'qemu-img map' provides a way to determine which extents of an image
come from the top layer vs. inherited from a backing chain. This is
useful information worth exposing over NBD. There is a proposal to
add a QMP command block-dirty-bitmap-populate which can create a dirty
bitmap that reflects allocation information, at which point the
qemu:dirty-bitmap:NAME metadata context can expose that information
via the creation of a temporary bitmap, but we can shorten the effort
by adding a new qemu:allocation-depth metadata context that does the
same thing without an intermediate bitmap (this patch does not
eliminate the need for that proposal, as it will have other uses as
well).
For this patch, I just encoded a tri-state value (unallocated, from
this layer, from any of the backing layers); an obvious extension
would be to provide the actual depth in bits 31-4 while keeping bits
1-0 as a tri-state (leaving bits 3-2 unused, for ease of reading depth
from a hex number). But adding this extension would require
bdrv_is_allocated_above to return a depth number.
While documenting things, remember that although the NBD protocol has
NBD_OPT_SET_META_CONTEXT, the rest of its documentation refers to
'metadata context', which is a more apt description of what is
actually being used by NBD_CMD_BLOCK_STATUS: the user is requesting
metadata by passing one or more context names. So I also touched up
some existing wording to prefer the term 'metadata context' where it
makes sense.
Note that this patch does not actually enable any way to request a
server to enable this context; that will come in the next patch.
Signed-off-by: Eric Blake
---
docs/interop/nbd.txt | 27 ++---
include/block/nbd.h | 12 --
nbd/server.c | 92
3 files changed, 116 insertions(+), 15 deletions(-)
diff --git a/docs/interop/nbd.txt b/docs/interop/nbd.txt
index f3b3cacc9621..a55e5a8776c8 100644
--- a/docs/interop/nbd.txt
+++ b/docs/interop/nbd.txt
@@ -17,19 +17,35 @@ namespace "qemu".
== "qemu" namespace ==
-The "qemu" namespace currently contains only one type of context,
-related to exposing the contents of a dirty bitmap alongside the
-associated disk contents. That context has the following form:
+The "qemu" namespace currently contains two available metadata context
+types. The first is related to exposing the contents of a dirty
+bitmap alongside the associated disk contents. That metadata context
+is named with the following form:
qemu:dirty-bitmap:
Each dirty-bitmap metadata context defines only one flag for extents
in reply for NBD_CMD_BLOCK_STATUS:
-bit 0: NBD_STATE_DIRTY, means that the extent is "dirty"
+bit 0: NBD_STATE_DIRTY, set when the extent is "dirty"
+
+The second is related to exposing the source of various extents within
+the image, with a single metadata context named:
+
+qemu:allocation-depth
+
+In the allocation depth context, bits 0 and 1 form a tri-state value:
+
+bits 0-1: 00: NBD_STATE_DEPTH_UNALLOC, the extent is unallocated
+ 01: NBD_STATE_DEPTH_LOCAL, the extent is allocated in the
+ top level of the image
+ 10: NBD_STATE_DEPTH_BACKING, the extent is inherited from a
+ backing layer
+ 11: invalid, never returned
For NBD_OPT_LIST_META_CONTEXT the following queries are supported
-in addition to "qemu:dirty-bitmap:":
+in addition to the specific "qemu:allocation-depth" and
+"qemu:dirty-bitmap:":
* "qemu:" - returns list of all available metadata contexts in the
namespace.
@@ -55,3 +71,4 @@ the operation of that feature.
NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE
* 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports,
NBD_CMD_FLAG_FAST_ZERO
+* 5.2: NBD_CMD_BLOCK_STATUS for "qemu:allocation-depth"
diff --git a/include/block/nbd.h b/include/block/nbd.h
index 3dd9a04546ec..0bbf92f02951 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -1,5 +1,5 @@
/*
- * Copyright (C) 2016-2019 Red Hat, Inc.
+ * Copyright (C) 2016-2020 Red Hat, Inc.
* Copyright (C) 2005 Anthony Liguori
*
* Network Block Device
@@ -47,7 +47,7 @@ typedef struct NBDOptionReply NBDOptionReply;
typedef struct NBDOptionReplyMetaContext {
NBDOptionReply h; /* h.type = NBD_REP_META_CONTEXT, h.length > 4 */
uint32_t context_id;
-/* meta context name follows */
+/* metadata context name follows */
} QEMU_PACKED NBDOptionReplyMetaContext;
/* Transmission phase structs
@@ -229,7 +229,7 @@ enum {
#define NBD_MAX_BUFFER_SIZE (32 * 1024 * 1024)
/*
- * Maximum size of a protocol string (export name, meta context name,
+ * Maximum size of a protocol string (export name, metadata context name,
* etc.). Use malloc rather than stack allocation for storage of a
* string.
*/
@@ -259,6 +259,12 @@ enum {
/* Extent flags for qemu:dirty-bitmap in NBD_REPLY_TYPE_BLOCK_STATUS */
#define NBD_STATE_DIRTY (1 <<
