Skip building directio on platforms that don't support Direct IO (#9059)

pkg/disk/directio_unsupported.go

@@ -1,7 +1,7 @@
 // +build !linux,!netbsd,!freebsd,!darwin
 
 /*
- * Minio Cloud Storage, (C) 2019 Minio, Inc.
+ * Minio Cloud Storage, (C) 2019-2020 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,9 +22,30 @@ import (
 	"os"
 )
 
-// OpenBSD and Windows not supported.
-// On OpenBSD O_DIRECT is not supported
-// On Windows there is no documentation on disabling O_DIRECT
+// OpenBSD, Windows, and illumos do not support O_DIRECT.
+// On Windows there is no documentation on disabling O_DIRECT.
+// For these systems we do not attempt to build the 'directio' dependency since
+// the O_DIRECT symbol may not be exposed resulting in a failed build.
+//
+//
+// On illumos an explicit O_DIRECT flag is not necessary for two primary
+// reasons. Note that ZFS is effectively the default filesystem on illumos
+// systems.
+//
+// One benefit of using DirectIO on Linux is that the page cache will not be
+// polluted with single-access data. The ZFS read cache (ARC) is scan-resistant
+// so there is no risk of polluting the entire cache with data accessed once.
+// Another goal of DirectIO is to minimize the mutation of data by the kernel
+// before issuing IO to underlying devices. ZFS users often enable features like
+// compression and checksumming which currently necessitates mutating data in
+// the kernel.
+//
+// DirectIO semantics for a filesystem like ZFS would be quite different than
+// the semantics on filesystems like XFS, and these semantics are not
+// implemented at this time.
+// For more information on why typical DirectIO semantics do not apply to ZFS
+// see this ZFS-on-Linux commit message:
+// https://github.com/openzfs/zfs/commit/a584ef26053065f486d46a7335bea222cb03eeea
 
 func OpenFileDirectIO(filePath string, flag int, perm os.FileMode) (*os.File, error) {
 	return os.OpenFile(filePath, flag, perm)
@@ -33,3 +54,9 @@ func OpenFileDirectIO(filePath string, flag int, perm os.FileMode) (*os.File, er
 func DisableDirectIO(f *os.File) error {
 	return nil
 }
+
+// AlignedBlock simply returns an unaligned buffer for systems that do not
+// support DirectIO.
+func AlignedBlock(BlockSize int) []byte {
+	return make([]byte, BlockSize)
+}