Am 11.01.2016 um 14:05 hat Vladimir Sementsov-Ogievskiy geschrieben:
The new feature for qcow2: storing bitmaps.
This patch adds new header extension to qcow2 - Bitmaps Extension. It
provides an ability to store virtual disk related bitmaps in a qcow2
image. For now there is only one type of such bitmaps: Dirty Tracking
Bitmap, which just tracks virtual disk changes from some moment.
Note: Only bitmaps, relative to the virtual disk, stored in qcow2 file,
should be stored in this qcow2 file. The size of each bitmap
(considering its granularity) is equal to virtual disk size.
Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
---
v7:
- Rewordings, grammar.
Max, Eric, John, thank you very much.
- add last paragraph: remaining bits in bitmap data clusters must be
zero.
- s/Bitmap Directory/bitmap directory/ and other names like this at
the request of Max.
v6:
- reword bitmap_directory_size description
- bitmap type: make 0 reserved
- extra_data_size: resize to 4bytes
Also, I've marked this field as "must be zero". We can always change
it, if we decide allowing managing app to specify any extra data, by
defining some magic value as a top of user extra data.. So, for now
non zeor extra_data_size should be considered as an error.
- swap name and extra_data to give good alignment to extra_data.
v5:
- 'Dirty bitmaps' renamed to 'Bitmaps', as we may have several types of
bitmaps.
- rewordings
- move upper bounds to "Notes about Qemu limits"
- s/should/must somewhere. (but not everywhere)
- move name_size field closer to name itself in bitmap header
- add extra data area to bitmap header
- move bitmap data description to separate section
docs/specs/qcow2.txt | 172
++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 171 insertions(+), 1 deletion(-)
diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 121dfc8..997239d 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -103,7 +103,18 @@ in the description of a field.
write to an image with unknown auto-clear
features if it
clears the respective bits from this field first.
- Bits 0-63: Reserved (set to 0)
+ Bit 0: Bitmaps extension bit
+ This bit indicates consistency for
the bitmaps
+ extension data.
+
+ It is an error if this bit is set
without the
+ bitmaps extension present.
+
+ If the bitmaps extension is present
but this
+ bit is unset, the bitmaps extension
data is
+ inconsistent.
It may as well be consistent, but we don't know.
Perhaps something like "must be considered inconsistent" or "is
potentially inconsistent".
+
+ Bits 1-63: Reserved (set to 0)
96 - 99: refcount_order
Describes the width of a reference count block
entry (width
@@ -123,6 +134,7 @@ be stored. Each extension has a structure like
the following:
0x00000000 - End of the header extension area
0xE2792ACA - Backing file format name
0x6803f857 - Feature name table
+ 0x23852875 - Bitmaps extension
other - Unknown header extension, can
be safely
ignored
@@ -166,6 +178,34 @@ the header extension data. Each entry look
like this:
terminated if it has full length)
+== Bitmaps extension ==
+
+The bitmaps extension is an optional header extension. It provides
the ability
+to store bitmaps related to a virtual disk. For now, there is only
one bitmap
+type: the dirty tracking bitmap, which tracks virtual disk changes
>from some
+point in time.
I have one major problem with this patch, and it starts here.
The spec talks about dirty tracking bitmaps all the way, but it never
really defines what a dirty tracking bitmap even contains. It has a few
hints here and there, but they aren't consistent.
Here's the first hint: They track "virtual disk changes", which implies
they track guest clusters rather than host clusters.
+The data of the extension should be considered consistent only if the
+corresponding auto-clear feature bit is set, see autoclear_features
above.
+
+The fields of the bitmaps extension are:
+
+ 0 - 3: nb_bitmaps
+ The number of bitmaps contained in the image.
Must be
+ greater than or equal to 1.
+
+ Note: Qemu currently only supports up to 65535
bitmaps per
+ image.
+
+ 4 - 7: bitmap_directory_size
+ Size of the bitmap directory in bytes. It is the
cumulative
+ size of all (nb_bitmaps) bitmap headers.
+
+ 8 - 15: bitmap_directory_offset
+ Offset into the image file at which the bitmap
directory
+ starts. Must be aligned to a cluster boundary.
+
+
== Host cluster management ==
qcow2 manages the allocation of host clusters by maintaining a
reference count
@@ -360,3 +400,133 @@ Snapshot table entry:
variable: Padding to round up the snapshot table entry
size to the
next multiple of 8.
+
+
+== Bitmaps ==
+
+As mentioned above, the bitmaps extension provides the ability to
store bitmaps
+related a virtual disk. This section describes how these bitmaps are
stored.
s/related/related to/
+Note: all bitmaps are related to the virtual disk stored in this image.
+
+=== Bitmap directory ===
+
+Each bitmap saved in the image is described in a bitmap directory
entry. The
+bitmap directory is a contiguous area in the image file, whose
starting offset
+and length are given by the header extension fields
bitmap_directory_offset and
+bitmap_directory_size. The entries of the bitmap directory have
variable
+length, depending on the length of the bitmap name and extra data.
These
+entries are also called bitmap headers.
+
+Structure of a bitmap directory entry:
+
+ Byte 0 - 7: bitmap_table_offset
+ Offset into the image file at which the bitmap
table
+ (described below) for the bitmap starts. Must be
aligned to
+ a cluster boundary.
+
+ 8 - 11: bitmap_table_size
+ Number of entries in the bitmap table of the
bitmap.
+
+ 12 - 15: flags
+ Bit
+ 0: in_use
+ The bitmap was not saved correctly and may be
+ inconsistent.
+
+ 1: auto
+ The bitmap must reflect all changes of the
virtual
+ disk by any application that would write to
this qcow2
+ file (including writes, snapshot switching,
etc.). The
+ type of this bitmap must be 'dirty tracking
bitmap'.
This suggests that we can represent snapshot switching in a dirty
tracking bitmap. Which is almost impossible if we track guest clusters,
except if loading a snapshot should mean that all bits in the bitmap are
set. However, that feels a bit useless and dirty tracking across
snapshots doesn't seem to make a whole lot of sense anyway.
Maybe what would make more sense is that a bitmap is tied to a specific
snapshot or bitmaps are included in a snapshot, so that you can revert
to the dirty status as it was when the snapshot was taken.
Or, if you really meant, that the tracking words on a host cluster
level, what clusters would be included in the bitmap? Would only the
virtual disk be included? VM state? Any metadata?
It seems we definitely need a new section on dirty tracking bitmaps that
describes what the bitmap actually means and how it's supposed to work
with snapshots. I guess we could also talk about how it works with other
changes to the image like resizing.
+ Bits 2 - 31 are reserved and must be 0.
+
+ 16: type
+ This field describes the sort of the bitmap.
+ Values:
+ 1: Dirty tracking bitmap
+
+ Values 0, 2 - 255 are reserved.
+
+ 17: granularity_bits
+ Granularity bits. Valid values: 0 - 63.
+
+ Note: Qemu currently doesn't support
granularity_bits
+ greater than 31.
+
+ Granularity is calculated as
+ granularity = 1 << granularity_bits
+
+ A bitmap's granularity is how many bytes of the
image
+ accounts for one bit of the bitmap.
+
+ 18 - 19: name_size
+ Size of the bitmap name. Must be non-zero.
+
+ Note: Qemu currently doesn't support values
greater than
+ 1023.
+
+ 20 - 23: extra_data_size
+ Size of type-specific extra data.
+
+ For now, as no extra data is defined,
extra_data_size is
+ reserved and must be zero.
+
+ variable: Type-specific extra data for the bitmap.
We talked about this in the other subthread.
+ variable: The name of the bitmap (not null terminated).
Must be
+ unique among all bitmap names within the bitmaps
extension.
+
+ variable: Padding to round up the bitmap directory entry
size to the
+ next multiple of 8.
+
+=== Bitmap table ===
+
+Bitmaps are stored using a one-level structure (as opposed to two-level
+structure like for refcounts and guest clusters mapping) for the
mapping of
+bitmap data to host clusters. This structure is called the bitmap
table.
+
+Each bitmap table has a variable size (stored in the bitmap
directory Entry)
+and may use multiple clusters, however, it must be contiguous in the
image
+file.
+
+Structure of a bitmap table entry:
+
+ Bit 0: Reserved and must be zero if bits 9 - 55 are
non-zero.
+ If bits 9 - 55 are zero:
+ 0: Cluster should be read as all zeros.
+ 1: Cluster should be read as all ones.
+
+ 1 - 8: Reserved and must be zero.
+
+ 9 - 55: Bits 9 - 55 of the host cluster offset. Must be
aligned to
+ a cluster boundary. If the offset is 0, the
cluster is
+ unallocated; in that case, bit 0 determines how
this
+ cluster should be treated when read from.
+
+ 56 - 63: Reserved and must be zero.
+
+=== Bitmap data ===
+
+As noted above, bitmap data is stored in separate clusters,
described by the
+bitmap table. Given an offset (in bytes) into the bitmap data, the
offset into
+the image file can be obtained as follows:
+
+ image_offset =
+ bitmap_table[bitmap_data_offset / cluster_size] +
+ (bitmap_data_offset % cluster_size)
+
+This offset is not defined if bits 9 - 55 of bitmap table entry are
zero (see
+above).
+
+Given an offset byte_nr into the virtual disk and the bitmap's
granularity, the
+bit offset into the bitmap can be calculated like this:
+
+ bit_offset =
+ image_offset(byte_nr / granularity / 8) * 8 +
+ (byte_nr / granularity) % 8
+
+If the size of the bitmap data is not a multiply of cluster size
then the last
+cluster of the bitmap data contains some unused tail bits. These
bits must be
+zero.
In which order are the bits stored in the bitmap?