[PATCH] mkfs: add discard support

[Christoph Hellwig]

Call the BLKDISCARD ioctl to mark the whole disk as unused before creating
a new filesystem.  This will allow SSDs, Arrays with thin provisioning support
and virtual machines to make smarter allocation decisions.

Signed-off-by: Christoph Hellwig <hch at lst.de>

Index: xfsprogs-dev/mkfs/xfs_mkfs.c
===================================================================
--- xfsprogs-dev.orig/mkfs/xfs_mkfs.c	2009-10-06 18:02:34.000000000 +0000
+++ xfsprogs-dev/mkfs/xfs_mkfs.c	2009-10-06 18:03:55.000000000 +0000
@@ -605,6 +605,29 @@ done:
 	free(buf);
 }
 
+#ifdef __linux__
+
+#ifndef BLKDISCARD
+#define BLKDISCARD	_IO(0x12,119)
+#endif
+
+static void
+discard_blocks(dev_t dev, __uint64_t nsectors)
+{
+	int fd = libxfs_device_to_fd(dev);
+	__uint64_t range[2] = { 0, nsectors << 9 };
+
+	/*
+	 * We intentionally ignore errors from the discard ioctl.  It is
+	 * not nessecary for the mkfs functionality but just an optimization.
+	 */
+	if (fd > 0)
+		ioctl(fd, BLKDISCARD, &range);
+}
+#else
+#define discard_blocks(dev, nsectors)
+#endif
+
 int
 main(
 	int			argc,
@@ -1645,6 +1668,12 @@ main(
 		}
 	}
 
+	discard_blocks(xi.ddev, xi.dsize);
+	if (xi.rtdev)
+		discard_blocks(xi.rtdev, xi.rtsize);
+	if (xi.logdev && xi.logdev != xi.ddev)
+		discard_blocks(xi.logdev, xi.logBBsize);
+
 	if (!liflag && !ldflag)
 		loginternal = xi.logdev == 0;
 	if (xi.logname)