[Libguestfs] [nbdkit PATCH 2/2] nozero: More efficient FUA handling

[Eric Blake]

The nozero filter can split a large request with FUA into several
smaller requests; optimize whether the FUA flag is passed on to the
next layer based on whether FUA is emulated with flush (only the last
write needs it) or is natively supported (every write needs it).
Missed in commit df0cc21d.

Signed-off-by: Eric Blake <eblake at redhat.com>
---
 filters/nozero/nozero.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/filters/nozero/nozero.c b/filters/nozero/nozero.c
index 035caae3..964cce9f 100644
--- a/filters/nozero/nozero.c
+++ b/filters/nozero/nozero.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2018 Red Hat Inc.
+ * Copyright (C) 2018-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -102,12 +102,22 @@ nozero_zero (struct nbdkit_next_ops *next_ops, void *nxdata,
              void *handle, uint32_t count, uint64_t offs, uint32_t flags,
              int *err)
 {
+  int writeflags = 0;
+  bool need_flush = false;
+
   assert (zeromode != NONE);
   flags &= ~NBDKIT_FLAG_MAY_TRIM;

   if (zeromode == NOTRIM)
     return next_ops->zero (nxdata, count, offs, flags, err);

+  if (flags & NBDKIT_FLAG_FUA) {
+    if (next_ops->can_fua (nxdata) == NBDKIT_FUA_EMULATE)
+      need_flush = true;
+    else
+      writeflags = NBDKIT_FLAG_FUA;
+  }
+
   while (count) {
     /* Always contains zeroes, but we can't use const or else gcc 9
      * will use .rodata instead of .bss and inflate the binary size.
@@ -115,7 +125,10 @@ nozero_zero (struct nbdkit_next_ops *next_ops, void *nxdata,
     static /* const */ char buffer[MAX_WRITE];
     uint32_t size = MIN (count, MAX_WRITE);

-    if (next_ops->pwrite (nxdata, buffer, size, offs, flags, err) == -1)
+    if (size == count && need_flush)
+      writeflags = NBDKIT_FLAG_FUA;
+
+    if (next_ops->pwrite (nxdata, buffer, size, offs, writeflags, err) == -1)
       return -1;
     offs += size;
     count -= size;
-- 
2.20.1