Optimize copying from local cache to /dev/fuse

[jiefenghuang]

What would you like to be added:

• todo: return readResultFd for read request for go-fuse, and copy directly from cache file to /dev/fuse；
• potential issue: unable to verify cache checksum

Why is this needed:

reduce memory copy and memory management overhead in user space

[polyrabbit]

It would be great if zero copy could be achieved. In our scenario, the higher the cache hit rate, the higher the CPU consumption — sometimes reaching up to 20 cores. The main CPU overhead comes from memory copying when reading cache files, especially when read amplification is severe. This can impact normal read requests.

[davies]

We have tried this and no much gain actually. In the best case, the overhead of moving 4k pages from page cache to FUSE is not much faster than copying, because FUSE does support huge page.