Document a known short read issue

Ref #65

src/vmod_slash.man.rst

@@ -1319,6 +1319,16 @@ get them implemented.
 FELLOW KNOWN ISSUES
 ===================
 
+* With `fellow` storage on XFS, spurious read errors - most likely
+  short reads - have been observed. While short reads are technically
+  legal to happen, handling them would complicate the `fellow` code
+  substantially, so a fix is currently not planned. If you require a
+  fix, please support the project and let us know, see _CONTRIBUTING_
+  in `README.rst`_ for details.
+
+  For best performance, it is recommended to use `fellow` storage on a
+  raw device.
+
 * On Linux with ``io_uring``, by default, `fellow` registers all of
   the memory cache as buffers using
   :ref:`io_uring_register_buffers(3)` to achieve optimal performance

src/vmod_slash.vcc

@@ -1235,6 +1235,16 @@ get them implemented.
 FELLOW KNOWN ISSUES
 ===================
 
+* With `fellow` storage on XFS, spurious read errors - most likely
+  short reads - have been observed. While short reads are technically
+  legal to happen, handling them would complicate the `fellow` code
+  substantially, so a fix is currently not planned. If you require a
+  fix, please support the project and let us know, see _CONTRIBUTING_
+  in `README.rst`_ for details.
+
+  For best performance, it is recommended to use `fellow` storage on a
+  raw device.
+
 * On Linux with ``io_uring``, by default, `fellow` registers all of
   the memory cache as buffers using
   :ref:`io_uring_register_buffers(3)` to achieve optimal performance