On 12/14/2015 03:05 PM, Max Reitz wrote:
On 14.12.2015 18:43, Vladimir Sementsov-Ogievskiy wrote:
The new feature for qcow2: storing dirty bitmaps.
Only dirty bitmaps relative to this qcow2 image should be stored in it.
Strings started from +# are RFC-strings, not to be commited of course.
Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
---
docs/specs/qcow2.txt | 151 ++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 150 insertions(+), 1 deletion(-)
Overall: Looks better to me. Good enough for me to ACK it, but I still
have some issues with it.
Let's evaluate the main point of critique I had: I really want this not
to be qemu-specific but potentially useful to all programs.
Pretty good: You do implicitly describe what a (dirty) bitmap looks like
by describing how to obtain the bit offset of a certain byte guest
offset. So it's not an opaque binary data dump anymore.
(Why only "pretty good"? I find the description to be a bit too
"implicit", I think a separate section describing the bitmap structure
would be better.)
Good: The bitmap actually describes the qcow2 file.
Not so good: While now any program knows how to read the bitmap and that
it does refer to this qcow2 file, it's interpretation is not so easy
still. Generally, a dirty bitmap has some reference point, that is the
state of the disk when the bitmap was cleared or created. For instance,
for incremental backups, whenever you create a backup based on a dirty
bitmap, the dirty bitmap is cleared and the backup target is then said
reference point.
I think it would be nice to put that reference point (i.e. the name of
an image file that contains the clean image) into the dirty bitmap
header, if possible.
This starts to get a little spooky, because QEMU doesn't necessarily
know where (or what) the reference is. QEMU doesn't even know where the
last incremental is.
It might be hard to store something meaningful here.
I suppose the management application could do it itself if it wants to.
(Note: I won't comment on orthography, because I feel like that is
something a native speaker should do. O:-))
diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 121dfc8..3c89580 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -103,7 +103,17 @@ 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: Dirty bitmaps bit.
+ This bit is responsible for Dirty bitmaps
+ extension consistency.
+ If it is set, but there is no Dirty bitmaps
+ extensions, this should be considered as an
+ error.
+ If it is not set, but there is a Dirty bitmaps
+ extension, its data should be considered as
+ inconsistent.
+
+ Bits 1-63: Reserved (set to 0)
96 - 99: refcount_order
Describes the width of a reference count block entry
(width
@@ -123,6 +133,7 @@ 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 - Dirty bitmaps
other - Unknown header extension, can be safely
ignored
@@ -166,6 +177,31 @@ the header extension data. Each entry look like this:
terminated if it has full length)
+== Dirty bitmaps ==
+
+Dirty bitmaps is an optional header extension. It provides an ability to store
+dirty bitmaps in a qcow2 image. The data of this extension should be considered
+as consistent only if corresponding auto-clear feature bit is set (see
+autoclear_features above).
+The fields of Dirty bitmaps extension are:
+
+ 0 - 3: nb_dirty_bitmaps
+ The number of dirty bitmaps contained in the image. Valid
+ values: 1 - 65535.
Again, I don't see a reason for why we should impose a strict upper
limit here. I'd prefer "Note that qemu currently only supports up to
65535 dirty bitmaps per image."
After discussing this with Eric, I agree.
It's hard to present justification for arbitrary limits if our only
concern is "That's just too many."
Let's list the pragmatic limit we intend to impose in QEMU, but allow
the format to use the fill width of this field.
+# Let's be strict, the feature should be deleted with deleting last bitmap.
+
+ 4 - 7: dirty_bitmap_directory_size
+ Size of the Dirty Bitmap Directory in bytes. It should be
+ equal to sum of sizes of all (nb_dirty_bitmaps) dirty bitmap
+ headers.
No, it "should" not be equal, it *must* be equal. But I think you can
just omit that last sentence, that would be just as fine.
It's informative, though. Just another clarifying detail that the bitmap
directory is the collection of all the dirty bitmap headers.
Replacing "should" with "must" is sufficient.
+# This field is necessary to effectively read Dirty Bitmap Directory, because
+# it's entries (which are dirty bitmap headers) may have different lengths.
+
+ 8 - 15: dirty_bitmap_directory_offset
+ Offset into the image file at which the Dirty 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 +396,116 @@ Snapshot table entry:
variable: Padding to round up the snapshot table entry size to the
next multiple of 8.
+
+
+== Dirty bitmaps ==
+
+The feature supports storing dirty bitmaps in a qcow2 image. All dirty bitmaps
+are relating to the virtual disk, stored in this image.
+
+=== Dirty Bitmap Directory ===
+
+Each dirty bitmap saved in the image is described in a Dirty Bitmap Directory
+entry. Dirty Bitmap Directory is a contiguous area in the image file, whose
+starting offset and length are given by the header extension fields
+dirty_bitmap_directory_offset and dirty_bitmap_directory_size. The entries of
+the bitmap directory have variable length, depending on the length of the
+bitmap name. These entries are also called dirty bitmap headers.
+
+Dirty Bitmap Directory Entry:
+
+ Byte 0 - 7: dirty_bitmap_table_offset
+ Offset into the image file at which the Dirty Bitmap Table
+ (described below) for the bitmap starts. Must be aligned to
+ a cluster boundary.
+
+ 8 - 11: dirty_bitmap_table_size
+ Number of entries in the Dirty 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 should be autoloaded as block dirty bitmap
+ and tracking should be started. Type of the bitmap
+ should be 'Dirty Tracking Bitmap'.
I find the wording a bit too qemu-specific. How about this:
This bitmap is the default dirty bitmap for the virtual disk represented
by this qcow2 image. It should track all write accesses immediately
after the image has been opened.
Closer; though we can certainly have more than one, so "a" may not be
appropriate.
"This bitmap is a dirty tracking bitmap for the virtual disk represented
by this qcow2 image."
And to avoid "should" again:
"All writes to this file must also be represented in this bitmap."
And I find the "should" in "Type of the bitmap should be..." a bit too
weak. Maybe "This flag is valid only for Dirty Tracking Bitmaps" is better.
Sure.
+
+ Bits 2 - 31 are reserved and must be 0.
+
+ 16 - 17: name_size
+ Size of the bitmap name. Valid values: 1 - 1023.
+
+ 18: type
+ This field describes the sort of the bitmap.
+ Values:
+ 0: Dirty Tracking Bitmap
If we allow different kinds of bitmaps, it should not be called "dirty
bitmap" everywhere anymore.
Agreed.
+
+ Values 1 - 255 are reserved.
+# This is mostly for error checking and information in qemu-img info output.
+# The other types may be, for example, "Backup Bitmap" - to make it possible
+# stop backup job on vm stop and resume it later. The another one is "Sector
+# Alloction Bitmap" (Fam, John, please comment).
I'm waiting for their comments because that sounds like "refcount table
with refcount_bits=1" to me. :-)
The idea is that we may allow for bitmaps to store other kinds of
information, like "allocated in this layer." That information is not
necessarily useful to qcow2, but it might be for other image formats. If
we ever do add such subtypes, we can always add a new reserved entry:
# 1: Reserved - Invalid for qcow2
# 2: Backup Progress Bitmap ...
# 3-255: Reserved
Backup progress bitmaps may indeed be useful and sane information to
store in a qcow2, though.
Fam may have more opinions as he's been working on this area of thought
recently.
+ 19: granularity_bits
+ Granularity bits. Valid values are: 0 - 31.
+# Now, qemu allows creating bitmaps with granularity as a 32bit value. And
+# there are no reasons of increasing it.
Good (implicit) question. I can't imagine any reason for wanting to have
a coarser granularity than 2 GB, but I do think there may be a need in
the future for some people.
Once again, I think we should discriminate between what is generally a
useful limitation and what is simply due to qemu not supporting anything
else right now.
Thus, I think it would be better to increase the range to 0 - 63 and
make a note that qemu only supports values up to 31 right now.
I suppose that won't hurt anything.
(I look forward to the future where the hard drives are so big and the
network bandwidth so bountiful that 2GB granularity is seen as "too
fine-grained!")
+
+ Granularity is calculated as
+ granularity = 1 << granularity_bits
+
+ Granularity of the bitmap is how many bytes of the image
+ accounts for one bit of the bitmap.
+# To be closer to qcow2 and its reality, I've decided to use byte-granularity
+# here, not sector-granularity.
I like that. But do note that qcow2 does align everything at least to
512 bytes, so having used sector granularity wouldn't have been too bad.
+
+ variable: The name of the bitmap (not null terminated). Should be
+ unique among all dirty bitmap names within the Dirty
+ bitmaps extension.
+
+ variable: Padding to round up the Dirty Bitmap Directory Entry size
+ to the next multiple of 8.
What I'd like here is variable additional information based on the
bitmap type. For some types, this may be absolutely necessary; for dirty
tracking bitmaps it depends on what we do about the reference point thing.
The reference point thing is the following: As mentioned at the top, I'd
like there to be some kind of description of what the clean state was.
As far as I know, this is generally a backup in the form of a file. In
that case, we could put that filename here.
We may also have exported that backup to an NBD server and we're not
sure (on the local end) where that data is anymore, though.
For local utility usage, when a reference is possible, we might be able
to list it as an optional nice thing, but I think requiring it might be
difficult.
I don't think not having a reference point description is a serious show
stopper. qemu itself does rely on the management layer to know which
bitmap to use when. But I think it would be pretty nice to have it here.
I'm not opposed to listing some "nice" information when available.
last_backup /path/to/incremental.5.qcow2
base_backup /path/to/reference.qcow2
+
+=== Dirty Bitmap Table ===
+
+Dirty bitmaps are stored using a one-level (not two-level like refcounts and
+guest clusters mapping) structure for the mapping of bitmaps to host clusters.
+It is called Dirty Bitmap Table.
+
+Each Dirty Bitmap Table has a variable size (stored in the Dirty Bitmap
+Directory Entry) and may use multiple clusters, however it must be contiguous
+in the image file.
+
+Given an offset (in bytes) into the bitmap, the offset into the image file can
+be obtained as follows:
+
+ byte_offset =
+ dirty_bitmap_table[offset / cluster_size] + (offset % cluster_size)
+
+Taking into account the granularity of the bitmap, an offset in bits into the
+image file, corresponding to byte number byte_nr of the image can be calculated
+like this:
+
+ bit_offset =
+ byte_offset(byte_nr / granularity / 8) * 8 + (byte_nr / granularity) %
8
+
+Note: the last formula is only for understanding the things, it is unlikely for
+it to be useful in a program.
I think this note is superfluous. All the pseudo-code in this file is
only that, pseudo-code. ;-)
Apart from that, I think this last formula should be in its own section
("Dirty Bitmaps" or simply "Bitmaps") which describes the structure of a
bitmap. Putting this term there should basically suffice.
I was about to say I'd like it to define the bit order, too (i.e. "bit
offset 0 is the LSb"), but, well, it just uses the bit order used
everywhere in qcow2.
+
+Dirty Bitmap Table entry:
+
+ Bit 0: Reserved and should 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 host cluster offset. Must be aligned to a
+ cluster boundary. If the offset is 0, the cluster is
+ unallocated, see bit 0 description.
+
+ 56 - 63: Reserved, must be 0.
Looks good.
Max
Thank you, Max & Vladimir.
--js