add documentation on SMB client EXDEV / EIO issue

README.md

@@ -288,8 +288,19 @@ A B C
 
 # Known Issues / Bugs
 
+#### Samba
+* Moving files or directories between directories on a SMB share fail with IO errors.    
+    
+    This isn't an issue with Samba but come SMB clients. GVFS-fuse v1.20.3 and prior (found in Ubuntu 14.04 among others) failed to handle certain error codes correctly. Particularly **EXDEV** which is returned by **rename** when the call is crossing mountpoints. When a program gets an **EXDEV** it needs to explicitly take an alternate action to accomplish it's goal. In the case of **mv** or similar it tries **rename** and on **EXDEV** falls back to a manual copying of data between the two locations and unlinking the source. In these older versions of GVFS-fuse if it received **EXDEV** it would translate that into **EIO**. This would cause **mv** or most any application attempting to move files around on that SMB share to fail with a IO error.    
+    
+    GVFS-fuse v1.22.0 and above fixed this issue but a large number of systems use the older release. On Ubuntu the version can be checked by issuing `apt-cache showpkg gvfs-fuse`. Most distros released in 2015 seem to have the updated release and will work fine but older systems may not. Upgrading gvfs-fuse or the distro in general will address the problem.    
+    
+    Interestingly it appears that OSX is similary affected but it is not yet known why or if there are any workarounds. Windows 7, Windows 8.1, Windows 10, and ChromeOS all work fine.
+
+#### Supplemental groups
 * Due to the overhead of [getgroups/setgroups](http://linux.die.net/man/2/setgroups) mergerfs utilizes a cache. This cache is opportunistic and per thread. Each thread will query the supplemental groups for a user when that particular thread needs to change credentials and will keep that data for the lifetime of the mount or thread. This means that if a user is added to a group it may not be picked up without the restart of mergerfs. However, since the high level FUSE API's (at least the standard version) thread pool dynamically grows and shrinks it's possible that over time a thread will be killed and later a new thread with no cache will start and query the new data.    
-* The gid cache uses fixed storage to simplify the design and be compatible with older systems which may not have C++11 compilers (as the original design required). There is enough storage for 256 users' supplemental groups. Each user is allowed upto 32 supplemental groups. Linux >= 2.6.3 allows upto 65535 groups per user but most other *nixs allow far less. NFS allowing only 16. The system does handle overflow gracefully. If the user has more than 32 supplemental groups only the first 32 will be used. If more than 256 users are using the system when an uncached user is found it will evict an existing user's cache at random. So long as there aren't more than 256 active users this should be fine. If either value is too low for your needs you will have to modify `gidcache.hpp` to increase the values. Note that doing so will increase the memory needed by each thread.
+    
+    The gid cache uses fixed storage to simplify the design and be compatible with older systems which may not have C++11 compilers (as the original design required). There is enough storage for 256 users' supplemental groups. Each user is allowed upto 32 supplemental groups. Linux >= 2.6.3 allows upto 65535 groups per user but most other *nixs allow far less. NFS allowing only 16. The system does handle overflow gracefully. If the user has more than 32 supplemental groups only the first 32 will be used. If more than 256 users are using the system when an uncached user is found it will evict an existing user's cache at random. So long as there aren't more than 256 active users this should be fine. If either value is too low for your needs you will have to modify `gidcache.hpp` to increase the values. Note that doing so will increase the memory needed by each thread.
 
 # FAQ