[src/trunk]: src/sys/ufs/ufs Write an essay explaining why ffs_write is one h...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/a1151e1479ff
branches:  trunk
changeset: 337035:a1151e1479ff
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Tue Mar 31 00:22:50 2015 +0000

description:
Write an essay explaining why ffs_write is one huge WAPBL transaction.

diffstat:

 sys/ufs/ufs/ufs_readwrite.c |  26 ++++++++++++++++++++++++--
 1 files changed, 24 insertions(+), 2 deletions(-)

diffs (47 lines):

diff -r c322d093c97b -r a1151e1479ff sys/ufs/ufs/ufs_readwrite.c
--- a/sys/ufs/ufs/ufs_readwrite.c       Tue Mar 31 00:03:58 2015 +0000
+++ b/sys/ufs/ufs/ufs_readwrite.c       Tue Mar 31 00:22:50 2015 +0000
@@ -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 @@
 
        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);