

	Status = ZwQueryValueKey(
	    regfd,
	    &entry,
	    KeyValueFullInformation,
	    buffer,
	    length,
	    &length);

		KEY_VALUE_FULL_INFORMATION *kv =
		    (KEY_VALUE_FULL_INFORMATION *)buffer;

			// Set up buffers to SET value.
			val = &buffer[kv->DataOffset];
			len = kv->DataLength;

Lists val as 0000000000000000. How odd:

dt kv
Local var @ 0xffffcc87fc49a8a8 Type _KEY_VALUE_FULL_INFORMATION*
0xffffbb85`ddf0b640 
   +0x000 TitleIndex       : 0
   +0x004 Type             : 0xb
   +0x008 DataOffset       : 0x30
   +0x00c DataLength       : 8
   +0x010 NameLength       : 0x16
   +0x014 Name             : [1]  "z"
0: kd> dw 0xffffbb85`ddf0b640
ffffbb85`ddf0b640  0000 0000 000b 0000 0030 0000 0008 0000
ffffbb85`ddf0b650  0016 0000 007a 0066 0073 005f 0061 0072
ffffbb85`ddf0b660  0063 005f 006d 0061 0078 6120 7876 2032
ffffbb85`ddf0b670  0000 0000 0000 0000 0000 0002 0000 0000

dt val
Local var @ 0xffffcc87fc49a8f0 Type void*
0xffffbb85`ddf0b670 
Void
0: kd> dw 0xffffbb85`ddf0b670
ffffbb85`ddf0b670  0000 0000 0000 0000 0000 0002 0000 0000

[lundman]

Ah ok so this is expected, it is due to:

	val = *(uint64_t *)(*ptr);

	if (val != 0 && (val < MIN_ARC_MAX || val <= arc_c_min ||
	    val >= arc_all_memory()))
		return (SET_ERROR(EINVAL));

If you set zfs_arc_max to an invalid amount, it will be ignored, and reset back to 0.
Now, why doesn't survive reboots

[lundman]

Ah neat, so we read in Registry as one of the first things, before we init SPL, so that Registry can contain tunables for SPL, this means total_memory has not yet been set (Used by arc_all_memory()) so, any value > 0 would fail and zfs_arc_max reset back to 0.

This is addressed in 65ae459

[leonik3]

Hello,
I also have a problem setting the values of the zfs_arc_max and zfs_arc_min parameters. I try to set different values in the parameters and never succeed. After a reboot it always returns to zero.

I have tried such decimal values for example:
1048576 (1024 × 1024 = 1 MiB)
1073741824 (1024 × 1024 * 1024 = 1 GiB)
4294967296 (1024 × 1024 * 1024 * 4 = 4 GiB)
17179869184 (1024 × 1024 * 1024 * 16 = 16 GiB)

What are the possible values?