You need to do this before you create a volume.
.Pp
Device specific options must come before corresponding devices, e.g.
-.Dl bcachefs format --tier 0 /dev/sdb --tier 1 /dev/sdc
+.Dl bcachefs format --group=ssd /dev/sda --group=hdd /dev/sdb
.Bl -tag -width Ds
.It Fl b , Fl -block Ns = Ns Ar size
block size, in bytes (e.g. 4k)
must be greater than the btree node size
.It Fl -discard
Enable discards on subsequent devices
-.It Fl t , Fl -tier Ar index
-Specifies the tier of subsequent devices, where
-.Ar index
-is a small integer and a smaller index indicates a faster tier; tier 0
-being the fastest.
-Currently only two tiers are supported.
.It Fl q , Fl -quiet
Only print errors
.El
Set bucket size
.It Fl -discard
Enable discards
-.It Fl t , Fl -tier Ns = Ns Ar number
-Higher tier (e.g. 1) indicates slower devices
.It Fl f , Fl -force
Use device even if it appears to already be formatted
.El