(38)Function not implemented: core_output_filter: writing data to the network
The Apache HTTP Server tries to use the sendfile syscall to let the operating system take care of delivering files to the network in the most efficient way possible. When this works, it often makes responses faster and allows the server to scale to a much higher number of simultaneous requests.
Apache tries to determine at compile time whether sendfile is available and functional. Regrettably, there are occasions where sendfile will be found at compile time but not work when Apache tries to use it. This happens, in particular, when serving files from certain network file-systems (NFS or SMB, for example) or other non-standard file-systems.
In cases like these, you may see the above error message. You can instruct Apache to fall back to the slower but more reliable method of serving files by disabling sendfile using the following directive in httpd.conf:
EnableSendfile off
When sendfile is not available, Apache attempts to memory-map files for quicker serving using the mmap syscall. This syscall can sometimes have similar problems, and can be similarly disabled with:
EnableMMAP off