# passthrough

In [Linux 6.9](https://kernelnewbies.org/Linux_6.9#Faster_FUSE_I.2FO)
a feature was added the ability to do IO "passthrough." Typically
`mergerfs` has to act as an active proxy for all read/write
requests. This results in, at times, significant overhead compared to
direct interaction with the underlying filesystems. Not because
`mergerfs` is doing anything particularly slow but because the
additional communication and data transfers required. With the
[passthrough](passthrough.md) feature `mergerfs` is able to instruct
the kernel to perform reads and writes directly on the underlying
file. Bypassing `mergerfs` entirely (for specifically reads and
writes) and thereby providing near native performance.

This performance does come at the cost of some functionality as
`mergerfs` no longer has control over reads and writes which means all
features related to read/write are affected.

* [moveonenospc](moveonenospc.md): Does not work because errors are
  not reported back to `mergerfs`.
* nullrw: Since `mergerfs` no longer receives read/write requests
  there is no read or write to ignore.
* [readahead](readahead.md): Still affects the `readahead` values of
  underlying filesystems and `mergerfs` itself but no longer relevant
  to `mergerfs` in that it isn't servicing IO requests.
* [fuse_msg_size](fuse_msg_size.md): The primary reason to increase
  the FUSE message size is to allow transfering more data per request
  which helps improve read and write performance. Without read/write
  requests being sent to `mergerfs` there is little reason to have
  larger message sizes since directory reading currently has smaller
  fixed buffer requirements.
* parallel-direct-writes: 
* cache.files: