Core: added debug logging of writev() in ngx_write_chain_to_file().

src/os/unix/ngx_files.c

diff --git a/src/os/unix/ngx_files.c b/src/os/unix/ngx_files.c
index d71aec3..65d2320 100644
--- a/src/os/unix/ngx_files.c
+++ b/src/os/unix/ngx_files.c
@@ -241,6 +241,9 @@
             return NGX_ERROR;
         }
 
+        ngx_log_debug2(NGX_LOG_DEBUG_CORE, file->log, 0,
+                       "writev: %d, %z", file->fd, n);
+
         file->sys_offset += n;
         file->offset += n;
         total += n;