If you want to know which blocks you do not have:
dtrace -n fbt::metaslab_group_alloc:entry'{ self->s = arg1; }' -n
fbt::metaslab_group_alloc:return'/arg1 != -1/{ self->s = 0 }' -n
fbt::metaslab_group_alloc:return'/self->s && (arg1 == -1)/{ @s =
quantize(self->s); self->s = 0; }' -n tick-10s'{ printa(@s); }'
and which blocks you do not have in some metaslabs:
dtrace -n fbt::space_map_alloc:entry'{ self->s = arg1; }' -n
fbt::space_map_alloc:return'/arg1 != -1/{ self->s = 0 }' -n
fbt::space_map_alloc:return'/self->s && (arg1 == -1)/{ @s = quantize(self->s);
self->s = 0; }' -n tick-10s'{ printa(@s); }'
If metaslabs_group_alloc looks like this
value ------------- Distribution ------------- count
65536 | 0
131072 |@@@@@@@@@@ 9065
262144 | 0
then you can set zfs record size to 64k
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