[Libguestfs] [nbdkit PATCH v2 04/24] plugins: Implement .pread emulation cache

[Eric Blake]

For our plugins which are reading from one or more local files,
calling .pread is likely to populate the kernel's file cache to our
advantage; these plugins are complicated enough that there is nothing
better like posix_fadvise() that we can try. Implementing .can_cache
is sufficient to let nbdkit do the desired work on our behalf.

Full list of plugins changed:
ext2, floppy, iso, linuxdisk, partitioning

Note that the tar plugin would likewise probably benefit from .pread
treatment; but for that, we'd first have to wire up .can_cache to the
perl language binding.

Signed-off-by: Eric Blake <eblake at redhat.com>
---
 plugins/ext2/ext2.c                 | 10 +++++++++-
 plugins/floppy/floppy.c             | 11 ++++++++++-
 plugins/iso/iso.c                   | 10 +++++++++-
 plugins/linuxdisk/linuxdisk.c       |  9 +++++++++
 plugins/partitioning/partitioning.c | 11 ++++++++++-
 5 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/plugins/ext2/ext2.c b/plugins/ext2/ext2.c
index 17f88fe..6698d99 100644
--- a/plugins/ext2/ext2.c
+++ b/plugins/ext2/ext2.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2017-2018 Red Hat Inc.
+ * Copyright (C) 2017-2019 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -212,6 +212,13 @@ ext2_can_fua (void *handle)
   return NBDKIT_FUA_NATIVE;
 }

+static int
+ext2_can_cache (void *handle)
+{
+  /* Let nbdkit call pread to populate the file system cache. */
+  return NBDKIT_CACHE_EMULATE;
+}
+
 /* It might be possible to relax this, but it's complicated.
  *
  * It's desirable for ‘nbdkit -r’ to behave the same way as
@@ -345,6 +352,7 @@ static struct nbdkit_plugin plugin = {
   .open              = ext2_open,
   .close             = ext2_close,
   .can_fua           = ext2_can_fua,
+  .can_cache         = ext2_can_cache,
   .get_size          = ext2_get_size,
   .pread             = ext2_pread,
   .pwrite            = ext2_pwrite,
diff --git a/plugins/floppy/floppy.c b/plugins/floppy/floppy.c
index ebdea5b..41a2364 100644
--- a/plugins/floppy/floppy.c
+++ b/plugins/floppy/floppy.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
@@ -128,6 +128,14 @@ floppy_can_multi_conn (void *handle)
   return 1;
 }

+/* Cache. */
+static int
+floppy_can_cache (void *handle)
+{
+  /* Let nbdkit call pread to populate the file system cache. */
+  return NBDKIT_CACHE_EMULATE;
+}
+
 /* Read data from the file. */
 static int
 floppy_pread (void *handle, void *buf, uint32_t count, uint64_t offset)
@@ -199,6 +207,7 @@ static struct nbdkit_plugin plugin = {
   .open              = floppy_open,
   .get_size          = floppy_get_size,
   .can_multi_conn    = floppy_can_multi_conn,
+  .can_cache         = floppy_can_cache,
   .pread             = floppy_pread,
   .errno_is_preserved = 1,
 };
diff --git a/plugins/iso/iso.c b/plugins/iso/iso.c
index 586f1f9..4728ff3 100644
--- a/plugins/iso/iso.c
+++ b/plugins/iso/iso.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
@@ -215,6 +215,13 @@ iso_can_multi_conn (void *handle)
   return 1;
 }

+static int
+iso_can_cache (void *handle)
+{
+  /* Let nbdkit call pread to populate the file system cache. */
+  return NBDKIT_CACHE_EMULATE;
+}
+
 /* Read data from the file. */
 static int
 iso_pread (void *handle, void *buf, uint32_t count, uint64_t offset)
@@ -249,6 +256,7 @@ static struct nbdkit_plugin plugin = {
   .open              = iso_open,
   .get_size          = iso_get_size,
   .can_multi_conn    = iso_can_multi_conn,
+  .can_cache         = iso_can_cache,
   .pread             = iso_pread,
   .errno_is_preserved = 1,
 };
diff --git a/plugins/linuxdisk/linuxdisk.c b/plugins/linuxdisk/linuxdisk.c
index 1ba7114..99dbc99 100644
--- a/plugins/linuxdisk/linuxdisk.c
+++ b/plugins/linuxdisk/linuxdisk.c
@@ -159,6 +159,14 @@ linuxdisk_can_multi_conn (void *handle)
   return 1;
 }

+/* Cache. */
+static int
+linuxdisk_can_cache (void *handle)
+{
+  /* Let nbdkit call pread to populate the file system cache. */
+  return NBDKIT_CACHE_EMULATE;
+}
+
 /* Read data from the virtual disk. */
 static int
 linuxdisk_pread (void *handle, void *buf, uint32_t count, uint64_t offset,
@@ -221,6 +229,7 @@ static struct nbdkit_plugin plugin = {
   .open              = linuxdisk_open,
   .get_size          = linuxdisk_get_size,
   .can_multi_conn    = linuxdisk_can_multi_conn,
+  .can_cache         = linuxdisk_can_cache,
   .pread             = linuxdisk_pread,
   .errno_is_preserved = 1,
 };
diff --git a/plugins/partitioning/partitioning.c b/plugins/partitioning/partitioning.c
index 630c6d2..90333bf 100644
--- a/plugins/partitioning/partitioning.c
+++ b/plugins/partitioning/partitioning.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
@@ -297,6 +297,14 @@ partitioning_can_multi_conn (void *handle)
   return 1;
 }

+/* Cache. */
+static int
+partitioning_can_cache (void *handle)
+{
+  /* Let nbdkit call pread to populate the file system cache. */
+  return NBDKIT_CACHE_EMULATE;
+}
+
 /* Read data. */
 static int
 partitioning_pread (void *handle, void *buf, uint32_t count, uint64_t offset)
@@ -426,6 +434,7 @@ static struct nbdkit_plugin plugin = {
   .open              = partitioning_open,
   .get_size          = partitioning_get_size,
   .can_multi_conn    = partitioning_can_multi_conn,
+  .can_cache         = partitioning_can_cache,
   .pread             = partitioning_pread,
   .pwrite            = partitioning_pwrite,
   .flush             = partitioning_flush,
-- 
2.20.1