Merge pull request #262 from trapexit/mmap-direct_io

add details regarding rtorrent, mmap, and direct_io

README.md

@@ -296,6 +296,7 @@ Find tooling to help with managing `mergerfs` at: https://github.com/trapexit/me
 
 * Detailed guides to setting up a backup solution using mergerfs and other technologies: https://github.com/trapexit/backup-and-recovery-howtos
 * If you don't see some directories / files you expect in a merged point be sure the user has permission to all the underlying directories. If `/drive0/a` has is owned by `root:root` with ACLs set to `0700` and `/drive1/a` is `root:root` and `0755` you'll see only `/drive1/a`. Use `mergerfs.fsck` to audit the drive for out of sync permissions.
+* Do *not* use `direct_io` if you expect applications (such as rtorrent) to [mmap](http://linux.die.net/man/2/mmap) files. It is not currently supported in FUSE w/ `direct_io` enabled.
 * Since POSIX gives you only error or success on calls its difficult to determine the proper behavior when applying the behavior to multiple targets. **mergerfs** will return an error only if all attempts of an action fail. Any success will lead to a success returned.
 * The recommended options are **defaults,allow_other**. The **allow_other** is to allow users who are not the one which executed mergerfs access to the mountpoint. **defaults** is described above and should offer the best performance. It's possible that if you're running on an older platform the **splice** features aren't available and could error. In that case simply use the other options manually.
 * If write performance is valued more than read it may be useful to enable **direct_io**. Best to benchmark with and without and choose appropriately.
@@ -305,6 +306,10 @@ Find tooling to help with managing `mergerfs` at: https://github.com/trapexit/me
 
 # KNOWN ISSUES / BUGS
 
+#### rtorrent fails with ENODEV (No such device)
+
+Be sure to turn off `direct_io`. rtorrent and some other applications use [mmap](http://linux.die.net/man/2/mmap) to read and write to files and offer no failback to traditional methods. FUSE does not currently support mmap while using `direct_io`. There will be a performance penalty on writes with `direct_io` off but it's the only way to get such applications to work.
+
 #### Trashing files occasionally fails
 
 This is the same issue as with Samba. `rename` returns `EXDEV` (in our case that will really only happen with path preserving policies like `epmfs`) and the software doesn't handle the situtation well. This is unfortunately a common failure of software which moves files around. The standard indicates that an implementation `MAY` choose to support non-user home directory trashing of files (which is a `MUST`). The implementation `MAY` also support "top directory trashes" which many probably do.