diff --git a/src/core/ngx_output_chain.c b/src/core/ngx_output_chain.c
index 391852f..7101699 100644
--- a/src/core/ngx_output_chain.c
+++ b/src/core/ngx_output_chain.c
@@ -13,6 +13,17 @@
 #define NGX_SENDFILE_LIMIT  4096
 #endif
 
+/*
+ * When DIRECTIO is enabled FreeBSD, Solaris, and MacOSX read directly
+ * to an application memory from a device if parameters are aligned
+ * to device sector boundary(512 bytes).  They fallback to usual read
+ * operation if the parameters are not aligned.
+ * Linux allows DIRECTIO only if the parameters are aligned to a filesystem
+ * sector boundary, otherwise it returns EINVAL.  The sector size is
+ * usually 512 bytes, however, on XFS it may be 4096 bytes.
+ */
+#define NGX_DIRECTIO_BLOCK  4096
+
 
 #define NGX_NONE            1
 
@@ -327,7 +338,7 @@
 
     ctx->directio = 1;
 
-    size = (size_t) (in->file_pos - (in->file_pos & ~511));
+    size = (size_t) (in->file_pos - (in->file_pos & ~(NGX_DIRECTIO_BLOCK - 1)));
 
     if (size == 0) {
 
@@ -338,7 +349,7 @@
         size = (size_t) bsize;
 
     } else {
-        size = 512 - size;
+        size = NGX_DIRECTIO_BLOCK - size;
 
         if ((off_t) size > bsize) {
             size = (size_t) bsize;
@@ -413,7 +424,7 @@
          * userland buffer direct usage conjunctly with directio
          */
 
-        b->start = ngx_pmemalign(ctx->pool, size, 512);
+        b->start = ngx_pmemalign(ctx->pool, size, NGX_DIRECTIO_BLOCK);
         if (b->start == NULL) {
             return NGX_ERROR;
         }