Re: [Qemu-devel] [PATCH v4 RFC] spec: add qcow2-dirty-bitmaps specification

[Max Reitz]

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.


(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."

> +# 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.

> +# 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.

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.

> +
> +                    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.

> +
> +                    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. :-)

> +             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.

> +
> +                    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.

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.

> +
> +=== 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

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