Cache: introduced min_free cache clearing. Clearing cache based on free space left on a file system is expected to allow better disk utilization in some cases, notably when disk space might be also used for something other than nginx cache (including nginx own temporary files) and while loading cache (when cache size might be inaccurate for a while, effectively disabling max_size cache clearing). Based on a patch by Adam Bambuch.

--- a/src/os/unix/ngx_files.h
+++ b/src/os/unix/ngx_files.h
@@ -349,6 +349,7 @@ ngx_int_t ngx_directio_off(ngx_fd_t fd);
 #endif
 
 size_t ngx_fs_bsize(u_char *name);
+off_t ngx_fs_available(u_char *name);
 
 
 #if (NGX_HAVE_OPENAT)