[Libguestfs] [nbdkit PATCH] file: Add trim support

[Eric Blake]

We already have support in the file driver for punching holes
during .zero with the may_trim flag (via FALLOC_FL_PUNCH_HOLE),
so we should use the same mechanism to support .trim.  Note that
the NBD spec says that trim is advisory (we can return success
even if we did nothing); but at the same time, it's nicer to
avoid advertising the feature if we know for sure we can't do
it, so we also need .can_trim.  Note that there's still an
element of runtime behavior, as FALLOC_FL_PUNCH_HOLE doesn't
work on all filesystems; there we fall back on the NBD
protocol allowing us to be advisory for all but a handful of
errno values that we can directly report back over NBD.

Signed-off-by: Eric Blake <eblake at redhat.com>
---

I couldn't test --filter=log results on trim commands without
at least one plugin that supports trim ;)

 plugins/file/file.c | 47 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 44 insertions(+), 3 deletions(-)

diff --git a/plugins/file/file.c b/plugins/file/file.c
index 1fe4191..081848b 100644
--- a/plugins/file/file.c
+++ b/plugins/file/file.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2013 Red Hat Inc.
+ * Copyright (C) 2013-2018 Red Hat Inc.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -175,6 +175,18 @@ file_get_size (void *handle)
   return statbuf.st_size;
 }

+static int
+file_can_trim (void *handle)
+{
+  /* Trim is advisory, but we prefer to advertise it only when we can
+   * actually (attempt to) punch holes.  */
+#ifdef FALLOC_FL_PUNCH_HOLE
+  return 1;
+#else
+  return 0;
+#endif
+}
+
 /* Read data from the file. */
 static int
 file_pread (void *handle, void *buf, uint32_t count, uint64_t offset)
@@ -219,7 +231,7 @@ file_pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset)
   return 0;
 }

-/* Write data to the file. */
+/* Write zeroes to the file. */
 static int
 file_zero (void *handle, uint32_t count, uint64_t offset, int may_trim)
 {
@@ -268,6 +280,33 @@ file_flush (void *handle)
   return 0;
 }

+/* Punch a hole in the file. */
+static int
+file_trim (void *handle, uint32_t count, uint64_t offset)
+{
+  int r = -1;
+#ifdef FALLOC_FL_PUNCH_HOLE
+  struct handle *h = handle;
+
+  /* Trim is advisory; we don't care if it fails for anything other
+   * than EIO or EPERM. */
+  r = fallocate (h->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+                 offset, count);
+  if (r < 0) {
+    if (errno != EPERM && errno != EIO) {
+      nbdkit_debug ("ignoring failed fallocate during trim: %m");
+      r = 0;
+    }
+    else
+      nbdkit_error ("fallocate: %m");
+  }
+#else
+  /* Based on .can_trim, this should not be reached. */
+  errno = EOPNOTSUPP;
+#endif
+  return r;
+}
+
 static struct nbdkit_plugin plugin = {
   .name              = "file",
   .longname          = "nbdkit file plugin",
@@ -279,10 +318,12 @@ static struct nbdkit_plugin plugin = {
   .open              = file_open,
   .close             = file_close,
   .get_size          = file_get_size,
+  .can_trim          = file_can_trim,
   .pread             = file_pread,
   .pwrite            = file_pwrite,
-  .zero              = file_zero,
   .flush             = file_flush,
+  .trim              = file_trim,
+  .zero              = file_zero,
   .errno_is_preserved = 1,
 };

-- 
2.14.3