Module Name: src
Committed By: riastradh
Date: Tue Mar 31 00:22:50 UTC 2015
Modified Files:
src/sys/ufs/ufs: ufs_readwrite.c
Log Message:
Write an essay explaining why ffs_write is one huge WAPBL transaction.
To generate a diff of this commit:
cvs rdiff -u -r1.116 -r1.117 src/sys/ufs/ufs/ufs_readwrite.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/ufs/ufs/ufs_readwrite.c
diff -u src/sys/ufs/ufs/ufs_readwrite.c:1.116 src/sys/ufs/ufs/ufs_readwrite.c:1.117
--- src/sys/ufs/ufs/ufs_readwrite.c:1.116 Sun Mar 29 14:39:41 2015
+++ src/sys/ufs/ufs/ufs_readwrite.c Tue Mar 31 00:22:50 2015
@@ -1,4 +1,4 @@
-/* $NetBSD: ufs_readwrite.c,v 1.116 2015/03/29 14:39:41 riastradh Exp $ */
+/* $NetBSD: ufs_readwrite.c,v 1.117 2015/03/31 00:22:50 riastradh Exp $ */
/*-
* Copyright (c) 1993
@@ -32,7 +32,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(1, "$NetBSD: ufs_readwrite.c,v 1.116 2015/03/29 14:39:41 riastradh Exp $");
+__KERNEL_RCSID(1, "$NetBSD: ufs_readwrite.c,v 1.117 2015/03/31 00:22:50 riastradh Exp $");
#ifdef LFS_READWRITE
#define FS struct lfs
@@ -328,6 +328,28 @@ WRITE(void *v)
KASSERT(vp->v_type == VREG);
+ /*
+ * XXX The entire write operation must occur in a single WAPBL
+ * transaction because it may allocate disk blocks, if
+ * appending or filling holes, which is allowed to happen only
+ * if the write fully succeeds.
+ *
+ * If ubc_uiomove fails in the middle with EFAULT, we can clean
+ * up at the end with UFS_TRUNCATE. But if the power fails in
+ * the middle, there would be nobody to deallocate the blocks,
+ * without an fsck to globally analyze the file system.
+ *
+ * If the increasingly inaccurately named WAPBL were augmented
+ * with rollback records for block allocations, then we could
+ * split this into multiple transactions and commit the
+ * allocations in the last one.
+ *
+ * But WAPBL doesn't have that notion now, so we'll have to
+ * live with gigantic transactions and WAPBL tentacles in
+ * genfs_getpages/putpages to cope with the possibility that
+ * the transaction may or may not be locked on entry to the
+ * page cache.
+ */
error = UFS_WAPBL_BEGIN(vp->v_mount);
if (error) {
fstrans_done(vp->v_mount);
