Add ability to proxy ioprio of client apps (#1534)

mkdocs/docs/config/func_readdir.md

@@ -29,3 +29,15 @@ of the `readdir` call. In v6.16 that was changed to be able to grow to
 the [fuse_msg_size](fuse_msg_size.md) which allows a lot more data to
 be sent per request and therefore much better performance in certain
 situations.
+
+
+## Technical Details
+
+* On Linux
+  [getdents](https://man7.org/linux/man-pages/man2/getdents.2.html) is
+  used instead of
+  [readdir](https://man7.org/linux/man-pages/man2/readdir.2.html) in
+  order to leverage larger buffers for better performance on larger
+  directory reads as well as have more control in concurrent policies.
+* [readdir](https://man7.org/linux/man-pages/man2/readdir.2.html) is
+  used on FreeBSD.

mkdocs/docs/config/options.md

@@ -162,6 +162,8 @@ config file.
 * **scheduling-priority=INT**: Set mergerfs' scheduling
   priority. Valid values range from -20 to 19. See `setpriority` man
   page for more details. (default: -10)
+* **[proxy-ioprio](proxy-ioprio.md)=BOOL**: Use ioprio value from
+  calling threads when reading and writing. (default: false)
 * **fsname=STR**: Sets the name of the filesystem as seen in
   **mount**, **df**, etc. Defaults to a list of the source paths
   concatenated together with the longest common prefix removed.

mkdocs/docs/config/proxy-ioprio.md

@@ -0,0 +1,53 @@
+# proxy-ioprio
+
+* type: `BOOL`
+* default: `false`
+* example: `proxy-ioprio=true`
+
+In Linux certain process schedulers have the ability to
+[prioritize](https://man7.org/linux/man-pages/man2/ioprio_set.2.html)
+based on a per thread [IO
+priority](https://www.kernel.org/doc/Documentation/block/ioprio.txt)
+assigned to the software by users using tools such as
+[ionice](https://man7.org/linux/man-pages/man1/ionice.1.html). Since
+such details are not provided by the FUSE protocol, users may not use
+schedulers which support IO priority, and the extra overhead to query
+the priority and set it within mergerfs it does not by default attempt
+to mirror/proxy the value. When enabled however, for all read and
+write requests, mergerfs will query the priority of the requesting
+process/thread and set the same value on the thread within mergerfs
+making the read/write.
+
+This may be useful in situation where the system has high IO load or
+processes are known to have varying priorities. See the [man
+page](https://man7.org/linux/man-pages/man2/ioprio_set.2.html) for
+specific details.
+
+Keep in mind that if no IO scheduler has been set for a thread then by
+default the IO priority will match the CPU nice value which for
+mergerfs is set by [scheduling-priority](options.md). [Nice
+value](https://man7.org/linux/man-pages/man2/getpriority.2.html)
+proxying may be added in a future release.
+
+
+## Conflicts With Other Options
+
+If using [IO passthrough](passthrough.md) there is no reason to set
+this value to `true`. However, since `passthrough` leads to mergerfs
+IO calls being bypassed entirely will have no impact on performance or
+behavior regardless.
+
+When using `passthrough` the IO priority of the client app would be
+used directly given the IO is passed through.
+
+
+## Supported Platforms
+
+Only Linux. This is not supported on FreeBSD.
+
+
+## Performance Impact
+
+Despite the additional syscalls requires the impact appears
+undetectable with larger buffer sizes. At very small buffer sizes
+(such as 512 bytes) there is a noticable but small impact (~1.5%).

mkdocs/mkdocs.yml

@@ -81,6 +81,7 @@ nav:
   - config/inodecalc.md
   - config/threads.md
   - config/pin-threads.md
+  - config/proxy-ioprio.md
   - config/link_cow.md
   - config/fuse_msg_size.md
   - config/follow-symlinks.md