-Block size for I/O units. Default: 4k. Values for reads and writes can be
-specified separately in the format \fIread\fR,\fIwrite\fR, either of
-which may be empty to leave that value at its default.
+Block size for I/O units. Default: 4k. Values for reads, writes, and trims
+can be specified separately in the format \fIread\fR,\fIwrite\fR,\fItrim\fR
+either of which may be empty to leave that value at its default. If a trailing
+comma isn't given, the remainder will inherit the last value set.