README: add note regarding space calculations

README.md

@@ -1,6 +1,6 @@
 % mergerfs(1) mergerfs user manual
 % Antonio SJ Musumeci <trapexit@spawn.link>
-% 2020-02-09
+% 2020-03-10
 
 # NAME
 
@@ -240,6 +240,8 @@ If all branches are filtered an error will be returned. Typically **EROFS** or *
 | newest | Pick the file / directory with the largest mtime. |
 | rand (random) | Calls **all** and then randomizes. Returns 1. |
 
+**NOTE:** If you are using an underlying filesystem that reserves blocks such as ext2, ext3, or ext4 be aware that mergerfs respects the reservation by using `f_bavail` (number of free blocks for unprivileged users) rather than `f_bfree` (number of free blocks) in policy calculations. **df** does NOT use `f_bavail`, it uses `f_bfree`, so direct comparisons between **df** output and mergerfs' policies is not appropriate.
+
 
 #### Defaults ####
 

man/mergerfs.1

@@ -1,7 +1,7 @@
 .\"t
 .\" Automatically generated by Pandoc 1.19.2.4
 .\"
-.TH "mergerfs" "1" "2020\-02\-09" "mergerfs user manual" ""
+.TH "mergerfs" "1" "2020\-03\-10" "mergerfs user manual" ""
 .hy
 .SH NAME
 .PP
@@ -642,6 +642,15 @@ Calls \f[B]all\f[] and then randomizes.
 Returns 1.
 T}
 .TE
+.PP
+\f[B]NOTE:\f[] If you are using an underlying filesystem that reserves
+blocks such as ext2, ext3, or ext4 be aware that mergerfs respects the
+reservation by using \f[C]f_bavail\f[] (number of free blocks for
+unprivileged users) rather than \f[C]f_bfree\f[] (number of free blocks)
+in policy calculations.
+\f[B]df\f[] does NOT use \f[C]f_bavail\f[], it uses \f[C]f_bfree\f[], so
+direct comparisons between \f[B]df\f[] output and mergerfs\[aq] policies
+is not appropriate.
 .SS Defaults
 .PP
 .TS